RE2HS(1)

NAME
re2hs - generate fast lexical analyzers for Haskell

SYNOPSIS

re2hs [ OPTIONS ] [ WARNINGS ] INPUT

Input can be either a file or - for stdin.

INTRODUCTION

re2hs works as a preprocessor. It reads the input file (which is usually a program in Haskell, but can be anything) and looks for blocks of code enclosed in special-form start/end markers. The text outside of these blocks is copied verbatim into the output file. The contents of the blocks are processed by re2hs. It translates them to code in Haskell and outputs the generated code in place of the block.

Here is an example of a small program that checks if a given string contains a decimal number:

-- re2hs $INPUT -o $OUTPUT -i
{-# LANGUAGE OverloadedStrings #-}
{-# OPTIONS_GHC -Wno-unused-record-wildcards #-}
import Data.ByteString (ByteString, index)
data State = State {
    _yyinput :: ByteString,
    _yycursor :: Int
}
%{
    re2c:YYFN = ["lexer;Bool", "State{..};State"];
    re2c:yyfill:enable = 0;
    [1-9][0-9]* { True }
    *           { False }
%}
main :: IO ()
main = case lexer State{_yyinput = "1234\0", _yycursor = 0} of
    True -> return ()
    False -> error "lexer failed!"

In the output re2hs replaced the block in the middle with the generated code:

-- Generated by re2hs
{-# LANGUAGE RecordWildCards #-}
-- re2hs $INPUT -o $OUTPUT -i
{-# LANGUAGE OverloadedStrings #-}
{-# OPTIONS_GHC -Wno-unused-record-wildcards #-}
import Data.ByteString (ByteString, index)
data State = State {
    _yyinput :: ByteString,
    _yycursor :: Int
}
yy0 :: State -> Bool
yy0 State{..} =
    let yych = index _yyinput _yycursor in
    let __ = _yycursor + 1 in let _yycursor = __ in
    case yych of
        _c | 0x31 <= _c && _c <= 0x39 ->
            yy2 State{..}
        _c | True ->
            yy1 State{..}
yy1 :: State -> Bool
yy1 State{..} =
    False
yy2 :: State -> Bool
yy2 State{..} =
    let yych = index _yyinput _yycursor in
    case yych of
        _c | 0x30 <= _c && _c <= 0x39 ->
            let __ = _yycursor + 1 in let _yycursor = __ in
            yy2 State{..}
        _c | True ->
            yy3 State{..}
yy3 :: State -> Bool
yy3 State{..} =
    True
lexer :: State -> Bool
lexer State{..} =
    yy0 State{..}
main :: IO ()
main = case lexer State{_yyinput = "1234\0", _yycursor = 0} of
    True -> return ()
    False -> error "lexer failed!"

BASICS

A re2hs program consists of a sequence of blocks intermixed with code in the target language. A block may contain definitions, configurations, rules, actions and directives in any order:

name = regular-expression ;
    A definition binds name to regular-expression. Names may contain alphanumeric characters and underscore. The regular expressions section gives an overview of re2hs syntax for regular expressions. Once defined, the name can be used in other regular expressions and in rules. Recursion in named definitions is not allowed, and each name should be defined before it is used. A block inherits named definitions from the global scope. Redefining a name that exists in the current scope is an error.

configuration = value ;
    A configuration allows one to change re2hs behavior and customize the generated code. For a full list of configurations supported by re2hs see the configurations section. Depending on a particular configuration, the value can be a keyword, a nonnegative integer number or a one-line string which should be enclosed in double or single quotes unless it consists of alphanumeric characters. A block inherits configurations from the global scope and may redefine them or add new ones. Configurations defined inside of a block affect the whole block, even if they appear at the end of it.

regular-expression code
    A rule binds regular-expression to its semantic action (a block of code in curly braces, or a block of code that starts with := and ends on a newline followed by any non-whitespace character). If the regular-expression matches, the associated code is executed. If multiple rules match, the longest match takes precedence. If multiple rules match the same string, the earliest one takes precedence. There are two special rules: the default rule * and the end of input rule $. Default rule should always be defined, it has the lowest priority regardless of its place in the block, and it matches any code unit (not necessarily a valid character, see the encoding support section). The end of input rule should be defined if the corresponding method for handling the end of input is used. With start conditions rules have more complex syntax.

!action code
    An action binds a user-defined block of code to a particular place in the generated finite state machine (in the same way as semantic actions bind code to the final states). See the actions section for a full list of predefined actions.

!directive ;
    A directive is one of the special predefined statements. Each directive has a unique purpose. See the directives section for details.

  Blocks

Block start and end markers are either /*!re2c and */, or %{ and %} (both styles are supported). Starting from version 2.2 blocks may have optional names that allow them to be referenced in other blocks. There are different kinds of blocks:

/*!re2c[:<name>] ... */ or %{[:<name>] ... %}
    A global block contains definitions, configurations, rules and directives. re2hs compiles regular expressions associated with each rule into a deterministic finite automaton, encodes it in the form of conditional jumps in the target language and replaces the block with the generated code. Names and configurations defined in a global block are added to the global scope and become visible to subsequent blocks. At the start of the program the global scope is initialized with command-line options.

/*!local:re2c[:<name>] ... */ or %{local[:<name>] ... %}
    A local block is like a global block, but the names and configurations in it have local scope (they do not affect other blocks).

/*!rules:re2c[:<name>] ... */ or %{rules[:<name>] ... %}
    A rules block is like a local block, but it does not generate any code by itself, nor does it add any definitions to the global scope -- it is meant to be reused in other blocks. This is a way of sharing code (more details in the reusable blocks section). Prior to re2hs version 2.2 rules blocks required -r --reusable option.

/*!use:re2c[:<name>] ... */ or %{use[:<name>] ... %}
    A use block that references a previously defined rules block. If the name is specified, re2hs looks for a rules blocks with this name. Otherwise the most recent rules block is used (either a named or an unnamed one). A use block can add definitions, configurations and rules of its own, which are added to those of the referenced rules block. Prior to re2hs version 2.2 use blocks required -r --reusable option.

/*!max:re2c[:<name1>[:<name2>...]] ... */ or %{max[:<name1>[:<name2>...]] ... %}
    A block that generates YYMAXFILL definition. An optional list of block names specifies which blocks should be included when computing YYMAXFILL value (if the list is empty, all blocks are included). By default the generated code is a macro-definition for C (#define YYMAXFILL <n>), or a global variable for Go (var YYMAXFILL int = <n>). It can be customized with an optional configuration format that specifies a template string where @@{max} (or @@ for short) is replaced with the numeric value of YYMAXFILL.

/*!maxnmatch:re2c[:<name1>[:<name2>...]] ... */ or %{maxnmatch[:<name1>[:<name2>...]] ... %}
    A block that generates YYMAXNMATCH definition (it requires -P --posix-captures option). An optional list of block names specifies which blocks should be included when computing YYMAXNMATCH value (if the list is empty, all blocks are included). By default the generated code is a macro-definition for C (#define YYMAXNMATCH <n>), or a global variable for Go (var YYMAXNMATCH int = <n>). It can be customized with an optional configuration format that specifies a template string where @@{max} (or @@ for short) is replaced with the numeric value of YYMAXNMATCH.

/*!stags:re2c[:<name1>[:<name2>...]] ... */, /*!mtags:re2c[:<name1>[:<name2>...]] ... */ or %{stags[:<name1>[:<name2>...]] ... %}, %{mtags[:<name1>[:<name2>...]] ... %{
    Blocks that specify a template piece of code that is expanded for each s-tag/m-tag variable generated by re2hs. An optional list of block names specifies which blocks should be included when computing the set of tag variables (if the list is empty, all blocks are included). There are two optional configurations: format and separator. Configuration format specifies a template string where @@{tag} (or @@ for short) is replaced with the name of each tag variable. Configuration separator specifies a piece of code used to join the generated format pieces for different tag variables.

/*!svars:re2c[:<name1>[:<name2>...]] ... */, /*!mvars:re2c[:<name1>[:<name2>...]] ... */ or %{svars[:<name1>[:<name2>...]] ... %}, %{mvars[:<name1>[:<name2>...]] ... %{
    Blocks that specify a template piece of code that is expanded for each s-tag/m-tag that is either explicitly mentioned by the rules (with --tags option) or implicitly generated by re2hs (with --captvars or --posix-captvars options). An optional list of block names specifies which blocks should be included when computing the set of tags (if the list is empty, all blocks are included). There are two optional configurations: format and separator. Configuration format specifies a template string where @@{tag} (or @@ for short) is replaced with the name of each tag. Configuration separator specifies a piece of code used to join the generated format pieces for different tags.

/*!getstate:re2c[:<name1>[:<name2>...]] ... */ or %{getstate[:<name1>[:<name2>...]] ... %}
    A block that generates conditional dispatch on the lexer state (it requires --storable-state option). An optional list of block names specifies which blocks should be included in the state dispatch. The default transition goes to the start label of the first block on the list. If the list is empty, all blocks are included, and the default transition goes to the first block in the file that has a start label. This block type is incompatible with the --loop-switch option, as it requires cross-block transitions that are unsupported without goto or function calls.

/*!conditions:re2c[:<name1>[:<name2>...]] ... */, /*!types:re2c... */ or %{conditions[:<name1>[:<name2>...]] ... %}, %{types... %}
    A block that generates condition enumeration (it requires --conditions option). An optional list of block names specifies which blocks should be included when computing the set of conditions (if the list is empty, all blocks are included). By default the generated code is an enumeration YYCONDTYPE. It can be customized with optional configurations format and separator. Configuration format specifies a template string where @@{cond} (or @@ for short) is replaced with the name of each condition, and @@{num} is replaced with a numeric index of that condition. Configuration separator specifies a piece of code used to join the generated format pieces for different conditions.

/*!include:re2c <file> */ or %{include <file> %}
    This block allows one to include <file>, which must be a double-quoted file path. The contents of the file are literally substituted in place of the block, in the same way as #include works in C/C++. This block can be used together with the --depfile option to generate build system dependencies on the included files.

/*!header:re2c:on*/ or %{header:on %}
    This block marks the start of header file. Everything after it and up to the following header:off block is processed by re2hs and written to the header file specified with -t --type-header option.

/*!header:re2c:off*/ or %{header:off %}
    This block marks the end of header file started with header:on*/ block.

/*!ignore:re2c ... */ or %{ignore ... %}
    A block which contents are ignored and removed from the output file.

  Configurations

Here is a full list of configurations supported by re2hs:

re2c:api, re2c:input
    Same as the --api option.

re2c:api:sigil
    Specify the marker ("sigil") that is used for argument placeholders in the API primitives. The default is @@. A placeholder starts with sigil followed by the argument name in curly braces. For example, if sigil is set to $, then placeholders will have the form ${name}. Single-argument APIs may use shorthand notation without the name in braces. This option can be overridden by options for individual API primitives, e.g. re2c:YYFILL@len for YYFILL.

re2c:api:style
    Specify API style. Possible values are functions (the default for C) and free-form (the default for Go and Rust). In functions style API primitives are generated with an argument list in parentheses following the name of the primitive. The arguments are provided only for autogenerated parameters (such as the number of characters passed to YYFILL), but not for the general lexer context, so the primitives behave more like macros in C/C++ or closures in Go and Rust. In free-form style API primitives do not have a fixed form: they should be defined as strings containing free-form pieces of code with interpolated variables of the form @@{var} or @@ (they correspond to arguments in function-like style). This configuration may be overridden for individual API primitives, see for example re2c:YYFILL:naked configuration for YYFILL.

re2c:bit-vectors, re2c:flags:bit-vectors, re2c:flags:b
    Same as the --bit-vectors option, but can be configured on per-block basis.

re2c:captures, re2c:leftmost-captures
    Same as the --leftmost-captures option, but can be configured on per-block basis.

re2c:captvars, re2c:leftmost-captvars
    Same as the --leftmost-captvars option, but can be configured on per-block basis.

re2c:case-insensitive, re2c:flags:case-insensitive
    Same as the --case-insensitive option, but can be configured on per-block basis.

re2c:case-inverted, re2c:flags:case-inverted
    Same as the --case-inverted option, but can be configured on per-block basis.

re2c:case-ranges, re2c:flags:case-ranges
    Same as the --case-ranges option, but can be configured on per-block basis.

re2c:computed-gotos, re2c:flags:computed-gotos, re2c:flags:g
    Same as the --computed-gotos option, but can be configured on per-block basis.

re2c:computed-gotos:relative, re2c:cgoto:relative
    Same as the --computed-gotos-relative option, but can be configured on per-block basis.

re2c:computed-gotos:threshold, re2c:cgoto:threshold
    If computed goto is used, this configuration specifies the complexity threshold that triggers the generation of jump tables instead of nested if statements and bitmaps. The default value is 9.

re2c:cond:abort
    If set to a positive integer value, the default case in the generated condition dispatch aborts program execution.

re2c:cond:goto
    Specifies a piece of code used for the autogenerated shortcut rules :=> in conditions. The default is goto @@;. The @@ placeholder is substituted with condition name (see configurations re2c:api:sigil and re2c:cond:goto@cond).

re2c:cond:goto@cond
    Specifies the sigil used for argument substitution in re2c:cond:goto definition. The default value is @@. Overrides the more generic re2c:api:sigil configuration.

re2c:cond:divider
    Defines the divider for condition blocks. The default value is /* *********************************** */. Placeholders are substituted with condition name (see re2c:api;sigil and re2c:cond:divider@cond).

re2c:cond:divider@cond
    Specifies the sigil used for argument substitution in re2c:cond:divider definition. The default is @@. Overrides the more generic re2c:api:sigil configuration.

re2c:cond:prefix, re2c:condprefix
    Specifies the prefix used for condition labels. The default is yyc_.

re2c:cond:enumprefix, re2c:condenumprefix
    Specifies the prefix used for condition identifiers. The default is yyc.

re2c:debug-output, re2c:flags:debug-output, re2c:flags:d
    Same as the --debug-output option, but can be configured on per-block basis.

re2c:empty-class, re2c:flags:empty-class
    Same as the --empty-class option, but can be configured on per-block basis.

re2c:encoding:ebcdic, re2c:flags:ecb, re2c:flags:e
    Same as the --ebcdic option, but can be configured on per-block basis.

re2c:encoding:ucs2, re2c:flags:wide-chars, re2c:flags:w
    Same as the --ucs2 option, but can be configured on per-block basis.

re2c:encoding:utf8, re2c:flags:utf-8, re2c:flags:8
    Same as the --utf8 option, but can be configured on per-block basis.

re2c:encoding:utf16, re2c:flags:utf-16, re2c:flags:x
    Same as the --utf16 option, but can be configured on per-block basis.

re2c:encoding:utf32, re2c:flags:unicode, re2c:flags:u
    Same as the --utf32 option, but can be configured on per-block basis.

re2c:encoding-policy, re2c:flags:encoding-policy
    Same as the --encoding-policy option, but can be configured on per-block basis.

re2c:eof
    Specifies the sentinel symbol used with the end-of-input rule $. The default value is -1 ($ rule is not used). Other possible values include all valid code units. Only decimal numbers are recognized.

re2c:header, re2c:flags:type-header, re2c:flags:t
    Specifies the name of the generated header file relative to the directory of the output file. Same as the --header option except that the file path is relative.

re2c:indent:string
    Specifies the string used for indentation. The default is a single tab character "\t". Indent string should contain whitespace characters only. To disable indentation entirely, set this configuration to an empty string.

re2c:indent:top
    Specifies the minimum amount of indentation to use. The default value is zero. The value should be a non-negative integer number.

re2c:invert-captures
    Same as the --invert-captures option, but can be configured on per-block basis.

re2c:label:prefix, re2c:labelprefix
    Specifies the prefix used for DFA state labels. The default is yy.

re2c:label:start, re2c:startlabel
    Controls the generation of a block start label. The default value is zero, which means that the start label is generated only if it is used. An integer value greater than zero forces the generation of start label even if it is unused by the lexer. A string value also forces start label generation and sets the label name to the specified string. This configuration applies only to the current block (it is reset to default for the next block).

re2c:label:yyFillLabel
    Specifies the prefix of YYFILL labels used with re2c:eof and in storable state mode.

re2c:label:yyloop
    Specifies the name of the label marking the start of the lexer loop with --loop-switch option. The default is yyloop.

re2c:label:yyNext
    Specifies the name of the optional label that follows YYGETSTATE switch in storable state mode (enabled with re2c:state:nextlabel). The default is yyNext.

re2c:lookahead, re2c:flags:lookahead
    Deprecated (see the deprecated --no-lookahead option).

re2c:monadic
    If set to non-zero, the generated lexer will use monadic notation (this configuration is specific to Haskell).

re2c:nested-ifs, re2c:flags:nested-ifs, re2c:flags:s
    Same as the --nested-ifs option, but can be configured on per-block basis.

re2c:posix-captures, re2c:flags:posix-captures, re2c:flags:P
    Same as the --posix-captures option, but can be configured on per-block basis.

re2c:posix-captvars
    Same as the --posix-captvars option, but can be configured on per-block basis.

re2c:tags, re2c:flags:tags, re2c:flags:T
    Same as the --tags option, but can be configured on per-block basis.

re2c:tags:expression
    Specifies the expression used for tag variables. By default re2hs generates expressions of the form yyt<N>. This might be inconvenient, for example if tag variables are defined as fields in a struct. All occurrences of @@{tag} or @@ are replaced with the actual tag name. For example, re2c:tags:expression = "s.@@"; results in expressions of the form s.yyt<N> in the generated code. See also re2c:api:sigil configuration.

re2c:tags:negative
    Specifies the constant expression that is used for negative tag value (typically this would be -1 if tags are integer offsets in the input string, or null pointer if they are pointers).

re2c:tags:prefix
    Specifies the prefix for tag variable names. The default is yyt.

re2c:sentinel
    Specifies the sentinel symbol used for the end-of-input checks (when bounds checks are disabled with re2c:yyfill:enable = 0; and re2c:eof is not set). This configuration does not affect code generation: its purpose is to verify that the sentinel is not allowed in the middle of a rule, and ensure that the lexer won't read past the end of buffer. The default value is -1` (in that case re2hs assumes that the sentinel is zero, which is the most common case). Only decimal numbers are recognized.

re2c:state:abort
    If set to a positive integer value, the default case in the generated state dispatch aborts program execution, and an explicit -1 case contains transition to the start of the block.

re2c:state:nextlabel
    Controls if the YYGETSTATE switch is followed by an yyNext label (the default value is zero, which corresponds to no label). Alternatively one can use re2c:label:start to generate a specific start label, or an explicit getstate block to generate the YYGETSTATE switch separately from the lexer block.

re2c:unsafe, re2c:flags:unsafe
    Same as the --no-unsafe option, but can be configured on per-block basis. If set to zero, it suppresses the generation of unsafe wrappers around YYPEEK. The default is non-zero (wrappers are generated). This configuration is specific to Rust.

re2c:YYBACKUP, re2c:define:YYBACKUP
    Defines generic API primitive YYBACKUP.

re2c:YYBACKUPCTX, re2c:define:YYBACKUPCTX
    Defines generic API primitive YYBACKUPCTX.

re2c:YYCONDTYPE, re2c:define:YYCONDTYPE
    Defines API primitive YYCONDTYPE.

re2c:YYCTYPE, re2c:define:YYCTYPE
    Defines API primitive YYCTYPE.

re2c:YYCTXMARKER, re2c:define:YYCTXMARKER
    Defines API primitive YYCTXMARKER.

re2c:YYCURSOR, re2c:define:YYCURSOR
    Defines API primitive YYCURSOR.

re2c:YYDEBUG, re2c:define:YYDEBUG
    Defines API primitive YYDEBUG.

re2c:YYFILL, re2c:define:YYFILL
    Defines API primitive YYFILL.

re2c:YYFILL@len, re2c:define:YYFILL@len
    Specifies the sigil used for argument substitution in YYFILL definition. Defaults to @@. Overrides the more generic re2c:api:sigil configuration.

re2c:YYFILL:naked, re2c:define:YYFILL:naked
    Overrides the more generic re2c:api:style configuration for YYFILL. Zero value corresponds to free-form API style.

re2c:YYFN
    Defines API primitive YYFN.

re2c:YYINPUT
    Defines API primitive YYINPUT.

re2c:YYGETCOND, re2c:define:YYGETCONDITION
    Defines API primitive YYGETCOND.

re2c:YYGETCOND:naked, re2c:define:YYGETCONDITION:naked
    Overrides the more generic re2c:api:style configuration for YYGETCOND. Zero value corresponds to free-form API style.

re2c:YYGETSTATE, re2c:define:YYGETSTATE
    Defines API primitive YYGETSTATE.

re2c:YYGETSTATE:naked, re2c:define:YYGETSTATE:naked
    Overrides the more generic re2c:api:style configuration for YYGETSTATE. Zero value corresponds to free-form API style.

re2c:YYGETACCEPT, re2c:define:YYGETACCEPT
    Defines API primitive YYGETACCEPT.

re2c:YYLESSTHAN, re2c:define:YYLESSTHAN
    Defines generic API primitive YYLESSTHAN.

re2c:YYLIMIT, re2c:define:YYLIMIT
    Defines API primitive YYLIMIT.

re2c:YYMARKER, re2c:define:YYMARKER
    Defines API primitive YYMARKER.

re2c:YYMTAGN, re2c:define:YYMTAGN
    Defines generic API primitive YYMTAGN.

re2c:YYMTAGP, re2c:define:YYMTAGP
    Defines generic API primitive YYMTAGP.

re2c:YYPEEK, re2c:define:YYPEEK
    Defines generic API primitive YYPEEK.

re2c:YYRESTORE, re2c:define:YYRESTORE
    Defines generic API primitive YYRESTORE.

re2c:YYRESTORECTX, re2c:define:YYRESTORECTX
    Defines generic API primitive YYRESTORECTX.

re2c:YYRESTORETAG, re2c:define:YYRESTORETAG
    Defines generic API primitive YYRESTORETAG.

re2c:YYSETCOND, re2c:define:YYSETCONDITION
    Defines API primitive YYSETCOND.

re2c:YYSETCOND@cond, re2c:define:YYSETCONDITION@cond
    Specifies the sigil used for argument substitution in YYSETCOND definition. The default value is @@. Overrides the more generic re2c:api:sigil configuration.

re2c:YYSETCOND:naked, re2c:define:YYSETCONDITION:naked
    Overrides the more generic re2c:api:style configuration for YYSETCOND. Zero value corresponds to free-form API style.

re2c:YYSETSTATE, re2c:define:YYSETSTATE
    Defines API primitive YYSETSTATE.

re2c:YYSETSTATE@state, re2c:define:YYSETSTATE@state
    Specifies the sigil used for argument substitution in YYSETSTATE definition. The default value is @@. Overrides the more generic re2c:api:sigil configuration.

re2c:YYSETSTATE:naked, re2c:define:YYSETSTATE:naked
    Overrides the more generic re2c:api:style configuration for YYSETSTATE. Zero value corresponds to free-form API style.

re2c:YYSETACCEPT, re2c:define:YYSETACCEPT
    Defines API primitive YYSETACCEPT.

re2c:YYSKIP, re2c:define:YYSKIP
    Defines generic API primitive YYSKIP.

re2c:YYSHIFT, re2c:define:YYSHIFT
    Defines generic API primitive YYSHIFT.

re2c:YYCOPYMTAG, re2c:define:YYCOPYMTAG
    Defines generic API primitive YYCOPYMTAG.

re2c:YYCOPYSTAG, re2c:define:YYCOPYSTAG
    Defines generic API primitive YYCOPYSTAG.

re2c:YYSHIFTMTAG, re2c:define:YYSHIFTMTAG
    Defines generic API primitive YYSHIFTMTAG.

re2c:YYSHIFTSTAG, re2c:define:YYSHIFTSTAG
    Defines generic API primitive YYSHIFTSTAG.

re2c:YYSTAGN, re2c:define:YYSTAGN
    Defines generic API primitive YYSTAGN.

re2c:YYSTAGP, re2c:define:YYSTAGP
    Defines generic API primitive YYSTAGP.

re2c:yyaccept, re2c:variable:yyaccept
    Defines API primitive yyaccept.

re2c:yybm, re2c:variable:yybm
    Defines API primitive yybm.

re2c:yybm:hex, re2c:variable:yybm:hex
    If set to nonzero, bitmaps for the --bit-vectors option are generated in hexadecimal format. The default is zero (bitmaps are in decimal format).

re2c:yych, re2c:variable:yych
    Defines API primitive yych.

re2c:yych:emit, re2c:variable:yych:emit
    If set to zero, yych definition is not generated. The default is non-zero.

re2c:yych:conversion, re2c:variable:yych:conversion
    If set to non-zero, re2hs automatically generates a conversion to YYCTYPE every time yych is read. The default is to zero (no conversion).

re2c:yych:literals, re2c:variable:yych:literals
    Specifies the form of literals that yych is matched against. Possible values are: char (character literals in single quotes, non-printable ones use escape sequences that start with backslash), hex (hexadecimal integers) and char_or_hex (a mixture of both, character literals for printable characters and hexadecimal integers for others).

re2c:yyctable, re2c:variable:yyctable
    Defines API primitive yyctable.

re2c:yynmatch, re2c:variable:yynmatch
    Defines API primitive yynmatch.

re2c:yypmatch, re2c:variable:yypmatch
    Defines API primitive yypmatch.

re2c:yytarget, re2c:variable:yytarget
    Defines API primitive yytarget.

re2c:yystable, re2c:variable:yystable
    Deprecated.

re2c:yystate, re2c:variable:yystate
    Defines API primitive yystate.

re2c:yyfill, re2c:variable:yyfill
    Defines API primitive yyfill.

re2c:yyfill:check
    If set to zero, suppresses the generation of pre-YYFILL check for the number of input characters (the YYLESSTHAN definition in generic API and the YYLIMIT-based comparison in C pointer API). The default is non-zero (generate the check).

re2c:yyfill:enable
    If set to zero, suppresses the generation of YYFILL (together with the check). This should be used when the whole input fits into one piece of memory (there is no need for buffering) and the end-of-input checks do not rely on the YYFILL checks (e.g. if a sentinel character is used). Use warnings (-W option) and re2c:sentinel configuration to verify that the generated lexer cannot read past the end of input. The default is non-zero (YYFILL is enabled).

re2c:yyfill:parameter
    If set to zero, suppresses the generation of parameter passed to YYFILL. The parameter is the minimum number of characters that must be supplied. Defaults to non-zero (the parameter is generated). This configuration can be overridden with re2c:YYFILL:naked or re2c:api:style.

re2c:yyfn:sep
    Specifies separator used in YYFN elements (defaults to semicolon).

re2c:yyfn:throw
    Specifies exceptions thrown by YYFN function (defaults to empty, which means no exceptions).

  Regular expressions

re2hs uses the following syntax for regular expressions:

"foo"
    Case-sensitive string literal.

'foo'
    Case-insensitive string literal.

[a-xyz], [^a-xyz]
    Character class (possibly negated).

.   Any character except newline.

R \ S
    Difference of character classes R and S.

R*  Zero or more occurrences of R.

R+  One or more occurrences of R.

R?  Optional R.

R{n}
    Repetition of R exactly n times.

R{n,}
    Repetition of R at least n times.

R{n,m}
    Repetition of R from n to m times.

(R) Just R; parentheses are used to override precedence. If submatch extraction is enabled, (R) is a capturing or a non-capturing group depending on --invert-captures option.

(!R)
    If submatch extraction is enabled, (!R) is a non-capturing or a capturing group depending on --invert-captures option.

R S Concatenation: R followed by S.

R | S
    Alternative: R or S.

R / S
    Lookahead: R followed by S, but S is not consumed.

name
    Regular expression defined as name (or literal string "name" in Flex compatibility mode).

{name}
    Regular expression defined as name in Flex compatibility mode.

@stag
    An s-tag: saves the last input position at which @stag matches in a variable named stag.

#mtag
    An m-tag: saves all input positions at which #mtag matches in a variable named mtag.

    Character classes and string literals may contain the following escape sequences: \a, \b, \f, \n, \r, \t, \v, \\, octal escapes \ooo and hexadecimal escapes \xhh, \uhhhh and \Uhhhhhhhh.

  Actions

Here is a list of predefined actions supported by re2hs:

!entry code
    Entry action binds a user-defined block of code to the start state of the current finite state machine. If start conditions are used, the entry action can be set individually for each condition. This action may be used to perform initialization, e.g. to save start location of a lexeme.

!pre_rule code
    Pre-rule action prepends a user-defined block of code to semantic actions of all rules in the current block (or condition, if start conditions are used). This action may be used to factor out the common part of all semantic actions (e.g. saving the end location of a lexeme).

!post_rule code
    Post-rule action appends a user-defined block of code to semantic actions of all rules in the current block (or condition, if start conditions are used). This action may be used to emit trap statements that guard against unintended control flow.

  Directives

Here is a full list of directives supported by re2hs:

!use:name ;
    An in-block use directive that merges a previously defined rules block with the specified name into the current block. Named definitions, configurations and rules of the referenced block are added to the current ones. Conflicts between overlapping rules and configurations are resolved in the usual way: the first rule takes priority, and the latest configuration overrides the preceding ones. One exception is the special rules *, $ and <!> for which a block-local definition always takes priority. A use directive can be placed anywhere inside of a block, and multiple use directives are allowed.

!include file ;
    This directive is the same as include block: it inserts file contents verbatim in place of the directive.

  Program interface

The generated code interfaces with the outer program with the help of primitives, collectively referred to as the API. Which primitives should be defined for a particular program depends on multiple factors, including the complexity of regular expressions, input representation, buffering and the use of various features. All the necessary primitives should be defined by the user in the form of macros, functions, variables or any other suitable form that makes the generated code syntactically and semantically correct. re2hs does not (and cannot) check the definitions, so if anything is missing or defined incorrectly, the generated program may have compile-time or run-time errors. This manual provides examples of API definitions in the most common cases.

re2hs has two API flavors that define the core set of primitives used by a program:

Record API
    Record API is the default API for the Haskell backend. This API consists of a binding yyrecord (the name can be overridden with re2c:yyrecord) that should be defined as a record with fields _yyinput, _yycursor, _yymarker, _yyctxmarker, _yylimit. Only the fields used by the generated code need to be defined, and their names can be configured.

Generic API
    This is the most flexible API. It is enabled with --api generic option or re2c:api = generic configuration. It contains primitives for generic operations: YYPEEK, YYSKIP, YYBACKUP, YYBACKUPCTX, YYSTAGP, YYSTAGN, YYMTAGP, YYMTAGN, YYRESTORE, YYRESTORECTX, YYRESTORETAG, YYCOPYSTAG, YYCOPYMTAG, YYSHIFT, YYSHIFTSTAG, YYSHIFTMTAG, YYLESSTHAN.

    Here is a full list of API primitives that may be used by the generated code in order to interface with the outer program.

YYCTYPE
    The type of the input characters (code units). For ASCII, EBCDIC and UTF-8 encodings it should be 1-byte unsigned integer. For UTF-16 or UCS-2 it should be 2-byte unsigned integer. For UTF-32 it should be 4-byte unsigned integer.

YYCURSOR
    An l-value that stores the current input position (a pointer or an integer offset in YYINPUT). Initially YYCURSOR should point to the first input character, and later it is advanced by the generated code. When a rule matches, YYCURSOR position is the one after the last matched character.

YYLIMIT
    An r-value that stores the end of input position (a pointer or an integer offset in YYINPUT). Initially YYLIMIT should point to the position after the last available input character. It is not changed by the generated code. The lexer compares YYCURSOR to YYLIMIT in order to determine if there are enough input characters left.

YYMARKER
    An l-value that stores the position of the latest matched rule (a pointer or an integer offset in YYINPUT). It is used to restore the YYCURSOR position if the longer match fails and the lexer needs to rollback. Initialization is not needed.

YYCTXMARKER
    An l-value that stores the position of the trailing context (a pointer or an integer offset in YYINPUT). No initialization is needed. YYCTXMARKER is needed only if the lookahead operator / is used.

YYFILL
    A generic API primitive with one variable len. YYFILL should provide at least len more input characters or fail. If re2c:eof is used, then len is always 1 and YYFILL should always return to the calling function; zero return value indicates success. If re2c:eof is not used, then YYFILL return value is ignored and it should not return on failure. The maximum value of len is YYMAXFILL.

YYFN
    A primitive that defines function prototype in --recursive-functions code model. Its value should be an array of one or more strings, where each string contains two or three components separated by the string specified in re2c:fn:sep configuration (typically a semicolon). The first array element defines function name and return type (empty for a void function). Subsequent elements define function arguments: first, the expression for the argument used in function body (usually just a name); second, argument type; third, an optional formal parameter (it defaults to the first component - usually both the argument and the parameter are the same identifier).

YYINPUT
    An r-value that stores the current input character sequence (string, buffer, etc.).

YYMAXFILL
    An integral constant equal to the maximum value of the argument to YYFILL. It can be generated with a max block.

YYLESSTHAN
    A generic API primitive with one variable len. It should be defined as an r-value of boolean type that equals true if and only if there are less than len input characters left.

YYPEEK
    A generic API primitive with no variables. It should be defined as an r-value of type YYCTYPE that is equal to the character at the current input position.

YYSKIP
    A generic API primitive that should advance the current input position by one code unit.

YYBACKUP
    A generic API primitive that should save the current input position (to be restored with YYRESTORE later).

YYRESTORE
    A generic API primitive that should restore the current input position to the value saved by YYBACKUP.

YYBACKUPCTX
    A generic API primitive that should save the current input position as the position of the trailing context (to be restored with YYRESTORECTX later).

YYRESTORECTX
    A generic API primitive that should restore the trailing context position saved with YYBACKUPCTX.

YYRESTORETAG
    A generic API primitive with one variable tag that should restore the trailing context position to the value of tag.

YYSTAGP
    A generic API primitive with one variable tag, where tag can be a pointer or an offset in YYINPUT (see submatch extraction section for details). YYSTAGP should set tag to the current input position.

YYSTAGN
    A generic API primitive with one variable tag, where tag can be a pointer or an offset in YYINPUT (see submatch extraction section for details). YYSTAGN should to set tag to a value that represents non-existent input position.

YYMTAGP
    A generic API primitive with one variable tag. YYMTAGP should append the current position to the submatch history of tag (see the submatch extraction section for details.)

YYMTAGN
    A generic API primitive with one variable tag. YYMTAGN should append a value that represents non-existent input position position to the submatch history of tag (see the submatch extraction section for details.)

YYSHIFT
    A generic API primitive with one variable shift that should shift the current input position by shift characters (the shift value may be negative).

YYCOPYSTAG
    A generic API primitive with two variables, lhs and rhs that should copy right-hand-side s-tag variable rhs to the left-hand-side s-tag variable lhs. For most languages this primitive has a default definition that assigns lhs to rhs.

YYCOPYMTAG
    A generic API primitive with two variables, lhs and rhs that should copy right-hand-side m-tag variable rhs to the left-hand-side m-tag variable lhs. For most languages this primitive has a default definition that assigns lhs to rhs.

YYSHIFTSTAG
    A generic API primitive with two variables, tag and shift that should shift tag by shift code units (the shift value may be negative).

YYSHIFTMTAG
    A generic API primitive with two variables, tag and shift that should shift the latest value in the history of tag by shift code units (the shift value may be negative).

YYMAXNMATCH
    An integral constant equal to the maximal number of POSIX capturing groups in a rule. It is generated with a maxnmatch block.

YYCONDTYPE
    The type of the condition enum. It can be generated either with conditions block or --header option.

YYGETACCEPT
    A primitive with one variable var that stores numeric selector of the accepted rule. For most languages this primitive has a default definition that reads from var.

YYSETACCEPT
    A primitive with two variables: var (an l-value that stores numeric selector of the accepted rule), and val (the value of selector). For most languages this primitive has a default definition that assigns var to val.

YYGETCOND
    An r-value of type YYCONDTYPE that is equal to the current condition identifier.

YYSETCOND
    A primitive with one variable cond that should set the current condition identifier to cond.

YYGETSTATE
    An r-value of integer type that is equal to the current lexer state. It should be initialized to -1.

YYSETSTATE
    A primitive with one variable state that should set the current lexer state to state.

YYDEBUG
    This primitive is generated only with -d, --debug-output option. Its purpose is to add logging to the generated code (typical YYDEBUG definition is a print statement). YYDEBUG statements are generated in every state and have two variables: state (either a DFA state index or -1) and symbol (the current input symbol).

yyaccept
    An l-value of unsigned integral type that stores the number of the latest matched rule. User definition is necessary only with --storable-state option.

yybm
    A table containing compressed bitmaps for up to 8 transitions (used with the --bitmaps option). The table contains 256 elements and is indexed by 1-byte code units. Each 8-bit element combines boolean values for up to 8 transitions. k-Th bit of n-th element is true iff n-th code unit is in the range of k-th transition. The idea of this bitmap is to replace many if branches or switch cases with one check of a single bit in the table.

yych
    An l-value of type YYCTYPE that stores the current input character. User definition is necessary only with -f --storable-state option.

yyctable
    Jump table generated for the initial condition dispatch (enabled with the combination of --conditions and --computed-gotos options).

yyfill
    An l-value that stores the result of YYFILL call (this may be necessary for pure functional languages, where YYFILL is a monadic function with complex return value).

yynmatch
    An l-value of unsigned integral type that stores the number of POSIX capturing groups in the matched rule. Used only with -P --posix-captures option.

yypmatch
    An array of l-values that are used to hold the tag values corresponding to the capturing parentheses in the matching rule. Array length must be at least yynmatch * 2 (usually YYMAXNMATCH * 2 is a good choice). Used only with -P --posix-captures option.

yystable
    Deprecated.

yystate
    An l-value used with the --loop-switch option to store the current DFA state.

yytarget
    Jump table that contains jump targets (label addresses) for all transitions from a state. This table is local to each state. Generation of yytarget tables is enabled with --computed-gotos option.

  Options

Some of the options have corresponding configurations, others are global and cannot be changed after re2c starts reading the input file. Debug options generally require building re2c in debug configuration. Internal options are useful for experimenting with the algorithms used in re2c.

-? --help -h
    Show help message.

--api <simple | record | generic>
    Specify the API used by the generated code to interface with used-defined code. Option simple shold be used in simple cases when there's no need for buffer refilling and storing lexer state. Option record should be used when lexer state needs to be stored in a record (struct, class, etc.). Option generic should be used in complex cases when the other two APIs are not flexible enough.

--bit-vectors -b
    Optimize conditional jumps using bit masks. This option implies --nested-ifs.

--captures, --leftmost-captures
    Enable submatch extraction with leftmost greedy capturing groups. The result is collected into an array yybmatch of capacity 2 * YYMAXNMATCH, and yynmatch is set to the number of groups for the matching rule.

--captvars, --leftmost-captvars
    Enable submatch extraction with leftmost greedy capturing groups. The result is collected into variables yytl<k>, yytr<k> for k-th capturing group.

--case-insensitive
    Treat single-quoted and double-quoted strings as case-insensitive.

--case-inverted
    Invert the meaning of single-quoted and double-quoted strings: treat single-quoted strings as case-sensitive and double-quoted strings as case-insensitive.

--case-ranges
    Collapse consecutive cases in a switch statements into a range of the form low ... high. This syntax is a C/C++ language extension that is supported by compilers like GCC, Clang and Tcc. The main advantage over using single cases is smaller generated code and faster generation time, although for some compilers like Tcc it also results in smaller binary size.

--computed-gotos -g
    Optimize conditional jumps using non-standard "computed goto" extension (which must be supported by the compiler). re2hs generates jump tables only in complex cases with a lot of conditional branches. Complexity threshold can be configured with cgoto:threshold configuration. Relative offsets can be enabled with cgoto:relative configuration. This option implies --bit-vectors.

--computed-gotos-relative
    Similar to --computed-gotos but generate relative offsets for jump tables instead (which must be supported by the compiler). This option implies --computed-gotos.

--conditions --start-conditions -c
    Enable support of Flex-like "conditions": multiple interrelated lexers within one block. This is an alternative to manually specifying different re2hs blocks connected with goto or function calls.

--depfile FILE
    Write dependency information to FILE in the form of a Makefile rule <output-file> : <input-file> [include-file ...]. This allows one to track build dependencies in the presence of include blocks/directives, so that updating include files triggers regeneration of the output file. This option depends on the --output option.

--ebcdic --ecb -e
    Generate a lexer that reads input in EBCDIC encoding. re2hs assumes that the character range is 0 -- 0xFF and character size is 1 byte.

--empty-class <match-empty | match-none | error>
    Define the way re2hs treats empty character classes. With match-empty (the default) empty class matches empty input (which is illogical, but backwards-compatible). With match-none empty class always fails to match. With error empty class raises a compilation error.

--encoding-policy <fail | substitute | ignore>
    Define the way re2hs treats Unicode surrogates. With fail re2hs aborts with an error when a surrogate is encountered. With substitute re2hs silently replaces surrogates with the error code point 0xFFFD. With ignore (the default) re2hs treats surrogates as normal code points. The Unicode standard says that standalone surrogates are invalid, but real-world libraries and programs behave in different ways.

--flex-syntax -F
    Partial support for Flex syntax: in this mode named definitions don't need the equal sign and the terminating semicolon, and when used they must be surrounded with curly braces. Names without curly braces are treated as double-quoted strings.

--goto-label
    Use "goto/label" code model: encode DFA in form of labeled code blocks connected with goto transitions across blocks. This is only supported for languages that have a goto statement.

--header --type-header -t HEADER
    Generate a HEADER file. The contents of the file can be specified using special blocks header:on and header:off. If conditions are used, the generated header will have a condition enum automatically appended to it (unless there is an explicit conditions block).

-I PATH
    Add PATH to the list of locations which are used when searching for include files. This option is useful in combination with include block or directive. re2hs looks for FILE in the directory of the parent file and in the include locations specified with -I option.

--input <default | custom>
    Deprecated alias for --api. Option default corresponds to simple (it is indeed the default for most backends, but not for all). Option custom corresponds to generic.

--input-encoding <ascii | utf8>
    Specify the way re2hs parses regular expressions. With ascii (the default) re2hs handles input as ASCII-encoded: any sequence of code units is a sequence of standalone 1-byte characters. With utf8 re2hs handles input as UTF8-encoded and recognizes multibyte characters.

--invert-captures
    Invert the meaning of capturing and non-capturing groups. By default (...) is capturing and (! ...) is non-capturing. With this option (! ...) is capturing and (...) is non-capturing.

--lang <none | c | d | go | haskell | java | js | ocaml | python | rust | swift | v | zig>
    Specify the target language. Supported languages are C, D, Go, Haskell, Java, JS, OCaml, Python, Rust, Swift, V, Zig (more languages can be added via user-defined syntax files, see the --syntax option). Option none disables default suntax configs, so that the target language is undefined.

--location-format <gnu | msvc>
    Specify location format in messages. With gnu locations are printed as 'filename:line:column: ...'. With msvc locations are printed as 'filename(line,column) ...'. The default is gnu.

--loop-switch
    Use "loop/switch" code model: encode DFA in form of a loop over a switch statement, where individual states are switch cases. State is stored in a variable yystate. Transitions between states update yystate to the case label of the destination state and continue execution to the head of the loop.

--nested-ifs -s
    Use nested if statements instead of switch statements in conditional jumps. This usually results in more efficient code with non-optimizing compilers.

--no-debug-info -i
    Do not output line directives. This may be useful when the generated code is stored in a version control system (to avoid huge autogenerated diffs on small changes).

--no-generation-date
    Suppress date output in the generated file.

--no-version
    Suppress version output in the generated file.

--no-unsafe
    Do not generate unsafe wrapper over YYPEEK (this option is specific to Rust). For performance reasons YYPEEK should avoid bounds-checking, as the lexer already performs end-of-input checks in a more efficient way. The user may choose to provide a safe YYPEEK definition, or a definition that is unsafe only in release builds, in which case the --no-unsafe option helps to avoid warnings about redundant unsafe blocks.

--output -o OUTPUT
    Specify the OUTPUT file.

--posix-captures, -P
    Enable submatch extraction with POSIX-style capturing groups. The result is collected into an array yybmatch of capacity 2 * YYMAXNMATCH, and yynmatch is set to the number of groups for the matching rule.

--posix-captvars
    Enable submatch extraction with POSIX-style capturing groups. The result is collected into variables yytl<k>, yytr<k> for k-th capturing group.

--recursive-functions
    Use code model based on co-recursive functions, where each DFA state is a separate function that may call other state-functions or itself.

--reusable -r
    Deprecated since version 2.2 (reusable blocks are allowed by default now).

--skeleton -S
    Ignore user-defined interface code and generate a self-contained "skeleton" program. Additionally, generate input files with strings derived from the regular grammar and compressed match results that are used to verify "skeleton" behavior on all inputs. This option is useful for finding bugs in optimizations and code generation. This option is supported only for C.

--storable-state -f
    Generate a lexer which can store its inner state. This is useful in push-model lexers which are stopped by an outer program when there is not enough input, and then resumed when more input becomes available. In this mode users should additionally define YYGETSTATE and YYSETSTATE primitives, and variables yych, yyaccept and state should be part of the stored lexer state.

--syntax FILE
    Load configurations from the specified FILE and apply them on top of the default syntax file. Note that FILE can define only a few configurations (if it's used to amend the default syntax file), or it can define a whole new language backend (in the latter case it is recommended to use --lang none option).

--tags -T
    Enable submatch extraction with tags.

--ucs2 --wide-chars -w
    Generate a lexer that reads UCS2-encoded input. re2hs assumes that the character range is 0 -- 0xFFFF and character size is 2 bytes. This option implies --nested-ifs.

--utf8 --utf-8 -8
    Generate a lexer that reads input in UTF-8 encoding. re2hs assumes that the character range is 0 -- 0x10FFFF and character size is 1 byte.

--utf16 --utf-16 -x
    Generate a lexer that reads UTF16-encoded input. re2hs assumes that the character range is 0 -- 0x10FFFF and character size is 2 bytes. This option implies --nested-ifs.

--utf32 --unicode -u
    Generate a lexer that reads UTF32-encoded input. re2hs assumes that the character range is 0 -- 0x10FFFF and character size is 4 bytes. This option implies --nested-ifs.

--verbose
    Output a short message in case of success.

--vernum -V
    Show version information in MMmmpp format (major, minor, patch).

--version -v
    Show version information.

--single-pass -1
    Deprecated. Does nothing (single pass is the default now).

--debug-output -d
    Emit YYDEBUG invocations in the generated code. This is useful to trace lexer execution.

--dump-adfa
    Debug option: output DFA after tunneling (in .dot format).

--dump-cfg
    Debug option: output control flow graph of tag variables (in .dot format).

--dump-closure-stats
    Debug option: output statistics on the number of states in closure.

--dump-dfa-det
    Debug option: output DFA immediately after determinization (in .dot format).

--dump-dfa-min
    Debug option: output DFA after minimization (in .dot format).

--dump-dfa-tagopt
    Debug option: output DFA after tag optimizations (in .dot format).

--dump-dfa-tree
    Debug option: output DFA under construction with states represented as tag history trees (in .dot format).

--dump-dfa-raw
    Debug option: output DFA under construction with expanded state-sets (in .dot format).

--dump-interf
    Debug option: output interference table produced by liveness analysis of tag variables.

--dump-nfa
    Debug option: output NFA (in .dot format).

--emit-dot -D
    Instead of normal output generate lexer graph in .dot format. The output can be converted to an image with the help of Graphviz (e.g. something like dot -Tpng -odfa.png dfa.dot).

--dfa-minimization <moore | table>
    Internal option: DFA minimization algorithm used by re2hs. The moore option is the Moore algorithm (it is the default). The table option is the "table filling" algorithm. Both algorithms should produce the same DFA up to states relabeling; table filling is simpler and much slower and serves as a reference implementation.

--eager-skip
    Internal option: make the generated lexer advance the input position eagerly -- immediately after reading the input symbol. This changes the default behavior when the input position is advanced lazily -- after transition to the next state.

--no-lookahead
    Internal option, deprecated. It used to enable TDFA(0) algorithm. Unlike TDFA(1), TDFA(0) algorithm does not use one-symbol lookahead. It applies register operations to the incoming transitions rather than the outgoing ones. Benchmarks showed that TDFA(0) algorithm is less efficient than TDFA(1).

--no-optimize-tags
    Internal option: suppress optimization of tag variables (useful for debugging).

--posix-closure <gor1 | gtop>
    Internal option: specify shortest-path algorithm used for the construction of epsilon-closure with POSIX disambiguation semantics: gor1 (the default) stands for Goldberg-Radzik algorithm, and gtop stands for "global topological order" algorithm.

--posix-prectable <complex | naive>
    Internal option: specify the algorithm used to compute POSIX precedence table. The complex algorithm computes precedence table in one traversal of tag history tree and has quadratic complexity in the number of TNFA states; it is the default. The naive algorithm has worst-case cubic complexity in the number of TNFA states, but it is much simpler than complex and may be slightly faster in non-pathological cases.

--stadfa
    Internal option, deprecated. It used to enable staDFA algorithm, which differs from TDFA in that register operations are placed in states rather than on transitions. Benchmarks showed that staDFA algorithm is less efficient than TDFA.

--fixed-tags <none | toplevel | all>
    Internal option: specify whether the fixed-tag optimization should be applied to all tags (all), none of them (none), or only those in toplevel concatenation (toplevel). The default is all. "Fixed" tags are those that are located within a fixed distance to some other tag (called "base"). In such cases only the base tag needs to be tracked, and the value of the fixed tag can be computed as the value of the base tag plus a static offset. For tags that are under alternative or repetition it is also necessary to check if the base tag has a no-match value (in that case fixed tag should also be set to no-match, disregarding the offset). For tags in top-level concatenation the check is not needed, because they always match.

  Warnings

Warnings can be invividually enabled, disabled and turned into an error.

-W  Turn on all warnings.

-Werror
    Turn warnings into errors. Note that this option alone doesn't turn on any warnings; it only affects those warnings that have been turned on so far or will be turned on later.

-W<warning>
    Turn on warning.

-Wno-<warning>
    Turn off warning.

-Werror-<warning>
    Turn on warning and treat it as an error (this implies -W<warning>).

-Wno-error-<warning>
    Don't treat this particular warning as an error. This doesn't turn off the warning itself.

-Wcondition-order
    Warn if the generated program makes implicit assumptions about condition numbering. One should use either --header option or conditions block to generate a mapping of condition names to numbers and then use the autogenerated condition names.

-Wempty-character-class
    Warn if a regular expression contains an empty character class. Trying to match an empty character class makes no sense: it should always fail. However, for backwards compatibility reasons re2hs permits empty character classes and treats them as empty strings. Use the --empty-class option to change the default behavior.

-Wmatch-empty-string
    Warn if a rule is nullable (matches an empty string). If the lexer runs in a loop and the empty match is unintentional, the lexer may unexpectedly hang in an infinite loop.

-Wswapped-range
    Warn if the lower bound of a range is greater than its upper bound. The default behavior is to silently swap the range bounds.

-Wundefined-control-flow
    Warn if some input strings cause undefined control flow in the lexer (the faulty patterns are reported). This is a dangerous and common mistake. It can be easily fixed by adding the default rule * which has the lowest priority, matches any code unit, and always consumes a single code unit.

-Wunreachable-rules
    Warn about rules that are shadowed by other rules and will never match.

-Wdeprecated-eof_rule
    Warn about standalone end of input rules $ that will be broken by the future changes and require fixing. At the moment these rules take precedence when conflicting with other rules, but after the introduction of generalized end of input symbol $ precedence order will change and these rules will become shadowed by other rules.

-Wuseless-escape
    Warn if a symbol is escaped when it shouldn't be. By default, re2hs silently ignores such escapes, but this may as well indicate a typo or an error in the escape sequence.

-Wnondeterministic-tags
    Warn if a tag has n-th degree of nondeterminism, where n is greater than 1.

-Wsentinel-in-midrule
    Warn if the sentinel symbol occurs in the middle of a rule --- this may cause reads past the end of buffer, crashes or memory corruption in the generated lexer. This warning is only applicable if the sentinel method of checking for the end of input is used. It is set to an error if re2c:sentinel configuration is used.

-Wundefined-syntax-config
    Warn if the syntax file specified with --syntax option is missing definitions of some configurations. This helps to maintain user-defined syntax files: if a new release adds configurations, old syntax file will raise a warning, and the user will be notified. If some configurations are unused and do not need a definition, they should be explicitly set to <undefined>.

  Syntax files

Support for different languages in re2c is based on the idea of syntax files. A syntax file is a configuration file that defines syntax of the target language -- not the whole language, but a small part of it that is used by the generated code. Syntax files make re2c very flexible, but they should not be used as a replacement for re2c: configurations: their purpose is to define syntax of the target language, not to customize one particular lexer. All supported languages have default syntax files that are part of the distribution (see include/syntax subdirectory); they are also embedded in the re2hs binary. Users may provide a custom syntax file that overrides a few configurations for one of supported languages, or they may choose to redefine all configurations (in that case --lang none option should be used). Syntax files contain configurations of four different kinds: feature lists, language configurations, inplace configurations and code templates.

Feature lists A few list configurations define various features supported by a given backend, so that re2hs may give a clear error if the user tries to enable an unsupported feature:

supported_apis
    A list of supported APIs with possible elements simple, record, generic.

supported_api_styles
    A list of supported API styles with possible elements functions, free-form.

supported_code_models
    A list of supported code models with possible elements goto-label, loop-switch, recursive-functions.

supported_targets
    A list of supported codegen targets with possible elements code, dot, skeleton.

supported_features
    A list of supported features with possible elements nested-ifs, bitmaps, computed-gotos, case-ranges, monadic, unsafe, tags, captures, captvars.

    Language configurations A few boolean configurations describe features of the target language that affect re2hs parser and code generator:

semicolons
    Non-zero if the language uses semicolons after statements.

backtick_quoted_strings
    Non-zero if the language has backtick-quoted strings.

single_quoted_strings
    Non-zero if the language has single-quoted strings.

indentation_sensitive
    Non-zero if the language is indentation sensitive.

wrap_blocks_in_braces
    Non-zero if compound statements must be wrapped in curly braces.

    Inplace configurations Syntax files define initial values of all re2c: configurations, as they may differ for different languages. See configurations section for a full list of all inplace configurations and their meaning.

    Code templates Code templates define syntax of the target language. They are written in a simple domain-specific language with the following formal grammar:

    code-template ::
          name '=' code-exprs ';'
        | CODE_TEMPLATE ';'
        | '<undefined>' ';'
    code-exprs ::
          <EMPTY>
        | code-exprs code-expr
    code-expr ::
          STRING
        | VARIABLE
        | optional
        | list
    optional ::
          '(' CONDITIONAL '?' code-exprs ')'
        | '(' CONDITIONAL '?' code-exprs ':' code-exprs ')'
    list ::
          '[' VARIABLE ':' code-exprs ']'
        | '[' VARIABLE '{' NUMBER '}' ':' code-exprs ']'
        | '[' VARIABLE '{' NUMBER ',' NUMBER '}' ':' code-exprs ']'

    A code template is a sequence of string literals, variables, optional elements and lists, or a reference to another code template, or a special value <undefined>. Variables are placeholders that are substituted during code generation phase. List variables are special: when expanding list templates, re2hs repeats expressions the right hand side of the column a few times, each time replacing occurrences of the list variable with a value specific to this repetition. Lists have optional bounds (negative values are counted from the end, e.g. -1 means the last element). Conditional names start with a dot. Both conditionals and variables may be either local (specific to the given code template) or global (allowed in all code templates). When re2hs reads syntax file, it checks that each code template uses only the variables and conditionals that are allowed in it.

    For example, the following code template defines if-then-else construct for a C-like language:

    code:if_then_else =
        [branch{0}: topindent "if " cond " {" nl
            indent [stmt: stmt] dedent]
        [branch{1:-1}: topindent "} else" (.cond ? " if " cond) " {" nl
            indent [stmt: stmt] dedent]
        topindent "}" nl;

    Here branch is a list variable: branch{0} expands to the first branch (which is special, as there is no else part), branch{1:-1} expands to all remaining branches (if any). stmt is also a list variable: [stmt: stmt] is a nested list that expands to a list of statements in the body of the current branch. topindent, indent, dedent and nl are global variables, and .cond is a local conditional (their meaning is described below). This code template could produce the following code:

    if x {
        // do something
    } else if y {
        // do something else
    } else {
        // don't do anything
    }

    Here's a list of all code templates supported by re2hs with their local variables and conditionals. Note that a particular definition may, but does not have to use local variables and conditionals. Any unused code templates should be set to <undefined>.

code:var_local
    Declaration or definition of a local variable. Supported variables: type (the type of the variable), name (its name) and init (initial value, if any). Conditionals: .init (true if there is an initializer).

code:var_global
    Same as code:var_local, except that it's used in top-level.

code:const_local
    Definition of a local constant. Supported variables: type (the type of the constant), name (its name) and init (initial value).

code:const_global
    Same as code:const_local, except that it's used in top-level.

code:array_local
    Definition of a local array (table). Supported variables: type (the type of array elements), name (array name), size (its size), row (a list variable that does not itself produce any code, but expands list expression as many times as there are rows in the table) and elem (a list variable that expands to all table elements in the current row -- it's meant to be nested in the row list). Supported conditional: .const (true if the array is immutable).

code:array_global
    Same as code:array_local, except that it's used in top-level.

code:array_elem
    Reference to an element of an array (table). Supported variables: array (the name of the array) and index (index of the element).

code:enum
    Definition of an enumeration (it may be defined using a special language construct for enumerations, or simply as a few standalone constants). Supported variables are type (user-defined enumeration type or type of the constants), elem (list variable that expands to the name of each member) and init (initializer for each member). Conditionals: .init (true if there is an initializer).

code:enum_elem
    Enumeration element (a member of a user-defined enumeration type or a name of a constant, depending on how code:enum is defined). Supported variables are name (the name of the element) and type (its type).

code:assign
    Assignment statement. Supported variables are lhs (left hand side) and rhs (right hand side).

code:type_int
    Signed integer type.

code:type_uint
    Unsigned integer type.

code:type_yybm
    Type of elements in the yybm table.

code:type_yytarget
    Type of elements in the yytarget table.

code:type_yyctable
    Type of elements in the yyctable table.

code:cmp_eq
    Operator "equals".

code:cmp_ne
    Operator "not equals".

code:cmp_lt
    Operator "less than".

code:cmp_gt
    Operator "greater than"

code:cmp_le
    Operator "less or equal"

code:cmp_ge
    Operator "greater or equal"

code:if_then_else
    If-then-else statement with one or more branches. Supported variables: branch (a list variable that does not itself produce any code, but expands list expression as many times as there are branches), cond (condition of the current branch) and stmt (a list variable that expands to all statements in the current branch). Conditionals: .cond (true if the current branch has a condition), .many (true if there's more than one branch).

code:if_then_else_oneline
    A specialization of code:if_then_else for the case when all branches have one-line statements. If this is <undefined>, code:if_then_else is used instead.

code:switch
    A switch statement with one or more cases. Supported variables: expr (the switched-on expression) and case (a list variable that expands to all cases-groups with their code blocks).

code:switch_cases
    A group of switch cases that maps to a single code block. Supported variables are case (a list variable that expands to all cases in this group) and stmt (a list variable that expands to all statements in the code block.

code:switch_cases_oneline
    A specialization of code:switch_cases for the case when the code block consists of a single one-line statement. If this is <undefined>, code:switch_cases is used instead.

code:switch_case_range
    A single switch case that covers a range of values (possibly consisting of a single value). Supported variable: val (a list variable that expands to all values in the range). Supported conditionals: .many (true if there's more than one value in the range) and .char_literals (true if this is a switch on character literals -- some languages provide special syntax for this case).

code:switch_case_default
    Default switch case.

code:loop
    A loop that runs forever (unless interrupted from the loop body). Supported variables: label (loop label), stmt (a list variable that expands to all statements in the loop body).

code:continue
    Continue statement. Supported variables: label (label from which to continue execution).

code:goto
    Goto statement. Supported variables: label (label of the jump target).

code:cgoto
    Computed goto statement. Supported variables: array (the table containing computed goto information), index (index of the element in the table) and base (base label, only used if .cgoto.relative is true).

code:cgoto:data
    Initializer expression for a single element in computed goto table. Supported variables: label (the label that is used to initialize the current element), type (underlying type of the elements in the table) and base (base label - only used if .cgoto.relative is true).

code:fndecl
    Function declaration. Supported variables: name (function name), type (return type), throw (exceptions thrown by this function, maps to re2c:yyfn:throw configuration), arg (a list variable that does not itself produce code, but expands list expression as many times as there are function arguments), argname (name of the current argument), argtype (type of the current argument). Conditional: .type (true if this is a non-void function).

code:fndef
    Like code:fndecl, but used for function definitions, so it has one additional list variable stmt that expands to all statements in the function body.

code:fncall
    Function call statement. Supported variables: name (function name), retval (l-value where the return value is stored, if any) and arg (a list variable that expands to all function arguments). Conditionals: .args (true if the function has arguments) and .retval (true if return value needs to be saved).

code:tailcall
    Tail call statement. Supported variables: name (function name), and arg (a list variable that expands to all function arguments). Conditionals: .args (true if the function has arguments) and .retval (true if this is a non-void function).

code:recursive_functions
    Program body with --recursive-functions code model. Supported variables: fn (a list variable that does not itself produce any code, but expands list expression as many times as there are functions), fndecl (declaration of the current function) and fndef (definition of the current function).

code:fingerprint
    The fingerprint at the top of the generated output file. Supported variables: ver (re2hs version that was used to generate this) and date (generation date).

code:line_info
    The format of line directives (if this is set to <undefined>, no directives are generated). Supported variables: line (line number) and file (filename).

code:abort
    A statement that aborts program execution.

code:yydebug
    YYDEBUG statement, possibly specialized for different APIs. Supported variables: YYDEBUG, yyrecord, yych (map to the corresponding re2c: configurations), state (DFA state number).

code:yypeek
    YYPEEK statement, possibly specialized for different APIs. Supported variables: YYPEEK, YYCTYPE, YYINPUT, YYCURSOR, yyrecord, yych (map to the corresponding re2c: configurations). Conditionals: .cast (true if re2c:yych:conversion is set to non-zero).

code:yyskip
    YYSKIP statement, possibly specialized for different APIs. Supported variables: YYSKIP, YYCURSOR, yyrecord (map to the corresponding re2c: configurations).

code:yybackup
    YYBACKUP statement, possibly specialized for different APIs. Supported variables: YYBACKUP, YYCURSOR, YYMARKER, yyrecord (map to the corresponding re2c: configurations).

code:yybackupctx
    YYBACKUPCTX statement, possibly specialized for different APIs. Supported variables: YYBACKUPCTX, YYCURSOR, YYCTXMARKER, yyrecord (map to the corresponding re2c: configurations).

code:yyskip_yypeek
    Combined code:yyskip and code:yypeek statement (defaults to code:yyskip followed by code:yypeek).

code:yypeek_yyskip
    Combined code:yypeek and code:yyskip statement (defaults to code:yypeek followed by code:yyskip).

code:yyskip_yybackup
    Combined code:yyskip and code:yybackup statement (defaults to code:yyskip followed by code:yybackup).

code:yybackup_yyskip
    Combined code:yybackup and code:yyskip statement (defaults to code:yybackup followed by code:yyskip).

code:yybackup_yypeek
    Combined code:yybackup and code:yypeek statement (defaults to code:yybackup followed by code:yypeek).

code:yyskip_yybackup_yypeek
    Combined code:yyskip, code:yybackup and code:yypeek statement (defaults to``code:yyskip`` followed by code:yybackup followed by code:yypeek).

code:yybackup_yypeek_yyskip
    Combined code:yybackup, code:yypeek and code:yyskip statement (defaults to``code:yybackup`` followed by code:yypeek followed by code:yyskip).

code:yyrestore
    YYRESTORE statement, possibly specialized for different APIs. Supported variables: YYRESTORE, YYCURSOR, YYMARKER, yyrecord (map to the corresponding re2c: configurations).

code:yyrestorectx
    YYRESTORECTX statement, possibly specialized for different APIs. Supported variables: YYRESTORECTX, YYCURSOR, YYCTXMARKER, yyrecord (map to the corresponding re2c: configurations).

code:yyrestoretag
    YYRESTORETAG statement, possibly specialized for different APIs. Supported variables: YYRESTORETAG, YYCURSOR, yyrecord (map to the corresponding re2c: configurations), tag (the name of tag variable used to restore position).

code:yyshift
    YYSHIFT statement, possibly specialized for different APIs. Supported variables: YYSHIFT, YYCURSOR, yyrecord (map to the corresponding re2c: configurations), offset (the number of code units to shift the current position).

code:yyshiftstag
    YYSHIFTSTAG statement, possibly specialized for different APIs. Supported variables: YYSHIFTSTAG, yyrecord, negative (map to the corresponding re2c: configurations), tag (tag variable which needs to be shifted), offset (the number of code units to shift). Conditionals: .nested (true if this is a nested tag -- in this case its value may equal to re2c:tags:negative, which should not be shifted).

code:yyshiftmtag
    YYSHIFTMTAG statement, possibly specialized for different APIs. Supported variables: YYSHIFTMTAG (maps to the corresponding re2c: configuration), tag (tag variable which needs to be shifted), offset (the number of code units to shift).

code:yystagp
    YYSTAGP statement, possibly specialized for different APIs. Supported variables: YYSTAGP, YYCURSOR, yyrecord (map to the corresponding re2c: configurations), tag (tag variable that should be updated).

code:yymtagp
    YYMTAGP statement, possibly specialized for different APIs. Supported variables: YYMTAGP (maps to the corresponding re2c: configuration), tag (tag variable that should be updated).

code:yystagn
    YYSTAGN statement, possibly specialized for different APIs. Supported variables: YYSTAGN, negative, yyrecord (map to the corresponding re2c: configurations), tag (tag variable that should be updated).

code:yymtagn
    YYMTAGN statement, possibly specialized for different APIs. Supported variables: YYMTAGN (maps to the corresponding re2c: configuration), tag (tag variable that should be updated).

code:yycopystag
    YYCOPYSTAG statement, possibly specialized for different APIs. Supported variables: YYCOPYSTAG, yyrecord (map to the corresponding re2c: configurations), lhs, rhs (left and right hand side tag variables of the copy operation).

code:yycopymtag
    YYCOPYMTAG statement, possibly specialized for different APIs. Supported variables: YYCOPYMTAG, yyrecord (map to the corresponding re2c: configurations), lhs, rhs (left and right hand side tag variables of the copy operation).

code:yygetaccept
    YYGETACCEPT statement, possibly specialized for different APIs. Supported variables: YYGETACCEPT, yyrecord (map to the corresponding re2c: configurations), var (maps to re2c:yyaccept configuration).

code:yysetaccept
    YYSETACCEPT statement, possibly specialized for different APIs. Supported variables: YYSETACCEPT, yyrecord (map to the corresponding re2c: configurations), var (maps to re2c:yyaccept configuration) and val (numeric value of the accepted rule).

code:yygetcond
    YYGETCOND statement, possibly specialized for different APIs. Supported variables: YYGETCOND, yyrecord (map to the corresponding re2c: configurations), var (maps to re2c:yycond configuration).

code:yysetcond
    YYSETCOND statement, possibly specialized for different APIs. Supported variables: YYSETCOND, yyrecord (map to the corresponding re2c: configurations), var (maps to re2c:yycond configuration) and val (numeric condition identifier).

code:yygetstate
    YYGETSTATE statement, possibly specialized for different APIs. Supported variables: YYGETSTATE, yyrecord (map to the corresponding re2c: configurations), var (maps to re2c:yystate configuration).

code:yysetstate
    YYSETSTATE statement, possibly specialized for different APIs. Supported variables: YYSETSTATE, yyrecord (map to the corresponding re2c: configurations), var (maps to re2c:yystate configuration) and val (state number).

code:yylessthan
    YYLESSTHAN statement, possibly specialized for different APIs. Supported variables: YYLESSTHAN, YYCURSOR, YYLIMIT, yyrecord (map to the corresponding re2c: configurations), need (the number of code units to check against). Conditional: .many (true if the need is more than one).

code:yybm_filter
    Condition that is used to filter out yych values that are not covered by the yybm table (used with --bitmaps option). Supported variable: yych (maps to re2c:yych configuration).

code:yybm_match
    The format of yybm table check (generated with --bitmaps option). Supported variables: yybm, yych (map to the corresponding re2c: configurations), offset (offset in the yybm table that needs to be added to yych) and mask (bit mask that should be applied to the table entry to retrieve the boolean value that needs to be checked)

code:yytarget_filter
    Condition that is used to filter out yych values that are not covered by the yytarget table (used with --computed-gotos option). Supported variable: yych (maps to re2c:yych configuration).

    Here's a list of all global variables that are allowed in syntax files:

nl  A newline.

indent
    A variable that does not produce any code, but has a side-effect of increasing indentation level.

dedent
    A variable that does not produce any code, but has a side-effect of decreasing indentation level.

topindent
    Indentation string for the current statement. Indentation level is tracked and automatically updated by the code generator.

    Here's a list of all global conditionals that are allowed in syntax files:

.api.simple
    True if simple API is used (--api simple or re2c:api = simple).

.api.generic
    True if generic API is used (--api generic or re2c:api = generic).

.api.record
    True if record API is used (--api record or re2c:api = record).

.api_style.functions
    True if function-like API style is used (re2c:api-style = functions).

.api_style.freeform
    True if free-form API style is used (re2c:api-style = free-form).

.case_ranges
    True if case ranges feature is enabled (--case-ranges or re2c:case-ranges = 1).

.cgoto.relative
    True if the relative form of computed goto is used (--computed-gotos-relative or re2c:cgoto:relative = 1).

.code_model.goto_label
    True if code model based on goto/label is used (--goto-label).

.code_model.loop_switch
    True if code model based on loop/switch is used (--loop-switch).

.code_model.recursive_functions
    True if code model based on recursive functions is used (--recursive-function).

.date
    True if the generated fingerprint should contain generation date.

.loop_label
    True if re2hs generated loops must have a label (re2c:label:yyloop is set to a nonempty string).

.monadic
    True if the generated code should be monadic (re2c:monadic = 1). This is only relevant for pure functional languages.

.start_conditions
    True if start conditions are enabled (--start-conditions).

.storable_state
    True if storable state is enabled (--storable-state).

.unsafe
    True if re2hs should use "unsafe" blocks in order to generate faster code (--unsafe, re2c:unsafe = 1). This is only relevant for languages that have "unsafe" feature.

.version
    True if the generated fingerprint should contain re2hs version.

.yyfn.throw
    True if re2c:yyfn:throw configuration is defined to a nonempty string.

HANDLING THE END OF INPUT

One of the main problems for the lexer is to know when to stop. There are a few terminating conditions:

•   the lexer may match some rule (including default rule *) and come to a final state

•   the lexer may fail to match any rule and come to a default state

•   the lexer may reach the end of input

    The first two conditions terminate the lexer in a "natural" way: it comes to a state with no outgoing transitions, and the matching automatically stops. The third condition, end of input, is different: it may happen in any state, and the lexer should be able to handle it. Checking for the end of input interrupts the normal lexer workflow and adds conditional branches to the generated program, therefore it is necessary to minimize the number of such checks. re2hs supports a few different methods for handling the end of input. Which one to use depends on the complexity of regular expressions, the need for buffering, performance considerations and other factors. Here is a list of methods:

•   Sentinel. This method eliminates the need for the end of input checks altogether. It is simple and efficient, but limited to the case when there is a natural "sentinel" character that can never occur in valid input. This character may still occur in invalid input, but it should not be allowed by the regular expressions, except perhaps as the last character of a rule. The sentinel is appended at the end of input and serves as a stop signal: when the lexer reads this character, it is either a syntax error or the end of input. In both cases the lexer should stop. This method is used if YYFILL is disabled with re2c:yyfill:enable = 0; and re2c:eof has the default value -1.

•   Sentinel with bounds checks. This method is generic: it allows one to handle any input without restrictions on the regular expressions. The idea is to reduce the number of end of input checks by performing them only on certain characters. Similar to the "sentinel" method, one of the characters is chosen as a "sentinel" and appended at the end of input. However, there is no restriction on where the sentinel may occur (in fact, any character can be chosen for a sentinel). When the lexer reads this character, it additionally performs a bounds check. If the current position is within bounds, the lexer resumes matching and handles the sentinel as a regular character. Otherwise it invokes YYFILL (unless it is disabled). If more input is supplied, the lexer will rematch the last character and continue as if the sentinel wasn't there. Otherwise it must be the real end of input, and the lexer stops. This method is used when re2c:eof has non-negative value (it should be set to the numeric value of the sentinel). YYFILL is optional.

•   Bounds checks with padding. This method is generic, and it may be faster than the "sentinel with bounds checks" method, but it is also more complex. The idea is to partition DFA states into strongly connected components (SCCs) and generate a single check per SCC for enough characters to cover the longest non-looping path in this SCC. This reduces the number of checks, but there is a problem with short lexemes at the end of input, as the check requires enough characters to cover the longest lexeme. This can be fixed by padding the input with a few fake characters that do not form a valid lexeme suffix (so that the lexer cannot match them). The length of padding should be YYMAXFILL, generated with a max block. If there is not enough input, the lexer invokes YYFILL which should supply at least the required number of characters or not return. This method is used if YYFILL is enabled and re2c:eof is -1 (this is the default configuration).

•   Custom checks. Generic API allows one to override basic operations like reading a character, which makes it possible to include the end-of-input checks as part of them. This approach is error-prone and should be used with caution. To use a custom method, enable generic API with --api custom or re2c:api = custom; and disable default bounds checks with re2c:yyfill:enable = 0; or re2c:yyfill:check = 0;.

    The following subsections contain an example of each method.

  Sentinel

This example uses a sentinel character to handle the end of input. The program counts space-separated words in a null-terminated string. The sentinel is null: it is the last character of each input string, and it is not allowed in the middle of a lexeme by any of the rules (in particular, it is not included in character ranges where it is easy to overlook). If a null occurs in the middle of a string, it is a syntax error and the lexer will match default rule *, but it won't read past the end of input or crash (use -Wsentinel-in-midrule warning and re2c:sentinel configuration to verify this). Configuration re2c:yyfill:enable = 0; suppresses the generation of bounds checks and YYFILL invocations.

-- re2hs $INPUT -o $OUTPUT
{-# OPTIONS_GHC -Wno-unused-record-wildcards #-}
{-# LANGUAGE OverloadedStrings #-}
import Control.Monad (when)
import Data.ByteString (ByteString, index)
data State = State {
    _yyinput :: ByteString,
    _yycursor :: Int,
    _count :: Int
}
-- expect a null-terminated string
%{
    re2c:YYFN = ["lexer;Int", "State{..};State"];
    re2c:yyfill:enable = 0;
    *      { (-1) }
    [\x00] { _count }
    [a-z]+ { lexer State{_count = _count + 1, ..} }
    [ ]+   { lexer State{..} }
%}
main :: IO ()
main = do
    let test s n = when (lexer st  /= n) $ error "failed"
                   where st = State{_yyinput = s, _yycursor = 0, _count = 0}
    test "\0" 0
    test "one two three\0" 3
    test "f0ur\0" (-1)

  Sentinel with bounds checks

This example uses sentinel with bounds checks to handle the end of input (this method was added in version 1.2). The program counts space-separated single-quoted strings. The sentinel character is null, which is specified with re2c:eof = 0; configuration. As in the sentinel method, null is the last character of each input string, but it is allowed in the middle of a rule (for example, 'aaa\0aa'\0 is valid input, but 'aaa\0 is a syntax error). Bounds checks are generated in each state that matches an input character, but they are scoped to the branch that handles null. Bounds checks are of the form YYLIMIT <= YYCURSOR or YYLESSTHAN(1) with generic API. If the check condition is true, lexer has reached the end of input and should stop (YYFILL is disabled with re2c:yyfill:enable = 0; as the input fits into one buffer, see the YYFILL with sentinel section for an example that uses YYFILL). Reaching the end of input opens three possibilities: if the lexer is in the initial state it will match the end-of-input rule $, otherwise it may fallback to a previously matched rule (including default rule *) or go to a default state, causing -Wundefined-control-flow.

-- re2hs $INPUT -o $OUTPUT
{-# OPTIONS_GHC -Wno-unused-record-wildcards #-}
{-# LANGUAGE OverloadedStrings #-}
import Control.Monad (when)
import qualified Data.ByteString as BS
import Data.Word
data State = State {
    _yyinput :: BS.ByteString,
    _yycursor :: Int,
    _yymarker :: Int,
    _yylimit :: Int,
    _count :: Int
}
-- expect a null-terminated string
%{
    re2c:YYFN = ["lexer;Int", "State{..};State"];
    re2c:YYCTYPE = "Word8";
    re2c:YYPEEK = "BS.index";
    re2c:eof = 0;
    re2c:yyfill:enable = 0;
    str = ['] ([^'\\] | [\\][^])* ['];
    *    { (-1) }
    $    { _count }
    str  { lexer State{_count = _count + 1, ..} }
    [ ]+ { lexer State{..} }
%}
main :: IO ()
main = do
    let test s n = do
            let st = State {
                    _yyinput = s,
                    _yycursor = 0,
                    _yymarker = 0,
                    _yylimit = BS.length s - 1, -- terminating null not included
                    _count = 0}
            when (lexer st /= n) $ error "failed"
    test "\0" 0
    test "'qu\0tes' 'are' 'fine: \\'' \0" 3
    test "'unterminated\\'\0" (-1)

  Bounds checks with padding

This example uses bounds checks with padding to handle the end of input (this method is enabled by default). The program counts space-separated single-quoted strings. There is a padding of YYMAXFILL null characters appended at the end of input, where YYMAXFILL value is autogenerated with a max block. It is not necessary to use null for padding --- any characters can be used as long as they do not form a valid lexeme suffix (in this example padding should not contain single quotes, as they may be mistaken for a suffix of a single-quoted string). There is a "stop" rule that matches the first padding character (null) and terminates the lexer (note that it checks if null is at the beginning of padding, otherwise it is a syntax error). Bounds checks are generated only in some states that are determined by the strongly connected components of the underlying automaton. Checks have the form (YYLIMIT - YYCURSOR) < n or YYLESSTHAN(n) with generic API, where n is the minimum number of characters that are needed for the lexer to proceed (it also means that the next bounds check will occur in at most n characters). If the check condition is true, the lexer has reached the end of input and will invoke YYFILL(n) that should either supply at least n input characters or not return. In this example YYFILL always fails and terminates the lexer with an error (which is fine because the input fits into one buffer). See the YYFILL with padding section for an example that refills the input buffer with YYFILL.

-- re2hs $INPUT -o $OUTPUT
{-# OPTIONS_GHC -Wno-unused-record-wildcards #-}
{-# LANGUAGE OverloadedStrings #-}
import Control.Exception
import Control.Monad (when)
import qualified Data.ByteString as BS
data State = State {
    _yyinput :: BS.ByteString,
    _yycursor :: Int,
    _yylimit :: Int,
    _count :: Int
}
data FillException = UnexpectedFill deriving (Show)
instance Exception FillException
yymaxfill :: Int
%{max %}
%{
    re2c:YYFN  = ["lexer;IO Int", "State{..};State"];
    re2c:YYPEEK = "BS.index";
    re2c:YYFILL = "throw UnexpectedFill";
    re2c:monadic = 1; // YYFILL requires monadic do-notation for `when` conditions
    str = ['] ([^'\\] | [\\][^])* ['];
    [\x00] {
        -- check that it is the sentinel, not some unexpected null
        return $ if _yycursor == BS.length _yyinput - yymaxfill + 1 then _count else (-1)
    }
    str  { lexer State{_count = _count + 1, ..} }
    [ ]+ { lexer State{..} }
    *    { return (-1) }
%}
main :: IO ()
main = do
    let test s n = do
            let buf = BS.concat [s, BS.replicate yymaxfill 0]
            let st = State {
                    _yyinput = buf,
                    _yycursor = 0,
                    _yylimit = BS.length buf,
                    _count = 0}
            m <- catch (lexer st) (\(_ :: FillException) -> return (-2))
            when (m /= n) $ error "failed"
    test "" 0
    test "'unterminated\\'" (-2)
    test "'qu\0tes' 'are' 'fine: \\'' " 3
    test "'unexpected' \0 'null'" (-1)

  Custom checks

This example uses a custom end-of-input handling method based on generic API. The program counts space-separated single-quoted strings. It is the same as the sentinel example, except that the input is not null-terminated. To cover up for the absence of a sentinel character at the end of input, YYPEEK is redefined to perform a bounds check before it reads the next input character. This is inefficient because checks are done very often. If the check condition fails, YYPEEK returns the real character, otherwise it returns a fake sentinel character.

-- re2hs $INPUT -o $OUTPUT
{-# OPTIONS_GHC -Wno-unused-record-wildcards #-}
{-# LANGUAGE OverloadedStrings #-}
import Control.Monad (when)
import qualified Data.ByteString as BS
data State = State {
    _str :: BS.ByteString,
    _cur :: Int,
    _lim :: Int,
    _cnt :: Int
}
-- Expect a string without terminating null.
%{
    re2c:api = generic;
    re2c:YYFN = ["lexer;Int", "State{..};State"];
    re2c:YYPEEK = "if _cur < _lim then BS.index _str _cur else 0";
    re2c:YYSKIP = "let cur = _cur + 1 in let _cur = cur in";
    re2c:yyfill:enable = 0;
    *      { (-1) }
    [\x00] { _cnt }
    [a-z]+ { lexer State{_cnt = _cnt + 1, ..} }
    [ ]+   { lexer State{..} }
%}
main :: IO ()
main = do
    let test s n =
            let st = State {_str = s, _cur = 0, _lim = BS.length s, _cnt = 0}
            in when (lexer st /= n) $ error "failed"
    test "" 0
    test "one two three " 3
    test "f0ur" (-1)

BUFFER REFILLING

The need for buffering arises when the input cannot be mapped in memory all at once: either it is too large, or it comes in a streaming fashion (like reading from a socket). The usual technique in such cases is to allocate a fixed-sized memory buffer and process input in chunks that fit into the buffer. When the current chunk is processed, it is moved out and new data is moved in. In practice it is somewhat more complex, because lexer state consists not of a single input position, but a set of interrelated positions:

•   cursor: the next input character to be read (YYCURSOR in C pointer API or YYSKIP/YYPEEK in generic API)

•   limit: the position after the last available input character (YYLIMIT in C pointer API, implicitly handled by YYLESSTHAN in generic API)

•   marker: the position of the most recent match, if any (YYMARKER in default API or YYBACKUP/YYRESTORE in generic API)

•   token: the start of the current lexeme (implicit in re2hs API, as it is not needed for the normal lexer operation and can be defined and updated by the user)

•   context marker: the position of the trailing context (YYCTXMARKER in C pointer API or YYBACKUPCTX/YYRESTORECTX in generic API)

•   tag variables: submatch positions (defined with stags and mtags blocks and generic API primitives YYSTAGP/YYSTAGN/YYMTAGP/YYMTAGN)

    Not all these are used in every case, but if used, they must be updated by YYFILL. All active positions are contained in the segment between token and cursor, therefore everything between buffer start and token can be discarded, the segment from token and up to limit should be moved to the beginning of buffer, and the free space at the end of buffer should be filled with new data. In order to avoid frequent YYFILL calls it is best to fill in as many input characters as possible (even though fewer characters might suffice to resume the lexer). The details of YYFILL implementation are slightly different depending on which EOF handling method is used: the case of EOF rule is somewhat simpler than the case of bounds-checking with padding. Also note that if -f --storable-state option is used, YYFILL has slightly different semantics (described in the section about storable state).

  YYFILL with sentinel

If EOF rule is used, YYFILL is a function-like primitive that accepts no arguments and returns a value which is checked against zero. YYFILL invocation is triggered by condition YYLIMIT <= YYCURSOR in C pointer API and YYLESSTHAN() in generic API. A non-zero return value means that YYFILL has failed. A successful YYFILL call must supply at least one character and adjust input positions accordingly. Limit must always be set to one after the last input position in buffer, and the character at the limit position must be the sentinel symbol specified by re2c:eof configuration. The pictures below show the relative locations of input positions in buffer before and after YYFILL call (sentinel symbol is marked with #, and the second picture shows the case when there is not enough input to fill the whole buffer).

               <-- shift -->
             >-A------------B---------C-------------D#-----------E->
             buffer       token    marker         limit,
                                                  cursor
>-A------------B---------C-------------D------------E#->
             buffer,  marker        cursor        limit
             token
               <-- shift -->
             >-A------------B---------C-------------D#--E (EOF)
             buffer       token    marker         limit,
                                                  cursor
>-A------------B---------C-------------D---E#........
             buffer,  marker       cursor limit
             token

Here is an example of a program that reads input file input.txt in chunks of 4096 bytes and uses EOF rule.

-- re2hs $INPUT -o $OUTPUT
{-# OPTIONS_GHC -Wno-unused-record-wildcards #-}
{-# LANGUAGE OverloadedStrings #-}
import Control.Monad
import Data.ByteString as BS
import Data.Word
import GHC.IO.Handle
import System.Directory
import System.IO
chunk_size :: Int
chunk_size = 4096
data State = State {
    _file :: !Handle,
    _yyinput :: !BS.ByteString,
    _yycursor :: !Int,
    _yymarker :: !Int,
    _yylimit :: !Int,
    _token :: !Int,
    _eof :: !Bool,
    _count :: !Int
}
%{
    re2c:YYFN = ["lexer;IO Int", "State{..};State;!State{..}"];
    re2c:YYCTYPE = "Word8";
    re2c:YYPEEK = "BS.index";
    re2c:YYFILL = "(State{..}, yyfill) <- fill State{..}";
    re2c:eof = 0;
    re2c:monadic = 1;
    str = ['] ([^'\\] | [\\][^])* ['];
    *    { return (-1) }
    $    { return _count }
    str  { lexer State{_token = _yycursor, _count = _count + 1, ..} }
    [ ]+ { lexer State{_token = _yycursor, ..} }
%}
fill :: State -> IO (State, Bool)
fill State{..} = do
    case _eof of
        True -> return (State{..}, False)
        False -> do
            -- Discard everything up to the current token, cut off terminating null,
            -- read new chunk from file and reappend terminating null at the end.
            chunk <- BS.hGet _file chunk_size
            return (State {
                _yyinput = BS.concat [(BS.init . BS.drop _token) _yyinput, chunk, "\0"],
                _yycursor = _yycursor - _token,
                _yymarker = _yymarker - _token,
                _yylimit = _yylimit - _token + BS.length chunk, -- exclude terminating null
                _token = 0,
                _eof = BS.null chunk, -- end of file?
                ..}, True)
main :: IO ()
main = do
    let fname = "input"
    -- Prepare input file.
    BS.writeFile fname $ BS.concat ["'qu\0tes' 'are' 'fine: \\'' " | _ <- [1..chunk_size]]
    let expect = 3 * chunk_size -- the total number of strings in file
    -- Run lexer on the prepared file.
    fh <- openFile fname ReadMode
    let st = State {
        _file = fh,
        _yyinput = BS.singleton 0,
        _yycursor = 0,
        _yymarker = 0,
        _token = 0,
        _yylimit = 0,
        _eof = False,
        _count = 0
    }
    result <- lexer st
    hClose fh
    -- Cleanup.
    removeFile fname
    -- Check result.
    when (result /= expect) $ error $ "expected " ++ show expect ++ ", got " ++ show result
    return ()

  YYFILL with padding

In the default case (when EOF rule is not used) YYFILL is a function-like primitive that accepts a single argument and does not return any value. YYFILL invocation is triggered by condition (YYLIMIT - YYCURSOR) < n in C pointer API and YYLESSTHAN(n) in generic API. The argument passed to YYFILL is the minimal number of characters that must be supplied. If it fails to do so, YYFILL must not return to the lexer (for that reason it is best implemented as a macro that returns from the calling function on failure). In case of a successful YYFILL invocation the limit position must be set either to one after the last input position in buffer, or to the end of YYMAXFILL padding (in case YYFILL has successfully read at least n characters, but not enough to fill the entire buffer). The pictures below show the relative locations of input positions in buffer before and after YYFILL invocation (YYMAXFILL padding on the second picture is marked with # symbols).

               <-- shift -->                 <-- need -->
             >-A------------B---------C-----D-------E---F--------G->
             buffer       token    marker cursor  limit
>-A------------B---------C-----D-------E---F--------G->
             buffer,  marker cursor               limit
             token
               <-- shift -->                 <-- need -->
             >-A------------B---------C-----D-------E-F        (EOF)
             buffer       token    marker cursor  limit
>-A------------B---------C-----D-------E-F###############
             buffer,  marker cursor                   limit
             token                        <- YYMAXFILL ->

Here is an example of a program that reads input file input.txt in chunks of 4096 bytes and uses bounds-checking with padding.

-- re2hs $INPUT -o $OUTPUT
{-# OPTIONS_GHC -Wno-unused-record-wildcards #-}
{-# LANGUAGE OverloadedStrings #-}
import Control.Monad
import qualified Data.ByteString as BS
import GHC.IO.Handle
import System.Directory
import System.IO
chunk_size :: Int
chunk_size = 4096
data State = State {
    _file :: !Handle,
    _yyinput :: !BS.ByteString,
    _yycursor :: !Int,
    _yylimit :: !Int,
    _token :: !Int,
    _eof :: !Bool,
    _count :: !Int
}
%{
    re2c:YYFN = ["lexer;IO Int", "State{..};State;!State{..}"];
    re2c:YYPEEK = "BS.index";
    // We have to turn off autogenerated YFILL check and write it manually as part of YYFILL
    // implementation, so that we can propagate the updated state out of it.
    re2c:yyfill:check = 0;
    re2c:YYFILL = "State{..} <- fill State{..} @@";
    re2c:monadic = 1;
    str = ['] ([^'\\] | [\\][^])* ['];
    *      { return (-1) }
    [\x00] { return $ if _yycursor == _yylimit - yymaxfill + 1 then _count else (-1) }
    str    { lexer State{_token = _yycursor, _count = _count + 1, ..} }
    [ ]+   { lexer State{_token = _yycursor, ..} }
%}
yymaxfill :: Int
%{max %}
fill :: State -> Int -> IO State
fill !st@State{..} !need =
    if _yylimit - _yycursor >= need then
        return st
    else case _eof of
        True -> error "fill failed"
        False -> do
            -- Discard everything up to the current token, cut off terminating null,
            -- read new chunk from file and reappend terminating null at the end.
            chunk <- BS.hGet _file chunk_size
            let !eof = BS.length chunk < need -- end of file ?
            let !buf = BS.concat [
                    BS.drop _token _yyinput,
                    chunk,
                    if eof then (BS.replicate yymaxfill 0) else BS.empty]
            return State {
                _yyinput = buf,
                _yycursor = _yycursor - _token,
                _yylimit = BS.length buf,
                _token = 0,
                _eof = eof,
                ..}
main :: IO ()
main = do
    let fname = "input"
    -- Prepare input file.
    BS.writeFile fname $ BS.concat ["'qu\0tes' 'are' 'fine: \\'' " | _ <- [1..chunk_size]]
    let expect = 3 * chunk_size -- the total number of strings in file
    -- Run lexer on the prepared file.
    fh <- openFile fname ReadMode
    let st = State {
        _file = fh,
        _yyinput = BS.empty,
        _yycursor = 0,
        _token = 0,
        _yylimit = 0,
        _eof = False,
        _count = 0
    }
    result <- lexer st
    hClose fh
    -- Cleanup.
    removeFile fname
    -- Check result.
    when (result /= expect) $ error $ "expected " ++ show expect ++ ", got " ++ show result
    return ()

FEATURES

  Multiple blocks

Sometimes it is necessary to have multiple interrelated lexers (for example, if there is a high-level state machine that transitions between lexer modes). This can be implemented using multiple connected re2hs blocks. Another option is to use start conditions.

The implementation of connections between blocks depends on the target language. In languages that have goto statement (such as C/C++ and Go) one can have all blocks in one function, each of them prefixed with a label. Transition from one block to another is a simple goto. In languages that do not have goto (such as Rust) it is necessary to use a loop with a switch on a state variable, similar to the yystate loop/switch generated by re2hs, or else wrap each block in a function and use function calls.

The example below uses multiple blocks to parse binary, octal, decimal and hexadecimal numbers. Each base has its own block. The initial block determines base and dispatches to other blocks. Common configurations are defined in a separate block at the beginning of the program; they are inherited by the other blocks.

-- re2hs $INPUT -o $OUTPUT -i
{-# OPTIONS_GHC -Wno-unused-record-wildcards #-}
{-# LANGUAGE OverloadedStrings #-}
import Control.Monad (when)
import Data.ByteString (ByteString, index)
data State = State {
    _yyinput :: !ByteString,
    _yycursor :: !Int,
    _yymarker :: !Int
}
peek_digit :: ByteString -> Int -> Int -> Int
peek_digit str idx offs = fromIntegral (index str (idx - 1)) - offs
%{
    re2c:yyfill:enable = 0;
%}
%{local
    re2c:YYFN = ["parse_bin;Int", "State{..};State", "num;Int"];
    [01] { parse_bin State{..} $ num * 2 + (peek_digit _yyinput _yycursor 48) }
    *    { num }
%}
%{local
    re2c:YYFN = ["parse_oct;Int", "State{..};State", "num;Int"];
    [0-7] { parse_oct State{..} $ num * 8 + (peek_digit _yyinput _yycursor 48) }
    *     { num }
%}
%{local
    re2c:YYFN = ["parse_dec;Int", "State{..};State", "num;Int"];
    [0-9] { parse_dec State{..} $ num * 10 + (peek_digit _yyinput _yycursor 48) }
    *     { num }
%}
%{local
    re2c:YYFN = ["parse_hex;Int", "State{..};State", "num;Int"];
    [0-9] { parse_hex State{..} $ num * 16 + (peek_digit _yyinput _yycursor 48) }
    [a-f] { parse_hex State{..} $ num * 16 + (peek_digit _yyinput _yycursor 87) }
    [A-F] { parse_hex State{..} $ num * 16 + (peek_digit _yyinput _yycursor 55) }
    *     { num }
%}
%{local
    re2c:YYFN = ["parse;Maybe Int", "State{..};State"];
    '0b' / [01]        { Just $ parse_bin State{..} 0 }
    "0"                { Just $ parse_oct State{..} 0 }
    "" / [1-9]         { Just $ parse_dec State{..} 0 }
    '0x' / [0-9a-fA-F] { Just $ parse_hex State{..} 0 }
    *                  { Nothing }
%}
test :: ByteString -> Maybe Int -> IO ()
test str expect = do
    let s = State {_yyinput = str, _yycursor = 0, _yymarker = 0}
    when (parse s /= expect) $ error "failed!"
main :: IO ()
main = do
    test "\0" Nothing
    test "1234567890\0" (Just 1234567890)
    test "0b1101\0" (Just 13)
    test "0x7Fe\0" (Just 2046)
    test "0644\0" (Just 420)
    test "9223372036854775807\0" (Just 9223372036854775807)

  Start conditions

Start conditions are enabled with --start-conditions option. They provide a way to encode multiple interrelated automata within the same re2hs block.

Each condition corresponds to a single automaton and has a unique name specified by the user and a unique internal number defined by re2hs. The numbers are used to switch between conditions: the generated code uses YYGETCOND and YYSETCOND primitives to get the current condition or set it to the given number. Use conditions block, --header option or re2c:header configuration to generate numeric condition identifiers. Configuration re2c:cond:enumprefix specifies the generated identifier prefix.

In condition mode every rule must be prefixed with a list of comma-separated condition names in angle brackets, or a wildcard <*> to denote all conditions. The rule syntax is extended as follows:

< condition-list > regular-expression code
    A rule that is merged to every condition on the condition-list. It matches regular-expression and executes the associated code.

< condition-list > regular-expression => condition code
    A rule that is merged to every condition on the condition-list. It matches regular-expression, sets the current condition to condition and executes the associated code.

< condition-list > regular-expression :=> condition
    A rule that is merged to every condition on the condition-list. It matches regular-expression and immediately transitions to condition (there is no semantic action).

< condition-list > !action code
    A rule that binds code to the place defined by action in every condition on the condition-list (see the actions section for various types of actions).

<! condition-list > code
    A rule that prepends code to semantic actions of all rules for every condition on the condition-list. This syntax is deprecated and the !pre_rule action should be used instead (it does exactly the same).

< > code
    A rule that creates a special entry condition with number zero and name "0" that executes code before jumping to other conditions. This syntax is deprecated, and the !entry action should be used instead (it provides a more fine-grained control, as the code can be specified on a per-condition basis, and one can jump directly to condition start without going through condition dispatch).

< > => condition code
    Same as the previous rule, except that it sets the next condition.

< > :=> condition
    Same as the previous rule, except that it has no associated code and immediately jumps to condition.

    The code re2hs generates for conditions depends on whether re2hs uses goto/label approach or loop/switch approach to encode the automata.

    In languages that have goto statement (such as C/C++ and Go) conditions are naturally implemented as blocks of code prefixed with labels of the form yyc_<cond>, where cond is a condition name (label prefix can be changed with re2c:cond:prefix). Transitions between conditions are implemented using goto and condition labels. Before all conditions re2hs generates an initial switch on YYGETSTATE that jumps to the start state of the current condition. The shortcut rules :=> bypass the initial switch and jump directly to the specified condition (re2c:cond:goto can be used to change the default behavior). The rules with semantic actions do not automatically jump to the next condition; this should be done by the user-defined action code.

    In languages that do not have goto (such as Rust) re2hs reuses the yystate variable to store condition numbers. Each condition gets a numeric identifier equal to the number of its start state, and a switch between conditions is no different than a switch between DFA states of a single condition. There is no need for a separate initial condition switch. (Since the same approach is used to implement storable states, YYGETCOND/YYSETCOND are redundant if both storable states and conditions are used).

    The program below uses start conditions to parse binary, octal, decimal and hexadecimal numbers. There is a single block where each base has its own condition, and the initial condition is connected to all of them. User-defined variable cond stores the current condition number; it is initialized to the number of the initial condition generated with conditions block.

    -- re2hs $INPUT -o $OUTPUT -ci
    {-# OPTIONS_GHC -Wno-unused-record-wildcards #-}
    {-# LANGUAGE OverloadedStrings #-}
    import Control.Monad (when)
    import Data.ByteString (ByteString, index)
    %{conditions %}
    data State = State {
        _yyinput :: !ByteString,
        _yycursor :: !Int,
        _yymarker :: !Int,
        _yycond :: !YYCONDTYPE
    }
    peek_digit :: ByteString -> Int -> Int -> Int
    peek_digit str idx offs = fromIntegral (index str (idx - 1)) - offs
    %{
        re2c:YYFN = ["parse;Maybe Int", "State{..};State", "_num;Int"];
        re2c:yyfill:enable = 0;
        <init> '0b' / [01]        :=> bin
        <init> "0"                :=> oct
        <init> "" / [1-9]         :=> dec
        <init> '0x' / [0-9a-fA-F] :=> hex
        <init> * { Nothing }
        <bin> [01]  { yyfnbin State{..} $ _num * 2 + (peek_digit _yyinput _yycursor 48) }
        <oct> [0-7] { yyfnoct State{..} $ _num * 8 + (peek_digit _yyinput _yycursor 48) }
        <dec> [0-9] { yyfndec State{..} $ _num * 10 + (peek_digit _yyinput _yycursor 48) }
        <hex> [0-9] { yyfnhex State{..} $ _num * 16 + (peek_digit _yyinput _yycursor 48) }
        <hex> [a-f] { yyfnhex State{..} $ _num * 16 + (peek_digit _yyinput _yycursor 87) }
        <hex> [A-F] { yyfnhex State{..} $ _num * 16 + (peek_digit _yyinput _yycursor 55) }
        <bin, oct, dec, hex> * { Just _num }
    %}
    test :: ByteString -> Maybe Int -> IO ()
    test str expect = do
        let s = State {
                _yyinput = str,
                _yycursor = 0,
                _yymarker = 0,
                _yycond = YYC_init}
        when (parse s 0 /= expect) $ error "failed!"
    main :: IO ()
    main = do
        test "\0" Nothing
        test "1234567890\0" (Just 1234567890)
        test "0b1101\0" (Just 13)
        test "0x7Fe\0" (Just 2046)
        test "0644\0" (Just 420)
        test "9223372036854775807\0" (Just 9223372036854775807)

  Storable state

With --storable-state option re2hs generates a lexer that can store its current state, return to the caller, and later resume operations exactly where it left off. The default mode of operation in re2hs is a "pull" model, in which the lexer "pulls" more input whenever it needs it. This may be unacceptable in cases when the input becomes available piece by piece (for example, if the lexer is invoked by the parser, or if the lexer program communicates via a socket protocol with some other program that must wait for a reply from the lexer before it transmits the next message). Storable state feature is intended exactly for such cases: it allows one to generate lexers that work in a "push" model. When the lexer needs more input, it stores its state and returns to the caller. Later, when more input becomes available, the caller resumes the lexer exactly where it stopped. There are a few changes necessary compared to the "pull" model:

•   Define YYSETSTATE() and YYGETSTATE(state) primitives.

•   Define yych, yyaccept (if used) and state variables as a part of persistent lexer state. The state variable should be initialized to -1.

•   YYFILL should return to the outer program instead of trying to supply more input. Return code should indicate that lexer needs more input.

•   The outer program should recognize situations when lexer needs more input and respond appropriately.

•   Optionally use getstate block to generate YYGETSTATE switch detached from the main lexer. This only works for languages that have goto (not in --loop-switch mode).

•   Use re2c:eof and the sentinel with bounds checks method to handle the end of input. Padding-based method may not work because it is unclear when to append padding: the current end of input may not be the ultimate end of input, and appending padding too early may cut off a partially read greedy lexeme. Furthermore, due to high-level program logic getting more input may depend on processing the lexeme at the end of buffer (which already is blocked due to the end-of-input condition).

    Here is an example of a "push" model lexer that simulates reading packets from a socket. The lexer loops until it encounters the end of input and returns to the calling function. The calling function provides more input by "sending" the next packet and resumes lexing. This process stops when all the packets have been sent, or when there is an error.

    -- re2hs $INPUT -o $OUTPUT -fi
    {-# OPTIONS_GHC -Wno-unused-record-wildcards #-}
    {-# LANGUAGE OverloadedStrings #-}
    import Control.Concurrent.Chan
    import Control.Monad
    import Data.ByteString as BS
    import Text.Printf
    debug :: IO () -> IO ()
    debug = when False
    data State = State {
        _pipe :: !(Chan BS.ByteString),
        _yyinput :: !BS.ByteString,
        _yycursor :: !Int,
        _yymarker :: !Int,
        _yylimit :: !Int,
        _token :: !Int,
        _eof :: !Bool,
        _yystate :: !Int,
        _recv :: !Int
    }
    data Status = End | Ready | Waiting | BadPacket deriving (Eq)
    %{
        re2c:YYFN = ["lexer;IO (State, Status)", "State{..};State;!State{..}"];
        re2c:YYPEEK = "BS.index";
        re2c:YYFILL = "return (State{..}, Waiting)";
        re2c:eof = 0;
        re2c:monadic = 1;
        packet = [a-z]+[;];
        *      { return (State{..}, BadPacket) }
        $      { return (State{..}, End) }
        packet { lexer State{_token = _yycursor, _recv = _recv + 1, ..} }
    %}
    fill :: State -> IO (State, Status)
    fill st@State{..} = do
        case _eof of
            True -> return (st, End)
            False -> do
                -- Discard everything up to the current token, cut off terminating null,
                -- read new chunk from file and reappend terminating null at the end.
                chunk <- readChan _pipe
                return (State {
                    _yyinput = BS.concat [(BS.init . BS.drop _token) _yyinput, chunk, "\0"],
                    _yycursor = _yycursor - _token,
                    _yymarker = _yymarker - _token,
                    _yylimit = _yylimit - _token + BS.length chunk, -- exclude terminating null
                    _token = 0,
                    _eof = BS.null chunk, -- end of file?
                    ..}, Ready)
    loop :: State -> [BS.ByteString] -> IO Status
    loop State{..} packets = do
        (State{..}, status) <- lexer State{..}
        case status of
            End -> do
                debug $ printf "done: got %d packets\n" _recv
                return End
            Waiting -> do
                debug $ printf "waiting...\n"
                packets' <- case packets of
                    [] -> do
                        writeChan _pipe BS.empty
                        return []
                    p:ps -> do
                        debug $ printf "sent packet '%s'\n" (show p)
                        writeChan _pipe p
                        return ps
                (State{..}, status') <- fill State{..}
                case status' of
                    Ready -> loop State{..} packets'
                    _ -> error "unexpected status after fill"
            BadPacket -> do
                debug $ printf "error: ill-formed packet\n"
                return BadPacket
            _ -> error "unexpected status"
    test :: [BS.ByteString] -> Status -> IO ()
    test packets expect = do
        pipe <- newChan -- emulate pipe using a chan of bytestrings
        let st = State {
            _pipe = pipe,
            _yyinput = BS.singleton 0, -- null sentinel triggers YYFILL
            _yycursor = 0,
            _yymarker = 0,
            _token = 0,
            _yylimit = 0,
            _eof = False,
            _yystate = -1,
            _recv = 0
        }
        status <- loop st packets
        when (status /= expect) $ error "failed"
        return ()
    main :: IO ()
    main = do
        test [] End
        test ["ze", "ro;o", "ne", ";t", "wo;thr", "e", "e", ";", "four;"] End
        test ["zer0;"] BadPacket

  Reusable blocks

Reusable blocks of the form /*!rules:re2c[:<name>] ... */ or %{rules[:<name>] ... %} can be reused any number of times and combined with other re2hs blocks. The <name> is optional. A rules block can be used in a use block or directive. The code for a rules block is generated at every point of use.

Use blocks are defined with /*!use:re2c[:<name>] ... */ or %{use[:<name>] ... %}. The <name> is optional: if it's not specified, the associated rules block is the most recent one (whether named or unnamed). A use block can add named definitions, configurations and rules of its own. An important use case for use blocks is a lexer that supports multiple input encodings: the same rules block is reused multiple times with encoding-specific configurations (see the example below).

In-block use directive !use:<name>; can be used from inside of a re2hs block. It merges the referenced block <name> into the current one. If some of the merged rules and configurations overlap with the previously defined ones, conflicts are resolved in the usual way: the earliest rule takes priority, and latest configuration overrides preceding ones. One exception are the special rules *, $ and (in condition mode) <!>, for which a block-local definition overrides any inherited ones. Use directive allows one to combine different re2hs blocks together in one block (see the example below).

Named blocks and in-block use directive were added in re2hs version 2.2. Since that version reusable blocks are allowed by default (no special option is needed). Before version 2.2 reuse mode was enabled with -r --reusable option. Before version 1.2 reusable blocks could not be mixed with normal blocks.

  Example of a !use directive

-- re2hs $INPUT -o $OUTPUT
{-# OPTIONS_GHC -Wno-unused-record-wildcards #-}
{-# LANGUAGE OverloadedStrings #-}
-- This example shows how to combine reusable re2c blocks: two blocks
-- ('colors' and 'fish') are merged into one. The 'salmon' rule occurs
-- in both blocks; the 'fish' block takes priority because it is used
-- earlier. Default rule * occurs in all three blocks; the local (not
-- inherited) definition takes priority.
import Control.Monad (when)
import Data.ByteString (ByteString, index)
data Answer = Color | Fish | Dunno deriving (Eq)
data State = State {
    _yyinput :: ByteString,
    _yycursor :: Int,
    _yymarker :: Int
}
%{rules:colors
    *                            { error "ah" }
    "red" | "salmon" | "magenta" { Color }
%}
%{rules:fish
    *                            { error "oh" }
    "haddock" | "salmon" | "eel" { Fish }
%}
%{
    re2c:YYFN = ["lexer;Answer", "State{..};State"];
    re2c:yyfill:enable = 0;
    !use:fish;
    !use:colors;
    * { Dunno } // overrides inherited '*' rules
%}
main :: IO ()
main = do
    let test str ans = do
            let st = State {_yyinput = str, _yycursor = 0, _yymarker = 0}
            when (lexer st /= ans) $ error "failed"
    test "salmon" Fish
    test "what?" Dunno

  Example of a /*!use:re2c ... */ block

-- re2hs $INPUT -o $OUTPUT --input-encoding utf8
{-# OPTIONS_GHC -Wno-unused-record-wildcards #-}
{-# LANGUAGE OverloadedStrings #-}
-- This example supports multiple input encodings: UTF-8 and UTF-32.
-- Both lexers are generated from the same rules block, and the use
-- blocks add only encoding-specific configurations.
import Control.Monad (when)
import Data.Array
import Data.Word
data State a = State {
    _yyinput :: a,
    _yycursor :: Int,
    _yymarker :: Int
}
%{rules
    re2c:yyfill:enable = 0;
    re2c:YYPEEK = "(!)";
    "∀x ∃y" { Just _yycursor }
    *       { Nothing }
%}
%{use
    re2c:YYFN = ["lex8;Maybe Int", "State{..};State (Array Int Word8)"];
    re2c:encoding:utf8 = 1;
    re2c:YYCTYPE = Word8;
%}
%{use
    re2c:YYFN = ["lex32;Maybe Int", "State{..};State (Array Int Int)"];
    re2c:encoding:utf32 = 1;
    re2c:YYCTYPE = Int;
%}
main :: IO ()
main = do
    let make_st l = State {
            _yyinput = listArray (0, length l - 1) l,
            _yycursor = 0,
            _yymarker = 0}
    let s8 = [0xe2, 0x88, 0x80, 0x78, 0x20, 0xe2, 0x88, 0x83, 0x79]
    when (lex8 (make_st s8) /= Just (length s8)) $ error "lex8 failed"
    let s32 = [0x2200, 0x78, 0x20, 0x2203, 0x79]
    when (lex32 (make_st s32) /= Just (length s32)) $ error "lex32 failed"

  Submatch extraction

re2hs has two options for submatch extraction.

Tags
    The first option is to use standalone tags of the form @stag or #mtag, where stag and mtag are arbitrary used-defined names. Tags are enabled with -T --tags option or re2c:tags = 1 configuration. Semantically tags are position markers: they can be inserted anywhere in a regular expression, and they bind to the corresponding position (or multiple positions) in the input string. S-tags bind to the last matching position, and m-tags bind to a list of positions (they may be used in repetition subexpressions, where a single position in a regular expression corresponds to multiple positions in the input string). All tags should be defined by the user, either manually or with the help of svars and mvars blocks. If there is more than one way tags can be matched against the input, ambiguity is resolved using leftmost greedy disambiguation strategy.

Captures
    The second option is to use capturing groups. They are enabled with --captures option or re2c:captures = 1 configuration. There are two flavours for different disambiguation policies, --leftmost-captures (the default) is for leftmost greedy policy, and, --posix-captures is for POSIX longest-match policy. In this mode all parenthesized subexpressions are considered capturing groups, and a bang can be used to mark non-capturing groups: (! ... ). With --invert-captures option or re2c:invert-captures = 1 configuration the meaning of bang is inverted. The number of groups for the matching rule is stored in a variable yynmatch (the whole regular expression is group number zero), and submatch results are stored in yypmatch array. Both yynmatch and yypmatch should be defined by the user, and yypmatch size must be at least [yynmatch * 2]. Use maxnmatch block to define YYMAXNMATCH, a constant that equals to the maximum value of yynmatch among all rules.

Captvars
    Another way to use capturing groups is the --captvars option or re2c:captvars = 1 configuration. The only difference with --captures is in the way the generated code stores submatch results: instead of yynmatch and yypmatch re2hs generates variables yytl<k> and yytr<k> for k-th capturing group (the user should declare these using an svars block). Captures with variables support two disambiguation policies: --leftmost-captvars or re2c:leftmost-captvars = 1 for leftmost greedy policy (the default one) and --posix-captvars or re2c:posix-captvars for POSIX longest-match policy.

    Under the hood all these options translate into tags and Tagged Deterministic Finite Automata with Lookahead. The core idea of TDFA is to minimize the overhead on submatch extraction. In the extreme, if there're no tags or captures in a regular expression, TDFA is just an ordinary DFA. If the number of tags is moderate, the overhead is barely noticeable. The generated TDFA uses a number of tag variables which do not map directly to tags: a single variable may be used for different tags, and a tag may require multiple variables to hold all its possible values. Eventually ambiguity is resolved, and only one final variable per tag survives. Tag variables should be defined using stags or mtags blocks. If lexer state is stored, tag variables should be part of it. They also need to be updated by YYFILL.

    S-tags support the following operations:

•   save input position to an s-tag: t = YYCURSOR with C pointer API or a user-defined operation YYSTAGP(t) with generic API

•   save default value to an s-tag: t = NULL with C pointer API or a user-defined operation YYSTAGN(t) with generic API

•   copy one s-tag to another: t1 = t2

    M-tags support the following operations:

•   append input position to an m-tag: a user-defined operation YYMTAGP(t) with both default and generic API

•   append default value to an m-tag: a user-defined operation YYMTAGN(t) with both default and generic API

•   copy one m-tag to another: t1 = t2

    S-tags can be implemented as scalar values (pointers or offsets). M-tags need a more complex representation, as they need to store a sequence of tag values. The most naive and inefficient representation of an m-tag is a list (array, vector) of tag values; a more efficient representation is to store all m-tags in a prefix-tree represented as array of nodes (v, p), where v is tag value and p is a pointer to parent node.

    Here is a simple example of using s-tags to parse semantic versions consisting of three numeric components: major, minor, patch (the latter is optional). See below for a more complex example that uses YYFILL.

    -- re2hs $INPUT -o $OUTPUT
    {-# OPTIONS_GHC -Wno-unused-record-wildcards #-}
    {-# LANGUAGE OverloadedStrings #-}
    import Control.Monad (when)
    import Data.ByteString (ByteString, index)
    data State = State {
        -- Final tag bindings available in semantic action.
        %{svars format = "\n@@ :: !Int,"; %}
        -- Intermediate tag bindings used by the lexer (must be autogenerated).
        %{stags format = "\n@@ :: !Int,"; %}
        _yyinput :: !ByteString,
        _yycursor :: !Int,
        _yymarker :: !Int
    }
    data SemVer = SemVer {
        major :: !Int,
        minor :: !Int,
        patch :: !Int
    } deriving (Eq)
    s2n :: ByteString -> Int -> Int -> Int
    s2n s i j = f i 0 where
        f k n = if k >= j then n else f (k + 1) (n * 10 + (fromIntegral (index s k) - 48))
    %{
        re2c:YYFN = ["parse;Maybe SemVer", "State{..};State"];
        re2c:tags = 1;
        re2c:yyfill:enable = 0;
        num = [0-9]+;
        @_1 num @_2 "." @_3 num @_4 ("." @_5 num)? [\x00] {
            Just SemVer {
                major = s2n _yyinput _1 _2,
                minor = s2n _yyinput _3 _4,
                patch = if _5 == (-1) then 0 else s2n _yyinput _5 (_yycursor - 1)
            }
        }
        * { Nothing }
    %}
    test :: ByteString -> Maybe SemVer -> IO ()
    test str expect = do
        let s = State {
            %{svars format = "\n@@ = (-1),"; %}
            %{stags format = "\n@@ = (-1),"; %}
            _yyinput = str,
            _yycursor = 0,
            _yymarker = 0
        }
        when (parse s /= expect) $ error "failed!"
    main :: IO ()
    main = do
        test "23.34\0" (Just SemVer {major = 23, minor = 34, patch = 0})
        test "1.2.99999\0" (Just SemVer {major = 1, minor = 2, patch = 99999})
        test "1.a\0" Nothing

    Here is a more complex example of using s-tags with YYFILL to parse a file with newline-separated semantic versions. Tag variables are part of the lexer state, and they are adjusted in YYFILL like other input positions. Note that it is necessary for s-tags because their values are invalidated after shifting buffer contents. It may not be necessary in a custom implementation where tag variables store offsets relative to the start of the input string rather than the buffer, which may be the case with m-tags.

    -- re2hs $INPUT -o $OUTPUT
    {-# OPTIONS_GHC -Wno-unused-record-wildcards #-}
    {-# LANGUAGE OverloadedStrings #-}
    import Control.Monad
    import Data.ByteString as BS
    import GHC.IO.Handle
    import System.Directory
    import System.IO
    chunk_size :: Int
    chunk_size = 4096
    data State = State {
        _file :: !Handle,
        _yyinput :: !BS.ByteString,
        _yycursor :: !Int,
        _yymarker :: !Int,
        _yylimit :: !Int,
        _token :: !Int,
        -- Final tag bindings available in semantic action.
        %{svars format = "\n@@ :: !Int,"; %}
        -- Intermediate tag bindings used by the lexer (must be autogenerated).
        %{stags format = "\n@@ :: !Int,"; %}
        _eof :: !Bool
    }
    data SemVer = SemVer {
        major :: !Int,
        minor :: !Int,
        patch :: !Int
    } deriving (Eq, Show)
    s2n :: BS.ByteString -> Int -> Int -> Int
    s2n s i j = f i 0 where
        f k n = if k >= j then n else f (k + 1) (n * 10 + (fromIntegral (BS.index s k) - 48))
    %{
        re2c:YYFN = ["lexer;IO [SemVer]", "State{..};State", "_vers;[SemVer]"];
        re2c:YYPEEK = "BS.index";
        re2c:YYFILL = "(State{..}, yyfill) <- fill State{..}";
        re2c:eof = 0;
        re2c:monadic = 1;
        re2c:tags = 1;
        num = [0-9]+;
        @_1 num @_2 "." @_3 num @_4 ("." @_5 num)? [\n] {
            let ver = SemVer {
                major = s2n _yyinput _1 _2,
                minor = s2n _yyinput _3 _4,
                patch = if _5 == (-1) then 0 else s2n _yyinput _5 (_yycursor - 1)
            }
            lexer State{..} (ver: _vers)
        }
        $ { return _vers }
        * { error "lexer failed" }
    %}
    fill :: State -> IO (State, Bool)
    fill State{..} = do
        case _eof of
            True -> return (State{..}, False)
            False -> do
                -- Discard everything up to the current token, cut off terminating null,
                -- read new chunk from file and reappend terminating null at the end.
                chunk <- BS.hGet _file chunk_size
                return (State{
                    _yyinput = BS.concat [(BS.init . BS.drop _token) _yyinput, chunk, "\0"],
                    _yycursor = _yycursor - _token,
                    _yymarker = _yymarker - _token,
                    _yylimit = _yylimit - _token + BS.length chunk, -- exclude terminating null
                    _token = 0,
                    _eof = BS.null chunk, -- end of file?
                    ..}, True)
    main :: IO ()
    main = do
        let fname = "input"
        -- Prepare input file.
        BS.writeFile fname $ BS.concat ["1.22.333\n" | _ <- [1..chunk_size]]
        let expect = [SemVer {major = 1, minor = 22, patch = 333} | _ <- [1..chunk_size]]
        -- Run lexer on the prepared file.
        fh <- openFile fname ReadMode
        let st = State {
            _file = fh,
            _yyinput = BS.singleton 0,
            _yycursor = 0,
            _yymarker = 0,
            _yylimit = 0,
            _token = 0,
            %{svars format = "\n@@ = (-1),"; %}
            %{stags format = "\n@@ = (-1),"; %}
            _eof = False
        }
        result <- lexer st []
        hClose fh
        -- Cleanup.
        removeFile fname
        -- Check result.
        when (result /= expect) $ error $ "expected " ++ show expect ++ ", got " ++ show result
        return ()

    Here is an example of using capturing groups to parse semantic versions.

    -- re2hs $INPUT -o $OUTPUT
    {-# OPTIONS_GHC -Wno-unused-record-wildcards #-}
    {-# LANGUAGE OverloadedStrings #-}
    import Control.Monad (when)
    import Data.ByteString (ByteString, index)
    import Data.Word (Word8)
    none :: Int
    none = -1
    data State = State {
        -- Final tag bindings available in semantic action.
        %{svars format = "\n@@ :: !Int,"; %}
        -- Intermediate tag bindings used by the lexer (must be autogenerated).
        %{stags format = "\n@@ :: !Int,"; %}
        _yyinput :: !ByteString,
        _yycursor :: !Int,
        _yymarker :: !Int
    }
    data SemVer = SemVer {
        major :: !Int,
        minor :: !Int,
        patch :: !Int
    } deriving (Eq)
    s2n :: ByteString -> Int -> Int -> Int
    s2n s i j = f i 0 where
        f k n = if k >= j then n else f (k + 1) (n * 10 + (fromIntegral (index s k) - 48))
    %{
        re2c:YYFN = ["parse;Maybe SemVer", "State{..};State"];
        re2c:YYCTYPE = "Word8";
        re2c:captvars = 1;
        re2c:yypmatch = _;
        re2c:yyfill:enable = 0;
        num = [0-9]+;
        (num) "." (num) ("." num)? [\x00] {
            Just SemVer {
                major = s2n _yyinput _yytl1 _yytr1,
                minor = s2n _yyinput _yytl2 _yytr2,
                patch = if _yytl3 == none then 0 else s2n _yyinput (_yytl3 + 1) _yytr3
            }
        }
        * { Nothing }
    %}
    test :: ByteString -> Maybe SemVer -> IO ()
    test str expect = do
        let s = State {
            %{svars format = "\n@@ = none,"; %}
            %{stags format = "\n@@ = none,"; %}
            _yyinput = str,
            _yycursor = 0,
            _yymarker = 0
        }
        when (parse s /= expect) $ error "failed!"
    main :: IO ()
    main = do
        test "23.34\0" (Just SemVer {major = 23, minor = 34, patch = 0})
        test "1.2.99999\0" (Just SemVer {major = 1, minor = 2, patch = 99999})
        test "1.a\0" Nothing

    Here is an example of using m-tags to parse a version with a variable number of components. Tag variables are stored in a trie.

    -- re2hs $INPUT -o $OUTPUT
    {-# OPTIONS_GHC -Wno-unused-record-wildcards #-}
    {-# LANGUAGE OverloadedStrings #-}
    import Control.Monad (when)
    import Data.ByteString (ByteString, index)
    data State = State {
        -- Final tag bindings available in semantic action.
        %{svars format = "\n@@ :: !Int,"; %}
        %{mvars format = "\n@@ :: ![Int],"; %}
        -- Intermediate tag bindings used by the lexer (must be autogenerated).
        %{stags format = "\n@@ :: !Int,"; %}
        %{mtags format = "\n@@ :: ![Int],"; %}
        _yyinput :: !ByteString,
        _yycursor :: !Int,
        _yymarker :: !Int
    }
    s2n :: ByteString -> Int -> Int -> Int
    s2n s i j = f i 0 where
        f k n = if k >= j then n else f (k + 1) (n * 10 + (fromIntegral (index s k) - 48))
    %{
        re2c:YYFN = ["parse;Maybe [Int]", "State{..};State"];
        re2c:YYMTAGP = "let tag = _yycursor : @@{tag} in let @@{tag} = tag in";
        re2c:YYMTAGN = ""; // alternatively could add -1 to the list
        re2c:tags = 1;
        re2c:yyfill:enable = 0;
        num = [0-9]+;
        @_1 num @_2 ("." #_3 num #_4)* [\x00] {
            Just $ (s2n _yyinput _1 _2) : (reverse $ zipWith (\i j -> s2n _yyinput i j) _3 _4)
        }
        * { Nothing }
    %}
    test :: ByteString -> Maybe [Int] -> IO ()
    test str expect = do
        let st = State {
            %{svars format = "\n@@ = (-1),"; %}
            %{stags format = "\n@@ = (-1),"; %}
            %{mvars format = "\n@@ = [],"; %}
            %{mtags format = "\n@@ = [],"; %}
            _yyinput = str,
            _yycursor = 0,
            _yymarker = 0
        }
        when (parse st /= expect) $ error "failed!"
    main :: IO ()
    main = do
        test "1\0" (Just [1])
        test "1.2.3.4.5.6.7\0" (Just [1, 2, 3, 4, 5, 6, 7])
        test "1.2.\0" Nothing

  Encoding support

It is necessary to understand the difference between code points and code units. A code point is a numeric identifier of a symbol. A code unit is the smallest unit of storage in the encoded text. A single code point may be represented with one or more code units. In a fixed-length encoding all code points are represented with the same number of code units. In a variable-length encoding code points may be represented with a different number of code units. Note that the "any" rule [^] matches any code point, but not necessarily any code unit (the only way to match any code unit regardless of the encoding is the default rule *). The generated lexer works with a stream of code units: yych stores a code unit, and YYCTYPE is the code unit type. Regular expressions, on the other hand, are specified in terms of code points. When re2hs compiles regular expressions to automata it translates code points to code units. This is generally not a simple mapping: in variable-length encodings a single code point range may get translated to a complex code unit graph. The following encodings are supported:

•   ASCII (enabled by default). It is a fixed-length encoding with code space [0-255] and 1-byte code points and code units.

•   EBCDIC (enabled with --ebcdic or re2c:encoding:ebcdic). It is a fixed-length encoding with code space [0-255] and 1-byte code points and code units.

•   UCS2 (enabled with --ucs2 or re2c:encoding:ucs2). It is a fixed-length encoding with code space [0-0xFFFF] and 2-byte code points and code units.

•   UTF8 (enabled with --utf8 or re2c:encoding:utf8). It is a variable-length Unicode encoding. Code unit size is 1 byte. Code points are represented with 1 -- 4 code units.

•   UTF16 (enabled with --utf16 or re2c:encoding:utf16). It is a variable-length Unicode encoding. Code unit size is 2 bytes. Code points are represented with 1 -- 2 code units.

•   UTF32 (enabled with --utf32 or re2c:encoding:utf32). It is a fixed-length Unicode encoding with code space [0-0x10FFFF] and 4-byte code points and code units.

    Include file include/unicode_categories.re provides re2hs definitions for the standard Unicode categories.

    Option --input-encoding specifies source file encoding, which can be used to enable Unicode literals in regular expressions. For example --input-encoding utf8 tells re2hs that the source file is in UTF8 (it differs from --utf8 which sets input text encoding). Option --encoding-policy specifies the way re2hs handles Unicode surrogates (code points in range [0xD800-0xDFFF]).

    Below is an example of a lexer for UTF8 encoded Unicode identifiers.

    -- re2hs $INPUT -o $OUTPUT --utf8 -i
    {-# OPTIONS_GHC -Wno-unused-record-wildcards #-}
    {-# LANGUAGE OverloadedStrings #-}
    import Control.Monad (when)
    import Data.ByteString (ByteString, index)
    %{include "unicode_categories.re" %}
    data State = State {
        _yyinput :: ByteString,
        _yycursor :: Int,
        _yymarker :: Int,
        _yyaccept :: Int
    }
    %{
        re2c:YYFN = ["lexer;Bool", "State{..};State"];
        re2c:yyfill:enable = 0;
        // Simplified "Unicode Identifier and Pattern Syntax"
        // (see https://unicode.org/reports/tr31)
        id_start    = L | Nl | [$_];
        id_continue = id_start | Mn | Mc | Nd | Pc | [\u200D\u05F3];
        identifier  = id_start id_continue*;
        identifier { True }
        *          { False }
    %}
    main :: IO ()
    main = do
        let st = State {
                _yyinput = "_Ыдентификатор\x00",
                _yycursor = 0,
                _yymarker = 0,
                _yyaccept = 0}
        when (not $ lexer st) $ error "failed"

  Include files

re2hs allows one to include other files using a block of the form /*!include:re2c FILE */ or %{include FILE %}, or an in-block directive !include FILE ;, where FILE is a path to the file to be included. re2hs looks for include files in the directory of the including file and in include locations, which can be specified with the -I option. Include blocks/directives in re2hs work in the same way as C/C++ #include: FILE contents are copy-pasted verbatim in place of the block/directive. Include files may have further includes of their own. Use --depfile option to track build dependencies of the output file on include files. re2hs provides some predefined include files that can be found in the include/ subdirectory of the project. These files contain definitions that may be useful to other projects (such as Unicode categories) and form something like a standard library for re2hs. Below is an example of using include files.

  Include file 1 (definitions.hs)

data Number = INum | FNum | NNaN deriving (Eq)
%{
    number = [1-9][0-9]*;
%}

  Include file 2 (extra_rules.re.inc)

// floating-point numbers
frac  = [0-9]* "." [0-9]+ | [0-9]+ ".";
exp   = 'e' [+-]? [0-9]+;
float = frac exp? | [0-9]+ exp;
float { FNum }

  Input file

-- re2hs $INPUT -o $OUTPUT -i
{-# OPTIONS_GHC -Wno-unused-record-wildcards #-}
{-# LANGUAGE OverloadedStrings #-}
import Control.Monad (when)
import Data.ByteString (ByteString, index)
%{include "definitions.hs" %}
data State = State {
    _yyinput :: ByteString,
    _yycursor :: Int,
    _yymarker :: Int,
    _yyaccept :: Int
}
%{
    re2c:YYFN = ["lexer;Number", "State{..};State"];
    re2c:yyfill:enable = 0;
    *      { NNaN }
    number { INum }
    !include "extra_rules.re.inc";
%}
main :: IO ()
main = do
    let test s n = do
            let st = State {
                    _yyinput = s,
                    _yycursor = 0,
                    _yymarker = 0,
                    _yyaccept = 0}
            when (lexer st /= n) $ error "failed"
    test "123\0" INum
    test "123.4567\0" FNum

  Header files

re2hs allows one to generate header file from the input .re file using --header option or re2c:header configuration and block pairs of the form /*!header:re2c:on*/ and /*!header:re2c:off*/, or %{header:on%} and %{header:off%}. The first block marks the beginning of header file, and the second block marks the end of it. Everything between these blocks is processed by re2hs, and the generated code is written to the file specified with --header option or re2c:header configuration (or stdout if neither option nor configuration is used). Autogenerated header file may be needed in cases when re2hs is used to generate definitions that must be visible from other translation units.

Here is an example of generating a header file that contains definition of the lexer state with tag variables (the number variables depends on the regular grammar and is unknown to the programmer).

  Input file

-- re2hs $INPUT -o $OUTPUT --header lexer/state.hs -i
{-# OPTIONS_GHC -Wno-unused-record-wildcards #-}
{-# LANGUAGE OverloadedStrings #-}
import Control.Monad (when)
import Data.ByteString (index)
import State
%{header:on %}
module State where
import Data.ByteString (ByteString)
data State = State {
    _yyinput :: !ByteString,
    _yycursor :: !Int,
    %{stags format = "\n@@{tag} :: !Int,"; %}
    _tag :: !Int
}
%{header:off %}
%{
    re2c:YYFN = ["lexer;Int", "State{..};State"];
    re2c:tags = 1;
    re2c:yyfill:enable = 0;
    re2c:header = "lexer/state.hs";
    [a]* @_tag [b]* { _tag }
%}
main :: IO ()
main = do
    let s = State {
        _yyinput = "ab\0",
        _yycursor = 0,
        %{stags format = "\n@@{tag} = -1,"; %}
        _tag = 0}
    when (lexer s /= 1) $ error "failed!"

  Header file

-- Generated by re2c
{-# LANGUAGE RecordWildCards #-}
module State where
import Data.ByteString (ByteString)
data State = State {
    _yyinput :: !ByteString,
    _yycursor :: !Int,

_yyt1 :: !Int,
    _tag :: !Int
}

  Skeleton programs

With the -S, --skeleton option, re2hs ignores all non-re2hs code and generates a self-contained C program that can be further compiled and executed. The program consists of lexer code and input data. For each constructed DFA (block or condition) re2hs generates a standalone lexer and two files: an .input file with strings derived from the DFA and a .keys file with expected match results. The program runs each lexer on the corresponding .input file and compares results with the expectations. Skeleton programs are very useful for a number of reasons:

•   They can check correctness of various re2hs optimizations (the data is generated early in the process, before any DFA transformations have taken place).

•   Generating a set of input data with good coverage may be useful for both testing and benchmarking.

•   Generating self-contained executable programs allows one to get minimized test cases (the original code may be large or have a lot of dependencies).

    The difficulty with generating input data is that for all but the most trivial cases the number of possible input strings is too large (even if the string length is limited). re2hs solves this difficulty by generating sufficiently many strings to cover almost all DFA transitions. It uses the following algorithm. First, it constructs a skeleton of the DFA. For encodings with 1-byte code unit size (such as ASCII, UTF-8 and EBCDIC) skeleton is just an exact copy of the original DFA. For encodings with multibyte code units skeleton is a copy of DFA with certain transitions omitted: namely, re2hs takes at most 256 code units for each disjoint continuous range that corresponds to a DFA transition. The chosen values are evenly distributed and include range bounds. Instead of trying to cover all possible paths in the skeleton (which is infeasible) re2hs generates sufficiently many paths to cover all skeleton transitions, and thus trigger the corresponding conditional jumps in the lexer. The algorithm implementation is limited by ~1Gb of transitions and consumes constant amount of memory (re2hs writes data to file as soon as it is generated).

  Visualization and debug

With the -D, --emit-dot option, re2hs does not generate code. Instead, it dumps the generated DFA in DOT format. One can convert this dump to an image of the DFA using Graphviz or another library. Note that this option shows the final DFA after it has gone through a number of optimizations and transformations. Earlier stages can be dumped with various debug options, such as --dump-nfa, --dump-dfa-raw etc. (see the full list of options).

SEE ALSO

You can find more information about re2c at the official website: http://re2c.org. Similar programs are flex(1), lex(1), quex(http://quex.sourceforge.net).

AUTHORS

re2hs was originally written by Peter Bumbulis (peter@csg.uwaterloo.ca) in 1993. Marcus Boerger and Dan Nuffer spent several years to turn the original idea into a production ready code generator. Since then it has been maintained and developed by multiple volunteers, most notably, Brian Young (bayoung@acm.org), Marcus Boerger, Dan Nuffer (nuffer@users.sourceforge.net), Ulya Trofimovich (skvadrik@gmail.com), Serghei Iakovlev, Sergei Trofimovich, Petr Skocik, ligfx raekye and PolarGoose.