re2go - Man Page

Note: This manual is for Go, but it refers to re2c as the general program.

re2c    [ OPTIONS ] [ WARNINGS ] INPUT
re2go   [ OPTIONS ] [ WARNINGS ] INPUT
re2rust [ OPTIONS ] [ WARNINGS ] INPUT

Input can be either a file or - for stdin.

re2c works as a preprocessor. It reads the input file (which is usually a program in the target language, but can be anything) and looks for blocks of code enclosed in special-form comments. The text outside of these blocks is copied verbatim into the output file. The contents of the blocks are processed by re2c. It translates them to code in the target language 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:

//go:generate re2go $INPUT -o $OUTPUT -i
package main

func lex(str string) {
    var cursor int
    /*!re2c
        re2c:define:YYCTYPE = byte;
        re2c:define:YYPEEK = "str[cursor]";
        re2c:define:YYSKIP = "cursor += 1";
        re2c:yyfill:enable = 0;

        number = [1-9][0-9]*;

        number { return }
        *      { panic("error!") }
    */
}

func main() {
    lex("1234\x00")
}

In the output everything between /*!re2c and */ has been replaced with the generated code:

// Code generated by re2c, DO NOT EDIT.
//go:generate re2go $INPUT -o $OUTPUT -i
package main

func lex(str string) {
    var cursor int
    
{
    var yych byte
    yych = str[cursor]
    switch (yych) {
    case '1','2','3','4','5','6','7','8','9':
        goto yy2
    default:
        goto yy1
    }
yy1:
    cursor += 1
    { panic("error!") }
yy2:
    cursor += 1
    yych = str[cursor]
    switch (yych) {
    case '0','1','2','3','4','5','6','7','8','9':
        goto yy2
    default:
        goto yy3
    }
yy3:
    { return }
}

}

func main() {
    lex("1234\x00")
}

A re2c program consists of a sequence of blocks intermixed with code in the target language. There are three main kinds of blocks:

/*!re2c[:<name>] ... */

A global block contains definitions, configurations, directives and rules. re2c 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. The :<name> part is optional: if specified, the name can be used to refer to the block in another part of the program.

/*!local:re2c[:<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>] ... */

A rules block is like a local block, but it does not generate any code and is meant to be reused in other blocks. This is a way of sharing code (more details in the reusable blocks section).

There are also many auxiliary blocks; see section blocks and directives for a full list of them. A block may contain the following kinds of statements:

<name> = <regular expression>;

A definition binds a name to a regular expression. Names may contain alphanumeric characters and underscore. The regular expressions section gives an overview of re2c 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 re2c behavior and customize the generated code. For a full list of configurations supported by re2c 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 a regular expression to a semantic action (a block of code in the target language). If the regular expression matches, the associated semantic action 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 $. The 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. If start conditions are used, rules have more complex syntax.

!<directive>;

A directive is one of the special predefined statements. Each directive has a unique purpose. For example, the !use directive merges a rules block into the current one (see the reusable blocks section), and the !include directive allows one to include an outer file (see the include files section).

The generated code interfaces with the outer program with the help of primitives -- symbolic names that can be defined as variables, functions or macros in the target language (collectively referred to as the API). The definition of primitives is left for the user: this gives them both freedom in customizing the lexer and responsibility to understand how it works. Not all primitives have to be defined --- only those used by a given program. The manual provides definitions for the most popular use cases. For a full list of primitives and their meaning see the API primitives section.

There are two API flavors that define the set of primitives used by re2c:

Pointer API

This API is based on C pointer arithmetic. It was historically the first, and for a long time the only one. It consists of pointer-like primitives YYCURSOR, YYMARKER, YYCTXMARKER, YYLIMIT (which are normally defined as pointers of type YYCTYPE*) and YYFILL. This API is enabled by default for C, and it cannot be used with other backends that do not support pointer arithmetic.

Generic API

This API is more flexible. It consists generic operations and does not assume any particular implementation. The primitives are YYPEEK, YYSKIP, YYBACKUP, YYBACKUPCTX, YYSTAGP, YYSTAGN, YYMTAGP, YYMTAGN, YYRESTORE, YYRESTORECTX, YYRESTORETAG, YYSHIFT, YYSHIFTSTAG, YYSHIFTMTAG, YYLESSTHAN and YYFILL. For the C backend generic API is enabled with --api custom option or re2c:api = custom; configuration; for Go and Rust it is enabled by default. Generic API was added in version 0.14.

There are two API styles that determine the form in which the primitives should be defined:

Free-form

Free-form style is enabled with configuration re2c:api:style = free-form;. It is the default for Go. In this style interface primitives should be defined as free-form pieces of code with interpolated variables of the form @@{var} or optionally just @@ if there is a single variable. The set of variables is specific to each primitive. Free-form style generic API can be defined in terms of integer variables cursor, limit, marker, ctxmarker and a string (or a byte slice) data as follows:

/*!re2c
  re2c:define:YYPEEK       = "data[cursor]";
  re2c:define:YYSKIP       = "cursor++";
  re2c:define:YYBACKUP     = "marker = cursor";
  re2c:define:YYRESTORE    = "cursor = marker";
  re2c:define:YYBACKUPCTX  = "ctxmarker = cursor";
  re2c:define:YYRESTORECTX = "cursor = ctxmarker";
  re2c:define:YYRESTORETAG = "cursor = ${tag}";
  re2c:define:YYLESSTHAN   = "limit - cursor < @@{len}";
  re2c:define:YYSTAGP      = "@@{tag} = cursor";
  re2c:define:YYSTAGN      = "@@{tag} = -1";
  re2c:define:YYSHIFT      = "cursor += @@{shift}";
  re2c:define:YYSHIFTSTAG  = "@@{tag} += @@{shift}";
*/

Function-like

Function-like style is enabled with configuration re2c:api:style = functions;. In this style primitives should be defined as functions or macros with parentheses, accepting the necessary arguments. This style is more restrictive than the free-form style, but it can be used with Go closures. For example, if the input is a string (or a byte slice) data, and integer variables cursor, limit, marker and ctxmarker represent input positions, then the primitives can be defined as follows:

YYPEEK := func() byte { return data[cursor] }
YYSKIP := func() { cursor++ }
YYBACKUP := func() { marker = cursor }
YYRESTORE := func() { cursor = marker }
YYBACKUPCTX := func() { ctxmarker = cursor }
YYRESTORECTX := func() { cursor = ctxmarker }
YYLESSTHAN := func(n int) bool { return limit-cursor < n }
YYSHIFT := func(n int) { cursor += n }

For YYFILL definition and instructions how to customize or disable end-of-input checks see the handling the end of input and buffer refilling sections.

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 --input <default | custom>

Specify the API used by the generated code to interface with used-defined code: default is the API based on pointer arithmetic (the default for C), and custom is the generic API (the default for Go and Rust).

--bit-vectors -b

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

--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. This option is supported only for C.

--computed-gotos -g

Optimize conditional jumps using non-standard "computed goto" extension (which must be supported by the compiler). re2c generates jump tables only in complex cases with a lot of conditional branches. Complexity threshold can be configured with cgoto:threshold configuration. This option implies --bit-vectors. It is supported only for C.

--conditions --start-conditions -c

Enable support of Flex-like "conditions": multiple interrelated lexers within one block. This is an alternative to manually specifying different re2c 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:re2c 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. re2c assumes that the character range is 0 -- 0xFF and character size is 1 byte.

--empty-class <match-empty | match-none | error>

Define the way re2c 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 re2c treats Unicode surrogates. With fail re2c aborts with an error when a surrogate is encountered. With substitute re2c silently replaces surrogates with the error code point 0xFFFD. With ignore (the default) re2c 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.

--header --type-header -t HEADER

Generate a HEADER file. The contents of the file can be specified with directives header:re2c:on and header:re2c:off. If conditions are used the header will have a condition enum automatically appended to it (unless there is an explicit conditions:re2c directive).

-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:re2c directive. re2c looks for FILE in the directory of the parent file and in the include locations specified with -I option.

--input-encoding <ascii | utf8>

Specify the way re2c parses regular expressions. With ascii (the default) re2c handles input as ASCII-encoded: any sequence of code units is a sequence of standalone 1-byte characters. With utf8 re2c 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 <c | go | rust>

Specify the output language. Supported languages are C, Go and Rust. The default is C for re2c, Go for re2go and Rust for re2rust.

--leftmost-captures

Enable submatch extraction with leftmost greedy capturing groups.

--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

Encode DFA in a form of a loop over a switch statement. Individual states are switch cases. The current state is stored in a variable yystate. Transitions between states update yystate to the case label of the destination state and continue to the head of the loop. This option is always enabled for Rust, as it has no goto statement and cannot use the goto/label approach which is the default for C and Go backends.

--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). This option is on by default for Rust, as it does not have line directives.

--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.

--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.

--tags -T

Enable submatch extraction with tags.

--ucs2 --wide-chars -w

Generate a lexer that reads UCS2-encoded input. re2c 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. re2c 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. re2c 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. re2c 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 re2c. 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 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 the ---header option or the conditions:re2c directive 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 re2c 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.

-Wuseless-escape

Warn if a symbol is escaped when it shouldn't be. By default, re2c 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.

Below is the list of re2c directives (syntactic constructs that mark the beginning and end of the code that should be processed by re2c). Named blocks were added in re2c version 2.2. They are exactly the same as unnamed blocks, except that the name can be used to reference a block in other parts of the program. More information on each directive can be found in the related sections.

/*!re2c[:<name>] ... */

A global re2c block with an optional name. The block may contain named definitions, configurations and rules in any order. Named definitions and configurations are defined in the global scope, so they are inherited by subsequent blocks. The code for a global block is generated at the point where the block is specified.

/*!local:re2c[:<name>] ... */

A local re2c block with an optional name. Unlike global blocks, definitions and configurations inside of a local block are not added into the global scope. In all other respects local blocks are the same as global blocks.

/*!rules:re2c[:<name>] ... */

A reusable block with an optional name. Rules blocks have the same structure as local or global blocks, but they do not produce any code and they can be reused multiple times in other blocks with the help of a !use:<name>; directive or a /*!use:re2c[:<name>] ... */ block. A rules block on its own does not add any definitions into the global scope. The code for it is generated at the point of use. Prior to re2c version 2.2 rules blocks required -r --reusable option.

/*!use:re2c[:<name>] ... */

A use block that references a previously defined rules block. If the name is specified, re2c 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 re2c version 2.2 use blocks required -r --reusable option.

!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.

/*!max:re2c[:<name1>[:<name2>...]] ... */

A directive 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>...]] ... */

A directive 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>...]] ... */

Directives that specify a template piece of code that is expanded for each s-tag/m-tag variable generated by re2c. 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.

/*!getstate:re2c[:<name1>[:<name2>...]] ... */

A directive 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 directive is incompatible with the --loop-switch option and Rust, as it requires cross-block transitions that are unsupported without the goto statement.

/*!conditions:re2c[:<name1>[:<name2>...]] ... */, /*!types:re2c... */

A directive 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> */

This directive 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 directive, in the same way as #include works in C/C++. This directive can be used together with the --depfile option to generate build system dependencies on the included files.

!include <file>;

This directive is the same as /*!include:re2c <file> */, except that it should be used inside of a re2c block.

/*!header:re2c:on*/

This directive marks the start of header file. Everything after it and up to the following /*!header:re2c:off*/ directive is processed by re2c and written to the header file specified with -t --type-header option.

/*!header:re2c:off*/

This directive marks the end of header file started with /*!header:re2c:on*/.

/*!ignore:re2c ... */

A block which contents are ignored and removed from the output file.

%{ ... %}

A global re2c block in the --flex-support mode. This is deprecated and exists for backward compatibility.

Here is a list of API primitives that may be used by the generated code in order to interface with the outer program. Which primitives are needed depends on multiple factors, including the complexity of regular expressions, input representation, buffering, the use of various features and so on. All the necessary primitives should be defined by the user in the form of macros, functions, variables, free-form pieces of code, or any other suitable form. re2c does not (and cannot) check the definitions, so if anything is missing or defined incorrectly the generated code will not compile.

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

A pointer-like l-value that stores the current input position (usually a pointer of type YYCTYPE*). Initially YYCURSOR should point to the first input character. It is advanced by the generated code. When a rule matches, YYCURSOR points to the position after the last matched character. It is used only in C pointer API.

YYLIMIT

A pointer-like r-value that stores the end of input position (usually a pointer of type YYCTYPE*). 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. YYLIMIT is used only in C pointer API.

YYMARKER

A pointer-like l-value (usually a pointer of type YYCTYPE*) that stores the position of the latest matched rule. It is used to restore the YYCURSOR position if the longer match fails and the lexer needs to rollback. Initialization is not needed. YYMARKER is used only in C pointer API.

YYCTXMARKER

A pointer-like l-value that stores the position of the trailing context (usually a pointer of type YYCTYPE*). No initialization is needed. It is used only in C pointer API, and only with the lookahead operator /.

YYFILL

A generic API primitive with one argument 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. The definition of YYFILL can be either function-like or free-form depending on the API style (see re2c:api:style and re2c:define:YYFILL:naked).

YYMAXFILL

An integral constant equal to the maximum value of the argument to YYFILL.  It can be generated with /*!max:re2c*/ directive.

YYLESSTHAN

A generic API primitive with one argument 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. The definition can be either function-like or free-form depending on the API style (see re2c:api:style).

YYPEEK

A generic API primitive with no arguments. It should be defined as an r-value of type YYCTYPE that is equal to the character at the current input position. The definition can be either function-like or free-form depending on the API style (see re2c:api:style).

YYSKIP

A generic API primitive with no arguments. YYSKIP should advance the current input position by one character. The definition can be either function-like or free-form depending on the API style (see re2c:api:style).

YYBACKUP

A generic API primitive with no arguments. YYBACKUP should save the current input position, which is later restored with YYRESTORE. The definition should be either function-like or free-form depending on the API style (see re2c:api:style).

YYRESTORE

A generic API primitive with no arguments. YYRESTORE should restore the current input position to the value saved by YYBACKUP. The definition should be either function-like or free-form depending on the API style (see re2c:api:style).

YYBACKUPCTX

A generic API primitive with zero arguments. YYBACKUPCTX should save the current input position as the position of the trailing context, which is later restored by YYRESTORECTX. The definition should be either function-like or free-form depending on the API style (see re2c:api:style).

YYRESTORECTX

A generic API primitive with no arguments. YYRESTORECTX should restore the trailing context position saved with YYBACKUPCTX. The definition should be either function-like or free-form depending on the API style (see re2c:api:style).

YYRESTORETAG

A generic API primitive with one argument tag. YYRESTORETAG should restore the trailing context position to the value of tag. The definition should be either function-like or free-form depending on the API style (see re2c:api:style).

YYSTAGP

A generic API primitive with one argument tag, where tag can be a pointer or an offset (see submatch extraction section for details). YYSTAGP should set tag to the current input position. The definition should be either function-like or free-form depending on the API style (see re2c:api:style).

YYSTAGN

A generic API primitive with one argument tag, where tag can be a pointer or an offset (see submatch extraction section for details). YYSTAGN should to set tag to a value that represents non-existent input position. The definition should be either function-like or free-form depending on the API style (see re2c:api:style).

YYMTAGP

A generic API primitive with one argument tag. YYMTAGP should append the current position to the submatch history of tag (see the submatch extraction section for details.) The definition should be either function-like or free-form depending on the API style (see re2c:api:style).

YYMTAGN

A generic API primitive with one argument 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.) The definition can be either function-like or free-form depending on the API style (see re2c:api:style).

YYSHIFT

A generic API primitive with one argument shift. YYSHIFT should shift the current input position by shift characters (the shift value may be negative). The definition can be either function-like or free-form depending on the API style (see re2c:api:style).

YYSHIFTSTAG

A generic  API primitive with two arguments, tag and shift. YYSHIFTSTAG should shift tag by shift characters (the shift value may be negative). The definition can be either function-like or free-form depending on the API style (see re2c:api:style).

YYSHIFTMTAG

A generic API primitive with two arguments, tag and shift. YYSHIFTMTAG should shift the latest value in the history of tag by shift characters (the shift value may be negative). The definition should be either function-like or free-form depending on the API style (see re2c:api:style).

YYMAXNMATCH

An integral constant equal to the maximal number of POSIX capturing groups in a rule. It is generated with /*!maxnmatch:re2c*/ directive.

YYCONDTYPE

The type of the condition enum. It should be generated either with the /*!types:re2c*/ directive or the -t --type-header option.

YYGETCONDITION

An API primitive with zero arguments. It should be defined as an r-value of type YYCONDTYPE that is equal to the current condition identifier. The definition can be either function-like or free-form depending on the API style (see re2c:api:style and re2c:define:YYGETCONDITION:naked).

YYSETCONDITION

An API primitive with one argument cond. The meaning of YYSETCONDITION is to set the current condition identifier to cond. The definition should be either function-like or free-form depending on the API style (see re2c:api:style and re2c:define:YYSETCONDITION@cond).

YYGETSTATE

An API primitive with zero arguments. It should be defined as an r-value of integer type that is equal to the current lexer state. Should be initialized to -1. The definition can be either function-like or free-form depending on the API style (see re2c:api:style and re2c:define:YYGETSTATE:naked).

YYSETSTATE

An API primitive with one argument state. The meaning of YYSETSTATE is to set the current lexer state to state. The definition should be either function-like or free-form depending on the API style (see re2c:api:style and re2c:define:YYSETSTATE@state).

YYDEBUG

A debug API primitive with two arguments. It can be used to debug the generated code (with -d --debug-output option). YYDEBUG should return no value and accept two arguments: state (either a DFA state index or -1) and symbol (the current input symbol).

yych

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

yyaccept

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

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.

re2c:api, re2c:flags: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:define: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:define: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: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: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: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:define:YYBACKUP

Defines generic API primitive YYBACKUP (see the API primitives section).

re2c:define:YYBACKUPCTX

Defines generic API primitive YYBACKUPCTX (see the API primitives section).

re2c:define:YYCONDTYPE

Defines YYCONDTYPE (see the API primitives section).

re2c:define:YYCTYPE

Defines YYCTYPE (see the API primitives section).

re2c:define:YYCTXMARKER

Defines API primitive YYCTXMARKER (see the API primitives section).

re2c:define:YYCURSOR

Defines API primitive YYCURSOR (see the API primitives section).

re2c:define:YYDEBUG

Defines API primitive YYDEBUG (see the API primitives section).

re2c:define:YYFILL

Defines API primitive YYFILL (see the API primitives section).

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:define:YYFILL:naked

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

re2c:define:YYGETCONDITION

Defines API primitive YYGETCONDITION (see the API primitives section).

re2c:define:YYGETCONDITION:naked

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

re2c:define:YYGETSTATE

Defines API primitive YYGETSTATE (see the API primitives section).

re2c:define:YYGETSTATE:naked

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

re2c:define:YYLESSTHAN

Defines generic API primitive YYLESSTHAN (see the API primitives section).

re2c:define:YYLIMIT

Defines API primitive YYLIMIT (see the API primitives section).

re2c:define:YYMARKER

Defines API primitive YYMARKER (see the API primitives section).

re2c:define:YYMTAGN

Defines generic API primitive YYMTAGN (see the API primitives section).

re2c:define:YYMTAGP

Defines generic API primitive YYMTAGP (see the API primitives section).

re2c:define:YYPEEK

Defines generic API primitive YYPEEK (see the API primitives section).

re2c:define:YYRESTORE

Defines generic API primitive YYRESTORE (see the API primitives section).

re2c:define:YYRESTORECTX

Defines generic API primitive YYRESTORECTX (see the API primitives section).

re2c:define:YYRESTORETAG

Defines generic API primitive YYRESTORETAG (see the API primitives section).

re2c:define:YYSETCONDITION

Defines API primitive YYSETCONDITION (see the API primitives section).

re2c:define:YYSETCONDITION@cond

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

re2c:define:YYSETCONDITION:naked

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

re2c:define:YYSETSTATE

Defines API primitive YYSETSTATE (see the API primitives section).

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:define:YYSETSTATE:naked

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

re2c:define:YYSKIP

Defines generic API primitive YYSKIP (see the API primitives section).

re2c:define:YYSHIFT

Defines generic API primitive YYSHIFT (see the API primitives section).

re2c:define:YYSHIFTMTAG

Defines generic API primitive YYSHIFTMTAG (see the API primitives section).

re2c:define:YYSHIFTSTAG

Defines generic API primitive YYSHIFTSTAG (see the API primitives section).

re2c:define:YYSTAGN

Defines generic API primitive YYSTAGN (see the API primitives section).

re2c:define:YYSTAGP

Defines generic API primitive YYSTAGP (see the API primitives section).

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:leftmost-captures

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

re2c:lookahead, re2c:flags:lookahead

Deprecated (see the deprecated --no-lookahead option).

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: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 re2c 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: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 re2c 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, changes the default case in YYGETSTATE switch: by default it aborts the program, 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:re2c directive 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:variable:yyaccept

Specifies the name of the yyaccept variable (see the API primitives section).

re2c:variable:yybm

Specifies the name of the yybm variable (used for bitmaps).

re2c:variable:yybm:hex, re2c: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:variable:yych

Specifies the name of the yych variable (see the API primitives section).

re2c:variable:yych:emit, re2c:yych:emit

If set to zero, yych definition is not generated. The default is non-zero.

re2c:variable:yych:conversion, re2c:yych:conversion

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

re2c:variable:yyctable

Specifies the name of the yyctable variable (the jump table generated for YYGETCONDITION switch with --computed-gotos option).

re2c:variable:yytarget

Specifies the name of the yytarget variable.

re2c:variable:yystable

Deprecated.

re2c:variable:yystate

Specifies the name of the yystate variable (used with the --loop-switch option to store the current DFA state).

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:define:YYFILL:naked or re2c:api:style.

re2c 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 the regular expression defined as name (or literal string "name" in Flex compatibility mode)
• {name} the 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.

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. re2c 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 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 /*!max:re2c*/. 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 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.

//go:generate re2go $INPUT -o $OUTPUT
package main

// Expect a null-terminated string.
func lex(str string) int {
    var cur int
    count := 0

    for { /*!re2c
        re2c:yyfill:enable = 0;
        re2c:define:YYCTYPE = byte;
        re2c:define:YYPEEK  = "str[cur]";
        re2c:define:YYSKIP  = "cur += 1";

        *      { return -1 }
        [\x00] { return count }
        [a-z]+ { count += 1; continue }
        [ ]+   { continue }
    */
    }
}

func main() {
    assert_eq := func(x, y int) { if x != y { panic("error") } }
    assert_eq(lex("\000"), 0)
    assert_eq(lex("one two three\000"), 3)
    assert_eq(lex("f0ur\000"), -1)
}

//go:generate re2go $INPUT -o $OUTPUT
package main

// Expects a null-terminated string.
func lex(str string) int {
    var cur, mar int
    lim := len(str) - 1 // lim points at the terminating null
    count := 0

    for { /*!re2c
        re2c:eof = 0;
        re2c:define:YYCTYPE    = byte;
        re2c:define:YYPEEK     = "str[cur]";
        re2c:define:YYSKIP     = "cur += 1";
        re2c:define:YYBACKUP   = "mar = cur";
        re2c:define:YYRESTORE  = "cur = mar";
        re2c:define:YYLESSTHAN = "lim <= cur";
        re2c:yyfill:enable = 0;

        str = ['] ([^'\\] | [\\][^])* ['];

        *    { return -1 }
        $    { return count }
        str  { count += 1; continue }
        [ ]+ { continue }
    */
    }
}

func main() {
    assert_eq := func(x, y int) { if x != y { panic("error") } }
    assert_eq(lex("\000"), 0)
    assert_eq(lex("'qu\000tes' 'are' 'fine: \\'' \000"), 3)
    assert_eq(lex("'unterminated\\'\000"), -1)
}

//go:generate re2go $INPUT -o $OUTPUT
package main

import "strings"

/*!max:re2c*/

// Expects YYMAXFILL-padded string.
func lex(str string) int {
    // Pad string with YYMAXFILL zeroes at the end.
    buf := str + strings.Repeat("\000", YYMAXFILL)

    var cur int
    lim := len(buf)
    count := 0

    for { /*!re2c
        re2c:define:YYCTYPE    = byte;
        re2c:define:YYPEEK     = "buf[cur]";
        re2c:define:YYSKIP     = "cur += 1";
        re2c:define:YYLESSTHAN = "lim - cur < @@";
        re2c:define:YYFILL     = "return -1";

        str = ['] ([^'\\] | [\\][^])* ['];

        [\x00] {
            // Check that it is the sentinel, not some unexpected null.
            if cur - 1 == len(str) { return count } else { return -1 }
        }
        str  { count += 1; continue }
        [ ]+ { continue }
        *    { return -1 }
    */
    }
}

func main() {
    assert_eq := func(x, y int) { if x != y { panic("error") } }
    assert_eq(lex(""), 0)
    assert_eq(lex("'qu\000tes' 'are' 'fine: \\'' "), 3)
    assert_eq(lex("'unterminated\\'"), -1)
    assert_eq(lex("'unexpected \000 null\\'"), -1)
}

//go:generate re2go $INPUT -o $OUTPUT
package main

// Returns "fake" terminating null if cursor has reached limit.
func peek(str string, cur int, lim int) byte {
    if cur >= lim {
        return 0 // fake null
    } else {
        return str[cur]
    }
}

// Expects a string without terminating null.
func lex(str string) int {
    var cur, mar int
    lim := len(str)
    count := 0

    for { /*!re2c
        re2c:eof = 0;
        re2c:define:YYCTYPE    = byte;
        re2c:define:YYLESSTHAN = "cur >= lim";
        re2c:define:YYPEEK     = "peek(str, cur, lim)";
        re2c:define:YYSKIP     = "cur += 1";
        re2c:define:YYBACKUP   = "mar = cur";
        re2c:define:YYRESTORE  = "cur = mar";
        re2c:yyfill:enable = 0;

        str = ['] ([^'\\] | [\\][^])* ['];

        *    { return -1 }
        $    { return count }
        str  { count += 1; continue }
        [ ]+ { continue }
    */
    }
}

func main() {
    assert_eq := func(x, y int) { if x != y { panic("error") } }
    assert_eq(lex(""), 0)
    assert_eq(lex("'qu\000tes' 'are' 'fine: \\'' "), 3)
    assert_eq(lex("'unterminated\\'"), -1)
}

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 re2c 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:re2c*/ and /*!mtags:re2c*/ directives and YYSTAGP/YYSTAGN/YYMTAGP/YYMTAGN in generic API)

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).

               <-- 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

//go:generate re2go $INPUT -o $OUTPUT
package main

import (
    "os"
    "strings"
)

const BUFSIZE int = 4096

type Input struct {
    file *os.File
    buf  []byte
    cur  int
    mar  int
    tok  int
    lim  int
    eof  bool
}

func fill(in *Input) int {
    if in.eof { return -1 } // unexpected EOF

    // Error: lexeme too long. In real life can reallocate a larger buffer.
    if in.tok < 1 { return -2 }

    // Shift buffer contents (discard everything up to the current token).
    copy(in.buf[0:], in.buf[in.tok:in.lim])
    in.cur -= in.tok
    in.mar -= in.tok
    in.lim -= in.tok
    in.tok = 0

    // Fill free space at the end of buffer with new data from file.
    n, _ := in.file.Read(in.buf[in.lim:BUFSIZE])
    in.lim += n
    in.buf[in.lim] = 0

    // If read less than expected, this is the end of input.
    in.eof = in.lim < BUFSIZE

    return 0
}

func lex(in *Input) int {
    count := 0
    for {
        in.tok = in.cur
    /*!re2c
        re2c:eof = 0;
        re2c:define:YYCTYPE    = byte;
        re2c:define:YYPEEK     = "in.buf[in.cur]";
        re2c:define:YYSKIP     = "in.cur += 1";
        re2c:define:YYBACKUP   = "in.mar = in.cur";
        re2c:define:YYRESTORE  = "in.cur = in.mar";
        re2c:define:YYLESSTHAN = "in.lim <= in.cur";
        re2c:define:YYFILL     = "fill(in) == 0";

        str = ['] ([^'\\] | [\\][^])* ['];

        *    { return -1 }
        $    { return count }
        str  { count += 1; continue }
        [ ]+ { continue }
    */
    }
}

func main() () {
    fname := "input"
    content := "'qu\000tes' 'are' 'fine: \\'' ";

    // Prepare input file: a few times the size of the buffer, containing
    // strings with zeroes and escaped quotes.
    f, _ := os.Create(fname)
    f.WriteString(strings.Repeat(content, BUFSIZE))
    f.Seek(0, 0)
    count := 3 * BUFSIZE // number of quoted strings written to file

    // Prepare lexer state: all offsets are at the end of buffer.
    in := &Input{
        file: f,
        // Sentinel at `lim` offset is set to zero, which triggers YYFILL.
        buf:  make([]byte, BUFSIZE+1),
        cur:  BUFSIZE,
        mar:  BUFSIZE,
        tok:  BUFSIZE,
        lim:  BUFSIZE,
        eof:  false,
    }

    // Run the lexer.
    if lex(in) != count { panic("error"); }

    // Cleanup: remove input file.
    f.Close();
    os.Remove(fname);
}

               <-- 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 ->

//go:generate re2go $INPUT -o $OUTPUT
package main

import (
    "os"
    "strings"
)

/*!max:re2c*/
const BUFSIZE int = 4096

type Input struct {
    file *os.File
    buf  []byte
    cur  int
    tok  int
    lim  int
    eof  bool
}

func fill(in *Input, need int) int {
    if in.eof { return -1 } // unexpected EOF

    // Error: lexeme too long. In real life can reallocate a larger buffer.
    if in.tok < need { return -2 }

    // Shift buffer contents (discard everything up to the current token).
    copy(in.buf[0:], in.buf[in.tok:in.lim])
    in.cur -= in.tok
    in.lim -= in.tok
    in.tok = 0

    // Fill free space at the end of buffer with new data from file.
    n, _ := in.file.Read(in.buf[in.lim:BUFSIZE])
    in.lim += n

    // If read less than expected, this is end of input => add zero padding
    // so that the lexer can access characters at the end of buffer.
    if in.lim < BUFSIZE {
        in.eof = true
        for i := 0; i < YYMAXFILL; i += 1 { in.buf[in.lim+i] = 0 }
        in.lim += YYMAXFILL
    }

    return 0
}

func lex(in *Input) int {
    count := 0
    for {
        in.tok = in.cur
    /*!re2c
        re2c:define:YYCTYPE    = byte;
        re2c:define:YYPEEK     = "in.buf[in.cur]";
        re2c:define:YYSKIP     = "in.cur += 1";
        re2c:define:YYLESSTHAN = "in.lim-in.cur < @@";
        re2c:define:YYFILL     = "if r := fill(in, @@); r != 0 { return r }";

        str = ['] ([^'\\] | [\\][^])* ['];

        [\x00] {
            // Check that it is the sentinel, not some unexpected null.
            if in.tok == in.lim - YYMAXFILL { return count } else { return -1 }
        }
        str  { count += 1; continue }
        [ ]+ { continue }
        *    { return -1 }
    */
    }
}

func main() () {
    fname := "input"
    content := "'qu\000tes' 'are' 'fine: \\'' ";

    // Prepare input file: a few times the size of the buffer, containing
    // strings with zeroes and escaped quotes.
    f, _ := os.Create(fname)
    f.WriteString(strings.Repeat(content, BUFSIZE))
    f.Seek(0, 0)
    count := 3 * BUFSIZE // number of quoted strings written to file

    // Prepare lexer state: all offsets are at the end of buffer.
    // This immediately triggers YYFILL, as the YYLESSTHAN condition is true.
    in := &Input{
        file: f,
        buf:  make([]byte, BUFSIZE+YYMAXFILL),
        cur:  BUFSIZE,
        tok:  BUFSIZE,
        lim:  BUFSIZE,
        eof:  false,
    }

    // Run the lexer.
    if lex(in) != count { panic("error"); }

    // Cleanup: remove input file.
    f.Close();
    os.Remove(fname);
}

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 re2c 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 re2c, 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.

//go:generate re2go $INPUT -o $OUTPUT -i
package main

import "errors"

const u32Limit uint64 = 1<<32
var (
    eSyntax   = errors.New("syntax error")
    eOverflow = errors.New("overflow error")
)

func parse_u32(str string) (uint32, error) {
    var cur, mar int
    result := uint64(0)

    add := func(base uint64, offset byte) {
        result = result * base + uint64(str[cur-1] - offset)
        if result >= u32Limit {
            result = u32Limit
        }
    }

    /*!re2c
        re2c:yyfill:enable = 0;
        re2c:define:YYCTYPE   = byte;
        re2c:define:YYPEEK    = "str[cur]";
        re2c:define:YYSKIP    = "cur += 1";
        re2c:define:YYSHIFT   = "cur += @@{shift}";
        re2c:define:YYBACKUP  = "mar = cur";
        re2c:define:YYRESTORE = "cur = mar";

        end = "\x00";

        '0b' / [01]        { goto bin }
        "0"                { goto oct }
        ""   / [1-9]       { goto dec }
        '0x' / [0-9a-fA-F] { goto hex }
        *                  { goto err }
    */
bin:
    /*!re2c
        end   { goto end }
        [01]  { add(2, '0'); goto bin }
        *     { goto err }
    */
oct:
    /*!re2c
        end   { goto end }
        [0-7] { add(8, '0'); goto oct }
        *     { goto err }
    */
dec:
    /*!re2c
        end   { goto end }
        [0-9] { add(10, '0'); goto dec }
        *     { goto err }
    */
hex:
    /*!re2c
        end   { goto end }
        [0-9] { add(16, '0');    goto hex }
        [a-f] { add(16, 'a'-10); goto hex }
        [A-F] { add(16, 'A'-10); goto hex }
        *     { goto err }
    */
end:
    if result < u32Limit {
        return uint32(result), nil
    } else {
        return 0, eOverflow
    }
err:
    return 0, eSyntax
}

func main() {
    test := func(num uint32, str string, err error) {
        if n, e := parse_u32(str); !(n == num && e == err) {
            panic("error")
        }
    }
    test(1234567890, "1234567890\000", nil)
    test(13, "0b1101\000", nil)
    test(0x7fe, "0x007Fe\000", nil)
    test(0644, "0644\000", nil)
    test(0, "9999999999\000", eOverflow)
    test(0, "123??\000", eSyntax)
}

Start conditions are enabled with --start-conditions option. They provide a way to encode multiple interrelated automata within the same re2c block.

Each condition corresponds to a single automaton and has a unique name specified by the user and a unique internal number defined by re2c. The numbers are used to switch between conditions: the generated code uses YYGETCONDITION and YYSETCONDITION primitives to get the current condition or set it to the given number. Use /*!conditions:re2c*/ directive or the --header option 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:

< cond-list > regexp action

A rule that is merged to every condition on the cond-list. It matches regexp and executes the associated action.

< cond-list > regexp => cond action

A rule that is merged to every condition on the cond-list. It matches regexp, sets the current condition to cond and executes the associated action.

< cond-list > regexp :=> cond

A rule that is merged to every condition on the cond-list. It matches regexp and immediately transitions to cond (there is no semantic action).

<! cond-list > action

The action is prepended to semantic actions of all rules for every condition on the cond-list. This may be used to deduplicate common code.

< > action

A rule that is merged to a special entry condition with number zero and name "0". It matches empty string and executes the action.

< > => cond action

A rule that is merged to a special entry condition with number zero and name "0". It matches empty string, sets the current condition to cond and executes the action.

< > :=> cond

A rule that is merged to a special entry condition with number zero and name "0". It matches empty string and immediately transitions to cond.

The code re2c generates for conditions depends on whether re2c 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 re2c 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) re2c 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, YYGETCONDITION/YYSETCONDITION 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:re2c*/.

//go:generate re2go -c $INPUT -o $OUTPUT -i
package main

import "errors"

var (
    eSyntax   = errors.New("syntax error")
    eOverflow = errors.New("overflow error")
)

/*!conditions:re2c*/

const u32Limit uint64 = 1<<32

func parse_u32(str string) (uint32, error) {
    var cur, mar int
    result := uint64(0)
    cond := yycinit

    add := func(base uint64, offset byte) {
        result = result * base + uint64(str[cur-1] - offset)
        if result >= u32Limit {
            result = u32Limit
        }
    }

    /*!re2c
        re2c:yyfill:enable = 0;
        re2c:define:YYCTYPE        = byte;
        re2c:define:YYPEEK         = "str[cur]";
        re2c:define:YYSKIP         = "cur += 1";
        re2c:define:YYSHIFT        = "cur += @@{shift}";
        re2c:define:YYBACKUP       = "mar = cur";
        re2c:define:YYRESTORE      = "cur = mar";
        re2c:define:YYGETCONDITION = "cond";
        re2c:define:YYSETCONDITION = "cond = @@";

        <*> * { return 0, eSyntax }

        <init> '0b' / [01]        :=> bin
        <init> "0"                :=> oct
        <init> ""   / [1-9]       :=> dec
        <init> '0x' / [0-9a-fA-F] :=> hex

        <bin, oct, dec, hex> "\x00" {
            if result < u32Limit {
                return uint32(result), nil
            } else {
                return 0, eOverflow
            }
        }

        <bin> [01]  { add(2, '0');     goto yyc_bin }
        <oct> [0-7] { add(8, '0');     goto yyc_oct }
        <dec> [0-9] { add(10, '0');    goto yyc_dec }
        <hex> [0-9] { add(16, '0');    goto yyc_hex }
        <hex> [a-f] { add(16, 'a'-10); goto yyc_hex }
        <hex> [A-F] { add(16, 'A'-10); goto yyc_hex }
    */
}

func main() {
    test := func(num uint32, str string, err error) {
        if n, e := parse_u32(str); !(n == num && e == err) {
            panic("error")
        }
    }
    test(1234567890, "1234567890\000", nil)
    test(13, "0b1101\000", nil)
    test(0x7fe, "0x007Fe\000", nil)
    test(0644, "0644\000", nil)
    test(0, "9999999999\000", eOverflow)
    test(0, "123??\000", eSyntax)
}

With --storable-state option re2c 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 re2c 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:re2c 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.

//go:generate re2go -f $INPUT -o $OUTPUT
package main

import (
    "fmt"
    "os"
)

// Use a small buffer to cover the case when a lexeme doesn't fit.
// In real world use a larger buffer.
const BUFSIZE int = 10

type State struct {
    file  *os.File
    buf   []byte
    cur   int
    mar   int
    tok   int
    lim   int
    state int
}

const (
    lexEnd = iota
    lexReady
    lexWaitingForInput
    lexPacketBroken
    lexPacketTooBig
)

func fill(st *State) int {
    shift := st.tok
    used := st.lim - st.tok
    free := BUFSIZE - used

    // Error: no space. In real life can reallocate a larger buffer.
    if free < 1 { return lexPacketTooBig }

    // Shift buffer contents (discard already processed data).
    copy(st.buf[0:], st.buf[shift:shift+used])
    st.cur -= shift
    st.mar -= shift
    st.lim -= shift
    st.tok -= shift

    // Fill free space at the end of buffer with new data.
    n, _ := st.file.Read(st.buf[st.lim:BUFSIZE])
    st.lim += n
    st.buf[st.lim] = 0 // append sentinel symbol

    return lexReady
}

func lex(st *State, recv *int) int {
    var yych byte
    /*!getstate:re2c*/
loop:
    st.tok = st.cur
    /*!re2c
        re2c:eof = 0;
        re2c:define:YYPEEK     = "st.buf[st.cur]";
        re2c:define:YYSKIP     = "st.cur += 1";
        re2c:define:YYBACKUP   = "st.mar = st.cur";
        re2c:define:YYRESTORE  = "st.cur = st.mar";
        re2c:define:YYLESSTHAN = "st.lim <= st.cur";
        re2c:define:YYFILL     = "return lexWaitingForInput";
        re2c:define:YYGETSTATE = "st.state";
        re2c:define:YYSETSTATE = "st.state = @@{state}";

        packet = [a-z]+[;];

        *      { return lexPacketBroken }
        $      { return lexEnd }
        packet { *recv = *recv + 1; goto loop }
    */
}

func test(expect int, packets []string) {
    // Create a "socket" (open the same file for reading and writing).
    fname := "pipe"
    fw, _ := os.Create(fname)
    fr, _ := os.Open(fname)

    // Initialize lexer state: `state` value is -1, all offsets are at the end
    // of buffer.
    st := &State{
        file:  fr,
        // Sentinel at `lim` offset is set to zero, which triggers YYFILL.
        buf:   make([]byte, BUFSIZE+1),
        cur:   BUFSIZE,
        mar:   BUFSIZE,
        tok:   BUFSIZE,
        lim:   BUFSIZE,
        state: -1,
    }

    // Main loop. The buffer contains incomplete data which appears packet by
    // packet. When the lexer needs more input it saves its internal state and
    // returns to the caller which should provide more input and resume lexing.
    var status int
    send := 0
    recv := 0
    for {
        status = lex(st, &recv)
        if status == lexEnd {
            break
        } else if status == lexWaitingForInput {
            if send < len(packets) {
                fw.WriteString(packets[send])
                send += 1
            }
            status = fill(st)
            if status != lexReady {
                break
            }
        } else if status == lexPacketBroken {
            break
        }
    }

    // Check results.
    if status != expect || (status == lexEnd && recv != send) {
        panic(fmt.Sprintf("got %d, want %d", status, expect))
    }

    // Cleanup: remove input file.
    fr.Close()
    fw.Close()
    os.Remove(fname)
}

func main() {
    test(lexEnd, []string{})
    test(lexEnd, []string{"zero;", "one;", "two;", "three;", "four;"})
    test(lexPacketBroken, []string{"??;"})
    test(lexPacketTooBig, []string{"looooooooooooong;"})
}

Reusable blocks are re2c blocks that can be reused any number of times and combined with other re2c blocks. They are defined with /*!rules:re2c[:<name>] ... */ (the <name> is optional). A rules block can be used in two contexts: either in a use block, or in a use directive inside of another block. The code for a rules block is generated at every point of use.

Use blocks are defined with /*!use:re2c[:<name>] ... */. The <name> is optional; if 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 re2c 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 re2c blocks together in one block (see the example below).

Named blocks and in-block use directive were added in re2c 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.

//go:generate re2go $INPUT -o $OUTPUT
package main

// 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.

const (
    Color = iota
    Fish
    Dunno
)

/*!rules:re2c:colors
    *                            { panic("eh!") }
    "red" | "salmon" | "magenta" { return Color }
*/

/*!rules:re2c:fish
    *                            { panic("oh!") }
    "haddock" | "salmon" | "eel" { return Fish }
*/

func lex(str string) int {
    var cur, mar int
    /*!re2c
        re2c:yyfill:enable = 0;
        re2c:define:YYCTYPE   = byte;
        re2c:define:YYPEEK    = "str[cur]";
        re2c:define:YYSKIP    = "cur += 1";
        re2c:define:YYBACKUP  = "mar = cur";
        re2c:define:YYRESTORE = "cur = mar";

        !use:fish;
        !use:colors;
        * { return Dunno }  // overrides inherited '*' rules
    */
}

func main() {
    assert_eq := func(x, y int) { if x != y { panic("error") } }
    assert_eq(lex("salmon"), Fish);
    assert_eq(lex("what?"), Dunno);
}

//go:generate re2go $INPUT -o $OUTPUT --input-encoding utf8
package main

// 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.
/*!rules:re2c
    re2c:yyfill:enable = 0;
    re2c:define:YYPEEK    = "str[cur]";
    re2c:define:YYSKIP    = "cur += 1";
    re2c:define:YYBACKUP  = "mar = cur";
    re2c:define:YYRESTORE = "cur = mar";

    "∀x ∃y" { return 0; }
    *       { return 1; }
*/

func lexUTF8(str []uint8) int {
    var cur, mar int
    /*!use:re2c
        re2c:encoding:utf8 = 1;
        re2c:define:YYCTYPE = uint8;
    */
}

func lexUTF32(str []uint32) int {
    var cur, mar int
    /*!use:re2c
        re2c:encoding:utf32 = 1;
        re2c:define:YYCTYPE = uint32;
    */
}

func main() {
    assert_eq := func(x, y int) { if x != y { panic("error") } }
    assert_eq(lexUTF8([]uint8{0xe2, 0x88, 0x80, 0x78, 0x20, 0xe2, 0x88, 0x83, 0x79}), 0)
    assert_eq(lexUTF32([]uint32{0x2200, 0x78, 0x20, 0x2203, 0x79}), 0)
}

re2c has two options for submatch extraction.

The first option is -T --tags. With this option one can use standalone tags of the form @stag and #mtag, where stag and mtag are arbitrary used-defined names. Tags can be used anywhere inside of a regular expression; semantically they are just position markers. Tags of the form @stag are called s-tags: they denote a single submatch value (the last input position where this tag matched). Tags of the form #mtag are called m-tags: they denote multiple submatch values (the whole history of repetitions of this tag). All tags should be defined by the user as variables with the corresponding names. With standalone tags re2c uses leftmost greedy disambiguation: submatch positions correspond to the leftmost matching path through the regular expression.

The second option is -P --posix-captures: it enables POSIX-compliant capturing groups. In this mode parentheses in regular expressions denote the beginning and the end of capturing groups; the whole regular expression is group number zero. The number of groups for the matching rule is stored in a variable yynmatch, 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]. re2c provides a directive /*!maxnmatch:re2c*/ that defines YYMAXNMATCH: a constant  equal to the maximal value of yynmatch among all rules. Note that re2c implements POSIX-compliant disambiguation: each subexpression matches as long as possible, and subexpressions that start earlier in regular expression have priority over those starting later. Capturing groups are translated into s-tags under the hood, therefore we use the word "tag" to describe them as well.

With both -P --posix-captures and T --tags options re2c uses efficient submatch extraction algorithm described in the Tagged Deterministic Finite Automata with Lookahead paper. The overhead on submatch extraction in the generated lexer grows with the number of tags --- if this number is moderate, the overhead is barely noticeable. In the lexer tags are implemented using a number of tag variables generated by re2c. There is no one-to-one correspondence between tag variables and tags: a single variable may be reused for different tags, and one tag may require multiple variables to hold all its ambiguous values. Eventually ambiguity is resolved, and only one final variable per tag survives. When a rule matches, all its tags are set to the values of the corresponding tag variables. The exact number of tag variables is unknown to the user; this number is determined by re2c. However, tag variables should be defined by the user as a part of the lexer state and updated by YYFILL, therefore re2c provides directives /*!stags:re2c*/ and /*!mtags:re2c*/ that can be used to declare, initialize and manipulate tag variables. These directives have two optional configurations: format = "@@"; (specifies the template where @@ is substituted with the name of each tag variable), and separator = ""; (specifies the piece of code used to join the generated pieces for different tag variables).

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.

//go:generate re2go $INPUT -o $OUTPUT
package main

import "reflect"

type SemVer struct { major, minor, patch int }

func s2n(s string) int { // convert pre-parsed string to a number
    n := 0
    for _, c := range s { n = n*10 + int(c-'0') }
    return n
}

func parse(str string) *SemVer {
    var cur, mar int

    // User-defined tag variables that are available in semantic action.
    var t1, t2, t3, t4, t5 int

    // Autogenerated tag variables used by the lexer to track tag values.
    /*!stags:re2c format = 'var @@ int'; separator = "\n\t"; */

    /*!re2c
        re2c:yyfill:enable = 0;
        re2c:define:YYCTYPE     = byte;
        re2c:define:YYPEEK      = "str[cur]";
        re2c:define:YYSKIP      = "cur += 1";
        re2c:define:YYBACKUP    = "mar = cur";
        re2c:define:YYRESTORE   = "cur = mar";
        re2c:define:YYSTAGP     = "@@{tag} = cur";
        re2c:define:YYSTAGN     = "@@{tag} = -1";
        re2c:define:YYSHIFTSTAG = "@@{tag} += @@{shift}";
        re2c:tags = 1;

        num = [0-9]+;

        @t1 num @t2 "." @t3 num @t4 ("." @t5 num)? [\x00] {
            major := s2n(str[t1:t2])
            minor := s2n(str[t3:t4])
            patch := 0
            if t5 != -1 { patch = s2n(str[t5:cur-1]) }
            return &SemVer{major, minor, patch}
        }
        * { return nil }
    */
}

func main() {
    assert_eq := func(x, y *SemVer) {
        if !reflect.DeepEqual(x, y) { panic("error") }
    }
    assert_eq(parse("23.34\000"), &SemVer{23, 34, 0})
    assert_eq(parse("1.2.9999\000"), &SemVer{1, 2, 9999})
    assert_eq(parse("1.a\000"), nil)
}

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.

//go:generate re2go $INPUT -o $OUTPUT --tags
package main

import (
    "os"
    "reflect"
    "strings"
)

const BUFSIZE int = 4095

type Input struct {
    file *os.File
    buf  []byte
    cur  int
    mar  int
    tok  int
    lim  int
    // Tag variables must be part of the lexer state passed to YYFILL.
    // They don't correspond to tags and should be autogenerated by re2c.
    /*!stags:re2c format = "\t@@ int\n"; */
    eof  bool
}

type SemVer struct { major, minor, patch int }

func s2n(s []byte) int { // convert pre-parsed string to a number
    n := 0
    for _, c := range s { n = n*10 + int(c-'0') }
    return n
}

func fill(in *Input) int {
    if in.eof { return -1 } // unexpected EOF

    // Error: lexeme too long. In real life can reallocate a larger buffer.
    if in.tok < 1 { return -2 }

    // Shift buffer contents (discard everything up to the current token).
    copy(in.buf[0:], in.buf[in.tok:in.lim])
    in.cur -= in.tok
    in.mar -= in.tok
    in.lim -= in.tok
    // Tag variables need to be shifted like other input positions. The check
    // for -1 is only needed if some tags are nested inside of alternative or
    // repetition, so that they can have -1 value.
    /*!stags:re2c format = "\tif in.@@ != -1 { in.@@ -= in.tok }\n"; */
    in.tok = 0

    // Fill free space at the end of buffer with new data from file.
    n, _ := in.file.Read(in.buf[in.lim:BUFSIZE])
    in.lim += n
    in.buf[in.lim] = 0

    // If read less than expected, this is the end of input.
    in.eof = in.lim < BUFSIZE

    return 0
}

func parse(in *Input) []SemVer {
    // User-defined local variables that store final tag values. They are
    // different from tag variables autogenerated with `stags:re2c`, as
    // they are set at the end of match and used only in semantic actions.
    var t1, t2, t3, t4 int
    vers := make([]SemVer, 0)
    for {
        in.tok = in.cur
    /*!re2c
        re2c:eof = 0;
        re2c:define:YYCTYPE     = byte;
        re2c:define:YYPEEK      = "in.buf[in.cur]";
        re2c:define:YYSKIP      = "in.cur += 1";
        re2c:define:YYBACKUP    = "in.mar = in.cur";
        re2c:define:YYRESTORE   = "in.cur = in.mar";
        re2c:define:YYLESSTHAN  = "in.lim <= in.cur";
        re2c:define:YYFILL      = "fill(in) == 0";
        re2c:define:YYSTAGP     = "@@{tag} = in.cur";
        re2c:define:YYSTAGN     = "@@{tag} = -1";
        re2c:define:YYSHIFTSTAG = "@@{tag} += @@{shift}";
        re2c:tags:expression    = "in.@@";

        num = [0-9]+;

        num @t1 "." @t2 num @t3 ("." @t4 num)? [\n] {
            major := s2n(in.buf[in.tok:t1])
            minor := s2n(in.buf[t2:t3])
            patch := 0
            if t4 != -1 { patch = s2n(in.buf[t4:in.cur-1]) }
            vers = append(vers, SemVer{major, minor, patch})
            continue
        }
        $ { return vers }
        * { return nil }
    */
    }
}


func main() () {
    fname := "input"
    content := "1.22.333\n";

    expect := make([]SemVer, 0, BUFSIZE)
    for i := 0; i < BUFSIZE; i += 1 { expect = append(expect, SemVer{1, 22, 333}) }

    // Prepare input file (make sure it exceeds buffer size).
    f, _ := os.Create(fname)
    f.WriteString(strings.Repeat(content, BUFSIZE))
    f.Seek(0, 0)

    // Initialize lexer state: all offsets are at the end of buffer.
    in := &Input{
        file: f,
        // Sentinel at `lim` offset is set to zero, which triggers YYFILL.
        buf:  make([]byte, BUFSIZE+1),
        cur:  BUFSIZE,
        mar:  BUFSIZE,
        tok:  BUFSIZE,
        lim:  BUFSIZE,
        eof:  false,
    }

    // Run the lexer and check results.
    if !reflect.DeepEqual(parse(in), expect) { panic("error"); }

    // Cleanup: remove input file.
    f.Close();
    os.Remove(fname);
}

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

//go:generate re2go $INPUT -o $OUTPUT
package main

import "reflect"

// Maximum number of capturing groups among all rules.
/*!maxnmatch:re2c*/

type SemVer struct { major, minor, patch int }

func s2n(s string) int { // convert pre-parsed string to a number
    n := 0
    for _, c := range s { n = n*10 + int(c-'0') }
    return n
}

func parse(str string) *SemVer {
    var cur, mar int

    // Allocate memory for capturing parentheses (twice the number of groups).
    yypmatch := make([]int, YYMAXNMATCH*2)
    var yynmatch int

    // Autogenerated tag variables used by the lexer to track tag values.
    /*!stags:re2c format = '\tvar @@ int\n'; */

    /*!re2c
        re2c:yyfill:enable = 0;
        re2c:define:YYCTYPE     = byte;
        re2c:define:YYPEEK      = "str[cur]";
        re2c:define:YYSKIP      = "cur += 1";
        re2c:define:YYBACKUP    = "mar = cur";
        re2c:define:YYRESTORE   = "cur = mar";
        re2c:define:YYSTAGP     = "@@{tag} = cur";
        re2c:define:YYSTAGN     = "@@{tag} = -1";
        re2c:define:YYSHIFTSTAG = "@@{tag} += @@{shift}";
        re2c:posix-captures = 1;

        num = [0-9]+;

        (num) "." (num) ("." num)? [\x00] {
            // `yynmatch` is the number of capturing groups
            if yynmatch != 4 { panic("expected 4 submatch groups") }

            // Even `yypmatch` values are for opening parentheses, odd values
            // are for closing parentheses, the first group is the whole match.
            major := s2n(str[yypmatch[2]:yypmatch[3]])
            minor := s2n(str[yypmatch[4]:yypmatch[5]])
            patch := 0
            if yypmatch[6] != -1 { patch = s2n(str[yypmatch[6]+1:yypmatch[7]]) }

            return &SemVer{major, minor, patch}
        }
        * { return nil }
    */
}

func main() {
    assert_eq := func(x, y *SemVer) {
        if !reflect.DeepEqual(x, y) { panic("error") }
    }
    assert_eq(parse("23.34\000"), &SemVer{23, 34, 0})
    assert_eq(parse("1.2.9999\000"), &SemVer{1, 2, 9999})
    assert_eq(parse("1.a\000"), nil)
}

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.

//go:generate re2go $INPUT -o $OUTPUT
package main

import "reflect"

const (
    mtagRoot int = -1
    tagNone int = -1
)

// An m-tag tree is a way to store histories with an O(1) copy operation.
// Histories naturally form a tree, as they have common start and fork at some
// point. The tree is stored as an array of pairs (tag value, link to parent).
// An m-tag is represented with a single link in the tree (array index).
type mtagElem struct {
    elem int
    pred int
}
type mtagTrie = []mtagElem

type Ver = []int // unbounded number of version components

func s2n(s string) int { // convert pre-parsed string to a number
    n := 0
    for _, c := range s { n = n*10 + int(c-'0') }
    return n
}

// Append a single value to an m-tag history.
func add_mtag(trie *mtagTrie, mtag int, value int) int {
    *trie = append(*trie, mtagElem{value, mtag})
    return len(*trie) - 1
}

// Recursively unwind tag histories and collect version components.
func unwind(trie mtagTrie, x int, y int, str string) Ver {
    // Reached the root of the m-tag tree, stop recursion.
    if x == mtagRoot && y == mtagRoot {
        return []int{}
    }

    // Unwind history further.
    ver := unwind(trie, trie[x].pred, trie[y].pred, str)

    // Get tag values. Tag histories must have equal length.
    if x == mtagRoot || y == mtagRoot {
        panic("tag histories have different length")
    }
    ex := trie[x].elem
    ey := trie[y].elem

    if ex != tagNone && ey != tagNone {
        // Both tags are valid string indices, extract component.
        ver = append(ver, s2n(str[ex:ey]))
    } else if !(ex == tagNone && ey == tagNone) {
        panic("both tags should be tagNone")
    }
    return ver
}

func parse(str string) []int {
    var cur, mar int
    trie := make([]mtagElem, 0)

    // User-defined tag variables that are available in semantic action.
    var t1, t2, t3, t4 int

    // Autogenerated tag variables used by the lexer to track tag values.
    /*!stags:re2c format = 'var @@ int'; separator = "\n\t"; */
    /*!mtags:re2c format = "\t@@ := mtagRoot\n"; */

    /*!re2c
        re2c:tags = 1;
        re2c:yyfill:enable = 0;
        re2c:define:YYCTYPE   = byte;
        re2c:define:YYPEEK    = "str[cur]";
        re2c:define:YYSKIP    = "cur += 1";
        re2c:define:YYBACKUP  = "mar = cur";
        re2c:define:YYRESTORE = "cur = mar";
        re2c:define:YYSTAGP   = "@@ = cur";
        re2c:define:YYSTAGN   = "@@ = tagNone";
        re2c:define:YYMTAGP   = "@@ = add_mtag(&trie, @@, cur)";
        re2c:define:YYMTAGN   = "@@ = add_mtag(&trie, @@, tagNone)";

        num = [0-9]+;

        @t1 num @t2 ("." #t3 num #t4)* [\x00] {
            ver := make([]int, 0)
            ver = append(ver, s2n(str[t1:t2]))
            ver = append(ver, unwind(trie, t3, t4, str)...)
            return ver
        }
        * { return nil }
    */
}

func main() {
    assert_eq := func(x, y []int) {
        if !reflect.DeepEqual(x, y) { panic("error") }
    }
    assert_eq(parse("1\000"), []int{1})
    assert_eq(parse("1.2.3.4.5.6.7\000"), []int{1, 2, 3, 4, 5, 6, 7})
    assert_eq(parse("1.\000"), nil)
}

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 re2c 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 re2c 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 re2c that the source file is in UTF8 (it differs from --utf8 which sets input text encoding). Option --encoding-policy specifies the way re2c handles Unicode surrogates (code points in range [0xD800-0xDFFF]).

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

//go:generate re2go $INPUT -o $OUTPUT -8 -s -i
package main

/*!include:re2c "unicode_categories.re" */

func lex(str string) int {
    var cur, mar int
    /*!re2c
        re2c:yyfill:enable = 0;
        re2c:define:YYCTYPE   = byte;
        re2c:define:YYPEEK    = "str[cur]";
        re2c:define:YYSKIP    = "cur += 1";
        re2c:define:YYBACKUP  = "mar = cur";
        re2c:define:YYRESTORE = "cur = mar";

        // 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 { return 0 }
        *          { return 1 }
    */
}

func main() {
    if lex("_Ыдентификатор\000") != 0 {
        panic("error")
    }
}

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

const (
    ResultOk = iota
    ResultFail
)

/*!re2c
    number = [1-9][0-9]*;
*/

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

float { return ResultOk }

//go:generate re2go $INPUT -o $OUTPUT -i
package main

/*!include:re2c "definitions.go" */

func lex(str string) int {
    var cur, mar int
    /*!re2c
        re2c:define:YYCTYPE   = byte;
        re2c:define:YYPEEK    = "str[cur]";
        re2c:define:YYSKIP    = "cur += 1";
        re2c:define:YYBACKUP  = "mar = cur";
        re2c:define:YYRESTORE = "cur = mar";
        re2c:yyfill:enable    = 0;

        *      { return ResultFail }
        number { return ResultOk }
        !include "extra_rules.re.inc";
    */
}

func main() {
    assert_eq := func(x, y int) { if x != y { panic("error") } }
    assert_eq(lex("123\000"), ResultOk)
    assert_eq(lex("123.4567\000"), ResultOk)
}

re2c allows one to generate header file from the input .re file using option -t, --type-header or configuration re2c:flags:type-header and directives /*!header:re2c:on*/ and /*!header:re2c:off*/. The first directive marks the beginning of header file, and the second directive marks the end of it. Everything between these directives is processed by re2c, and the generated code is written to the file specified by the -t --type-header option (or stdout if this option was not used). Autogenerated header file may be needed in cases when re2c is used to generate definitions of constants, variables and structs 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).

//go:generate re2go $INPUT -o $OUTPUT -i --header lexer/state.go
package main

import "./lexer" // the package is generated by re2c

/*!header:re2c:on*/
package lexer

type State struct {
    Data string
    Cur /*!stags:re2c format=", @@"; */ int
}
/*!header:re2c:off*/

func lex(st *lexer.State) int {
    var t int
    /*!re2c
        re2c:header = "lexer/state.go";
        re2c:yyfill:enable = 0;
        re2c:define:YYCTYPE  = byte;
        re2c:define:YYPEEK   = "st.Data[st.Cur]";
        re2c:define:YYSKIP   = "st.Cur++";
        re2c:define:YYSTAGP  = "@@ = st.Cur";
        re2c:tags            = 1;
        re2c:tags:expression = "st.@@";
        re2c:tags:prefix     = "Tag";

        [a]* @t [b]* { return t }
    */
}

func main() {
    st := &lexer.State{Data:"ab\x00",}
    if lex(st) != 1 {
        panic("error")
    }
}

// Code generated by re2c, DO NOT EDIT.

package lexer

type State struct {
    Data string
    Cur, Mar, Tag1 int
}

With the -S, --skeleton option, re2c ignores all non-re2c 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) re2c 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 re2c 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). re2c 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, re2c 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) re2c 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 (re2c writes data to file as soon as it is generated).

With the -D, --emit-dot option, re2c 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).

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).

re2c was originaly written by Peter Bumbulis in 1993. Since then it has been developed and maintained by multiple volunteers; mots notably, Brain Young, Marcus Boerger, Dan Nuffer and Ulya Trofimovich.