RE2C(1) RE2C(1)
NAME
re2c - generate fast lexical analyzers for C/C++, Go and Rust
SYNOPSIS
Note: examples are in C++ (but can be easily adapted to C).
re2c [ OPTIONS ] [ WARNINGS ] INPUT
re2go [ OPTIONS ] [ WARNINGS ] INPUT
re2rust [ OPTIONS ] [ WARNINGS ] INPUT
Input can be either a file or - for stdin.
INTRODUCTION
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:
// re2c $INPUT -o $OUTPUT -i --case-ranges
#include <assert.h>
bool lex(const char *s) {
const char *YYCURSOR = s;
/*!re2c
re2c:yyfill:enable = 0;
re2c:define:YYCTYPE = char;
number = [1-9][0-9]*;
number { return true; }
* { return false; }
*/
}
int main() {
assert(lex("1234"));
return 0;
}
In the output everything between /*!re2c and */ has been replaced with
the generated code:
/* Generated by re2c */
// re2c $INPUT -o $OUTPUT -i --case-ranges
#include <assert.h>
bool lex(const char *s) {
const char *YYCURSOR = s;
{
char yych;
yych = *YYCURSOR;
switch (yych) {
case '1' ... '9': goto yy2;
default: goto yy1;
}
yy1:
++YYCURSOR;
{ return false; }
yy2:
yych = *++YYCURSOR;
switch (yych) {
case '0' ... '9': goto yy2;
default: goto yy3;
}
yy3:
{ return true; }
}
}
int main() {
assert(lex("1234"));
return 0;
}
SYNTAX
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, direc-
tives and rules. re2c compiles regular expressions associ-
ated 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 an-
other part of the program.
/*!local:re2c[:<name>] ... */
A local block is like a global block, but the names and con-
figurations 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 sec-
tion).
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 reg-
ular expressions. Once defined, the name can be used in other
regular expressions and in rules. Recursion in named defini-
tions 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 cus-
tomize 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. Con-
figurations 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 expres-
sion 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 neces-
sarily 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 condi-
tions 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 direc-
tive 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).
PROGRAM INTERFACE
The generated code interfaces with the outer program with the help of
primitives -- symbolic names that can be defined as variables, func-
tions 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 under-
stand 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 histori-
cally the first, and for a long time the only one. It con-
sists of pointer-like primitives YYCURSOR, YYMARKER, YYCTX-
MARKER, 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 sup-
port 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 primi-
tives should be defined:
Free-form
Free-form style is enabled with configuration re2c:api:style
= free-form;. In this style interface primitives should be
defined as free-form pieces of code with interpolated vari-
ables of the form @@{var} or optionally just @@ if there is a
single variable. The set of variables is specific to each
primitive. Generic API can be defined in terms of pointers
cursor, limit, marker and ctxmarker as follows:
/*!re2c
re2c:define:YYPEEK = "*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} = NULL;";
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. For historical reasons this API
style is the default for C/C++ backend. Generic API can be
defined in terms of pointers cursor, limit, marker and ctx-
marker as follows:
#define YYPEEK() *cursor
#define YYSKIP() ++cursor
#define YYBACKUP() marker = cursor
#define YYRESTORE() cursor = marker
#define YYBACKUPCTX() ctxmarker = cursor
#define YYRESTORECTX() cursor = ctxmarker
#define YYRESTORETAG(tag) cursor = tag
#define YYLESSTHAN(len) limit - cursor < len
#define YYSTAGP(tag) tag = cursor
#define YYSTAGN(tag) tag = NULL
#define YYSHIFT(shift) cursor += shift
#define YYSHIFTSTAG(tag, shift) tag += shift
For YYFILL definition and instructions how to customize or disable
end-of-input checks see the handling the end of input and buffer re-
filling sections.
OPTIONS
Some of the options have corresponding configurations, others are
global and cannot be changed after re2c starts reading the input file.
Debug options generally require building re2c in debug configuration.
Internal options are useful for experimenting with the algorithms used
in re2c.
-? --help -h
Show help message.
--api --input <default | custom>
Specify the API used by the generated code to interface with
used-defined code: default is the API based on pointer arith-
metic (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-insensi-
tive.
--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 exten-
sion 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 sup-
ported only for C.
--computed-gotos -g
Optimize conditional jumps using non-standard "computed goto"
extension (which must be supported by the compiler). re2c gener-
ates 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-vec-
tors. 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 al-
lows one to track build dependencies in the presence of in-
clude: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 as-
sumes 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 sub-
stitute 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 with-
out curly braces are treated as double-quoted strings.
--header --type-header -t HEADER
Generate a HEADER file. The contents of the file can be speci-
fied with directives header:re2c:on and header:re2c:off. If
conditions are used the header will have a condition enum auto-
matically appended to it (unless there is an explicit condi-
tions: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 in-
clude: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.
--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.
--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. Individ-
ual 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 condi-
tional 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 gen-
erated code is stored in a version control system (to avoid huge
autogenerated diffs on small changes). This option is on by de-
fault 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 spe-
cific 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 de-
fault 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 in-
puts. 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 use-
ful 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, yyac-
cept 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 as-
sumes 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 clo-
sure.
--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 for-
mat).
--dump-dfa-tree
Debug option: output DFA under construction with states repre-
sented 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 ta-
ble option is the "table filling" algorithm. Both algorithms
should produce the same DFA up to states relabeling; table fill-
ing is simpler and much slower and serves as a reference imple-
mentation.
--eager-skip
Internal option: make the generated lexer advance the input po-
sition eagerly -- immediately after reading the input symbol.
This changes the default behavior when the input position is ad-
vanced lazily -- after transition to the next state. This option
is implied by --no-lookahead.
--no-lookahead
Internal option: use TDFA(0) instead of TDFA(1). This option
has effect only with --tags or --posix-captures options.
--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 seman-
tics: 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 ta-
ble in one traversal of tag history tree and has quadratic com-
plexity 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: use staDFA algorithm for submatch extraction.
The main difference with TDFA is that tag operations in staDFA
are placed in states, not on transitions.
--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 off-
set. For tags that are under alternative or repetition it is
also necessary to check if the base tag has a no-match value (in
that case fixed tag should also be set to no-match, disregarding
the offset). For tags in top-level concatenation the check is
not needed, because they always match.
WARNINGS
Warnings can be invividually enabled, disabled and turned into an er-
ror.
-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<warn-
ing>).
-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 condi-
tion 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 rea-
sons re2c permits empty character classes and treats them as
empty strings. Use the --empty-class option to change the de-
fault 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 indi-
cate 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 applica-
ble 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.
BLOCKS AND DIRECTIVES
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 speci-
fied.
/*!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 pro-
duce 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, con-
figurations 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 def-
initions, 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 pre-
ceding ones. One exception is the special rules *, $ and <!> for
which a block-local definition always takes priority. A use di-
rective 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-defini-
tion 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 YYMAXN-
MATCH 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 ex-
panded for each s-tag/m-tag variable generated by re2c. An op-
tional list of block names specifies which blocks should be in-
cluded when computing the set of tag variables (if the list is
empty, all blocks are included). There are two optional config-
urations: 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 incompat-
ible 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 condi-
tions (if the list is empty, all blocks are included). By de-
fault 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 #in-
clude 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 depre-
cated and exists for backward compatibility.
API PRIMITIVES
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 fea-
tures 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 gen-
erated 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 gener-
ated 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 YY-
CURSOR to YYLIMIT in order to determine if there are enough in-
put 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 YY-
MAXFILL. 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 de-
fined as an r-value of boolean type that equals true if and only
if there are less than len input characters left. The defini-
tion 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 func-
tion-like or free-form depending on the API style (see
re2c:api:style).
YYSKIP A generic API primitive with no arguments. YYSKIP should ad-
vance the current input position by one character. The defini-
tion 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 YYRE-
STORE. 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 re-
store the current input position to the value saved by YYBACKUP.
The definition should be either function-like or free-form de-
pending 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 defini-
tion 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 de-
pending 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 de-
tails). YYSTAGP should set tag to the current input position.
The definition should be either function-like or free-form de-
pending 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 de-
tails). 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 posi-
tion to the submatch history of tag (see the submatch extraction
section for details.) The definition can be either func-
tion-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 func-
tion-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 func-
tion-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 def-
inition 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 cap-
turing groups in a rule. It is generated with /*!maxn-
match:re2c*/ directive.
YYCONDTYPE
The type of the condition enum. It should be generated either
with the /*!types:re2c*/ directive or the -t --type-header op-
tion.
YYGETCONDITION
An API primitive with zero arguments. It should be defined as
an r-value of type YYCONDTYPE that is equal to the current con-
dition 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 YYSET-
CONDITION is to set the current condition identifier to cond.
The definition should be either function-like or free-form de-
pending on the API style (see re2c:api:style and re2c:define:YY-
SETCONDITION@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 YYSET-
STATE is to set the current lexer state to state. The defini-
tion should be either function-like or free-form depending on
the API style (see re2c:api:style and re2c:define:YYSET-
STATE@state).
YYDEBUG
A debug API primitive with two arguments. It can be used to de-
bug 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 charac-
ter. 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 corre-
sponding to the capturing parentheses in the matching rule. Ar-
ray length must be at least yynmatch * 2 (usually YYMAXNMATCH *
2 is a good choice). Used only with -P --posix-captures option.
CONFIGURATIONS
re2c:api, re2c:flags:input
Same as the --api option.
re2c:api:sigil
Specify the marker ("sigil") that is used for argument place-
holders 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 nota-
tion without the name in braces. This option can be overridden
by options for individual API primitives, e.g. re2c:define:YY-
FILL@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 func-
tions style API primitives are generated with an argument list
in parentheses following the name of the primitive. The argu-
ments are provided only for autogenerated parameters (such as
the number of characters passed to YYFILL), but not for the gen-
eral lexer context, so the primitives behave more like macros in
C/C++ or closures in Go and Rust. In free-form style API primi-
tives 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 argu-
ments in function-like style). This configuration may be over-
ridden for individual API primitives, see for example re2c:de-
fine: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 com-
plexity 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 @@ place-
holder 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 de-
fault 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 primi-
tives 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 sec-
tion).
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 YY-
FILL. 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 sec-
tion).
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 primi-
tives section).
re2c:define:YYRESTORETAG
Defines generic API primitive YYRESTORETAG (see the API primi-
tives 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 YYSETCON-
DITION 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 YY-
SETCONDITION. Zero value corresponds to free-form API style.
re2c:define:YYSETSTATE
Defines API primitive YYSETSTATE (see the API primitives sec-
tion).
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 YY-
SETSTATE. 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 primi-
tives section).
re2c:define:YYSHIFTSTAG
Defines generic API primitive YYSHIFTSTAG (see the API primi-
tives 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 sin-
gle tab character "\t". Indent string should contain whitespace
characters only. To disable indentation entirely, set this con-
figuration 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 num-
ber.
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:nextla-
bel). The default is yyNext.
re2c:lookahead, re2c:flags:lookahead
Same as inverted --no-lookahead option, but can be configured on
per-block basis.
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 in-
convenient, 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 ex-
plicit -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). Al-
ternatively 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 (wrap-
pers are generated). This configuration is specific to Rust.
re2c:variable:yyaccept
Specifies the name of the yyaccept variable (see the API primi-
tives 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 gen-
erated 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 con-
version).
re2c:variable:yyctable
Specifies the name of the yyctable variable (the jump table gen-
erated 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 (YY-
FILL 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 gener-
ated). This configuration can be overridden with re2c:de-
fine:YYFILL:naked or re2c:api:style.
REGULAR EXPRESSIONS
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 or for
POSIX-style submatch
• 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 hexa-
decimal escapes \xhh, \uhhhh and \Uhhhhhhhh.
HANDLING THE END OF INPUT
One of the main problems for the lexer is to know when to stop. There
are a few terminating conditions:
• the lexer may match some rule (including default rule *) and come to
a final state
• the lexer may fail to match any rule and come to a default state
• the lexer may reach the end of input
The first two conditions terminate the lexer in a "natural" way: it
comes to a state with no outgoing transitions, and the matching auto-
matically 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 de-
pends on the complexity of regular expressions, the need for buffering,
performance considerations and other factors. Here is a list of meth-
ods:
• 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 oc-
cur in valid input. This character may still occur in invalid input,
but it should not be allowed by the regular expressions, except per-
haps 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 dis-
abled with re2c:yyfill:enable = 0; and re2c:eof has the default value
-1.
• Sentinel with bounds checks. This method is generic: it allows one
to handle any input without restrictions on the regular expressions.
The idea is to reduce the number of end of input checks by performing
them only on certain characters. Similar to the "sentinel" method,
one of the characters is chosen as a "sentinel" and appended at the
end of input. However, there is no restriction on where the sentinel
may occur (in fact, any character can be chosen for a sentinel).
When the lexer reads this character, it additionally performs a
bounds check. If the current position is within bounds, the lexer
resumes matching and handles the sentinel as a regular character.
Otherwise it invokes YYFILL (unless it is disabled). If more input is
supplied, the lexer will rematch the last character and continue as
if the sentinel wasn't there. Otherwise it must be the real end of
input, and the lexer stops. This method is used when re2c:eof has
non-negative value (it should be set to the numeric value of the sen-
tinel). 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 con-
nected 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 in-
put, the lexer invokes YYFILL which should supply at least the re-
quired number of characters or not return. This method is used if
YYFILL is enabled and re2c:eof is -1 (this is the default configura-
tion).
• Custom checks. Generic API allows one to override basic operations
like reading a character, which makes it possible to include the
end-of-input checks as part of them. This approach is error-prone
and should be used with caution. To use a custom method, enable
generic API with --api custom or re2c:api = custom; and disable de-
fault bounds checks with re2c:yyfill:enable = 0; or re2c:yyfill:check
= 0;.
The following subsections contain an example of each method.
Sentinel
This example uses a sentinel character to handle the end of input. The
program counts space-separated words in a null-terminated string. The
sentinel is null: it is the last character of each input string, and it
is not allowed in the middle of a lexeme by any of the rules (in par-
ticular, it is not included in character ranges where it is easy to
overlook). If a null occurs in the middle of a string, it is a syntax
error and the lexer will match default rule *, but it won't read past
the end of input or crash (use -Wsentinel-in-midrule warning and
re2c:sentinel configuration to verify this). Configuration re2c:yy-
fill:enable = 0; suppresses the generation of bounds checks and YYFILL
invocations.
// re2c $INPUT -o $OUTPUT
#include <assert.h>
// Expect a null-terminated string.
static int lex(const char *YYCURSOR) {
int count = 0;
for (;;) {
/*!re2c
re2c:define:YYCTYPE = char;
re2c:yyfill:enable = 0;
* { return -1; }
[\x00] { return count; }
[a-z]+ { ++count; continue; }
[ ]+ { continue; }
*/
}
}
int main() {
assert(lex("") == 0);
assert(lex("one two three") == 3);
assert(lex("f0ur") == -1);
return 0;
}
Sentinel with bounds checks
This example uses sentinel with bounds checks to handle the end of in-
put (this method was added in version 1.2). The program counts
space-separated single-quoted strings. The sentinel character is null,
which is specified with re2c:eof = 0; configuration. As in the sentinel
method, null is the last character of each input string, but it is al-
lowed in the middle of a rule (for example, 'aaa\0aa'\0 is valid input,
but 'aaa\0 is a syntax error). Bounds checks are generated in each
state that matches an input character, but they are scoped to the
branch that handles null. Bounds checks are of the form YYLIMIT <= YY-
CURSOR or YYLESSTHAN(1) with generic API. If the check condition is
true, lexer has reached the end of input and should stop (YYFILL is
disabled with re2c:yyfill:enable = 0; as the input fits into one buf-
fer, see the YYFILL with sentinel section for an example that uses YY-
FILL). Reaching the end of input opens three possibilities: if the
lexer is in the initial state it will match the end-of-input rule $,
otherwise it may fallback to a previously matched rule (including de-
fault rule *) or go to a default state, causing
-Wundefined-control-flow.
// re2c $INPUT -o $OUTPUT
#include <assert.h>
// Expect a null-terminated string.
static int lex(const char *str, unsigned int len) {
const char *YYCURSOR = str, *YYLIMIT = str + len, *YYMARKER;
int count = 0;
for (;;) {
/*!re2c
re2c:define:YYCTYPE = char;
re2c:yyfill:enable = 0;
re2c:eof = 0;
str = ['] ([^'\\] | [\\][^])* ['];
* { return -1; }
$ { return count; }
str { ++count; continue; }
[ ]+ { continue; }
*/
}
}
#define TEST(s, r) assert(lex(s, sizeof(s) - 1) == r)
int main() {
TEST("", 0);
TEST("'qu\0tes' 'are' 'fine: \\'' ", 3);
TEST("'unterminated\\'", -1);
return 0;
}
Bounds checks with padding
This example uses bounds checks with padding to handle the end of input
(this method is enabled by default). The program counts space-separated
single-quoted strings. There is a padding of YYMAXFILL null characters
appended at the end of input, where YYMAXFILL value is autogenerated
with /*!max:re2c*/. It is not necessary to use null for padding --- any
characters can be used as long as they do not form a valid lexeme suf-
fix (in this example padding should not contain single quotes, as they
may be mistaken for a suffix of a single-quoted string). There is a
"stop" rule that matches the first padding character (null) and termi-
nates the lexer (note that it checks if null is at the beginning of
padding, otherwise it is a syntax error). Bounds checks are generated
only in some states that are determined by the strongly connected com-
ponents of the underlying automaton. Checks have the form (YYLIMIT -
YYCURSOR) < n or YYLESSTHAN(n) with generic API, where n is the minimum
number of characters that are needed for the lexer to proceed (it also
means that the next bounds check will occur in at most n characters).
If the check condition is true, the lexer has reached the end of input
and will invoke YYFILL(n) that should either supply at least n input
characters or not return. In this example YYFILL always fails and ter-
minates the lexer with an error (which is fine because the input fits
into one buffer). See the YYFILL with padding section for an example
that refills the input buffer with YYFILL.
// re2c $INPUT -o $OUTPUT
#include <assert.h>
#include <stdlib.h>
#include <string.h>
/*!max:re2c*/
static int lex(const char *str, unsigned int len) {
// Make a copy of the string with YYMAXFILL zeroes at the end.
char *buf = (char*) malloc(len + YYMAXFILL);
memcpy(buf, str, len);
memset(buf + len, 0, YYMAXFILL);
const char *YYCURSOR = buf, *YYLIMIT = buf + len + YYMAXFILL;
int count = 0;
loop:
/*!re2c
re2c:api:style = free-form;
re2c:define:YYCTYPE = char;
re2c:define:YYFILL = "goto fail;";
str = ['] ([^'\\] | [\\][^])* ['];
[\x00] {
// Check that it is the sentinel, not some unexpected null.
if (YYCURSOR - 1 == buf + len) goto exit; else goto fail;
}
str { ++count; goto loop; }
[ ]+ { goto loop; }
* { goto fail; }
*/
fail:
count = -1;
exit:
free(buf);
return count;
}
#define TEST(s, r) assert(lex(s, sizeof(s) - 1) == r)
int main() {
TEST("", 0);
TEST("'qu\0tes' 'are' 'fine: \\'' ", 3);
TEST("'unterminated\\'", -1);
TEST("'unexpected \0 null\\'", -1);
return 0;
}
Custom checks
This example uses a custom end-of-input handling method based on
generic API. The program counts space-separated single-quoted strings.
It is the same as the sentinel with bounds checks example, except that
the input is not null-terminated (this method can be used if padding is
not an option, not even a single character). To cover up for the ab-
sence of sentinel character at the end of input, YYPEEK is redefined to
perform a bounds check before it reads the next input character. This
is inefficient because checks are done very often. If the check condi-
tion fails, YYPEEK returns the real character, otherwise it returns a
fake sentinel character.
// re2c $INPUT -o $OUTPUT
#include <assert.h>
#include <stdlib.h>
#include <string.h>
static int lex(const char *str, unsigned int len) {
// For the sake of example create a string without terminating null.
char *buf = (char*) malloc(len);
memcpy(buf, str, len);
const char *cur = buf, *lim = buf + len, *mar;
int count = 0;
for (;;) {
/*!re2c
re2c:yyfill:enable = 0;
re2c:eof = 0;
re2c:api = custom;
re2c:api:style = free-form;
re2c:define:YYCTYPE = char;
re2c:define:YYLESSTHAN = "cur >= lim";
re2c:define:YYPEEK = "cur < lim ? *cur : 0"; // fake null
re2c:define:YYSKIP = "++cur;";
re2c:define:YYBACKUP = "mar = cur;";
re2c:define:YYRESTORE = "cur = mar;";
str = ['] ([^'\\] | [\\][^])* ['];
* { count = -1; break; }
$ { break;; }
str { ++count; continue; }
[ ]+ { continue; }
*/
}
free(buf);
return count;
}
#define TEST(s, r) assert(lex(s, sizeof(s) - 1) == r)
int main() {
TEST("", 0);
TEST("'qu\0tes' 'are' 'fine: \\'' ", 3);
TEST("'unterminated\\'", -1);
return 0;
}
BUFFER REFILLING
The need for buffering arises when the input cannot be mapped in memory
all at once: either it is too large, or it comes in a streaming fashion
(like reading from a socket). The usual technique in such cases is to
allocate a fixed-sized memory buffer and process input in chunks that
fit into the buffer. When the current chunk is processed, it is moved
out and new data is moved in. In practice it is somewhat more complex,
because lexer state consists not of a single input position, but a set
of interrelated positions:
• cursor: the next input character to be read (YYCURSOR in C pointer
API or YYSKIP/YYPEEK in generic API)
• limit: the position after the last available input character (YYLIMIT
in C pointer API, implicitly handled by YYLESSTHAN in generic API)
• marker: the position of the most recent match, if any (YYMARKER in
default API or YYBACKUP/YYRESTORE in generic API)
• token: the start of the current lexeme (implicit in 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 buf-
fer 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 seman-
tics (described in the section about storable state).
YYFILL with sentinel
If EOF rule is used, YYFILL is a function-like primitive that accepts
no arguments and returns a value which is checked against zero. YYFILL
invocation is triggered by condition YYLIMIT <= YYCURSOR in C pointer
API and YYLESSTHAN() in generic API. A non-zero return value means that
YYFILL has failed. A successful YYFILL call must supply at least one
character and adjust input positions accordingly. Limit must always be
set to one after the last input position in buffer, and the character
at the limit position must be the sentinel symbol specified by re2c:eof
configuration. The pictures below show the relative locations of input
positions in buffer before and after YYFILL call (sentinel symbol is
marked with #, and the second picture shows the case when there is not
enough input to fill the whole buffer).
<-- shift -->
>-A------------B---------C-------------D#-----------E->
buffer token marker limit,
cursor
>-A------------B---------C-------------D------------E#->
buffer, marker cursor limit
token
<-- shift -->
>-A------------B---------C-------------D#--E (EOF)
buffer token marker limit,
cursor
>-A------------B---------C-------------D---E#........
buffer, marker cursor limit
token
Here is an example of a program that reads input file input.txt in
chunks of 4096 bytes and uses EOF rule.
// re2c $INPUT -o $OUTPUT
#include <assert.h>
#include <stdio.h>
#include <string.h>
#define BUFSIZE 4095
struct Input {
FILE *file;
char buf[BUFSIZE + 1], *lim, *cur, *mar, *tok; // +1 for sentinel
bool eof;
};
static int fill(Input &in) {
if (in.eof) return 1;
const size_t shift = in.tok - in.buf;
const size_t used = in.lim - in.tok;
// Error: lexeme too long. In real life could reallocate a larger buffer.
if (shift < 1) return 2;
// Shift buffer contents (discard everything up to the current token).
memmove(in.buf, in.tok, used);
in.lim -= shift;
in.cur -= shift;
in.mar -= shift;
in.tok -= shift;
// Fill free space at the end of buffer with new data from file.
in.lim += fread(in.lim, 1, BUFSIZE - used, in.file);
in.lim[0] = 0;
in.eof = in.lim < in.buf + BUFSIZE;
return 0;
}
static int lex(Input &in) {
int count = 0;
for (;;) {
in.tok = in.cur;
/*!re2c
re2c:api:style = free-form;
re2c:define:YYCTYPE = char;
re2c:define:YYCURSOR = in.cur;
re2c:define:YYMARKER = in.mar;
re2c:define:YYLIMIT = in.lim;
re2c:define:YYFILL = "fill(in) == 0";
re2c:eof = 0;
str = ['] ([^'\\] | [\\][^])* ['];
* { return -1; }
$ { return count; }
str { ++count; continue; }
[ ]+ { continue; }
*/
}
}
int main() {
const char *fname = "input";
const char content[] = "'qu\0tes' 'are' 'fine: \\'' ";
// Prepare input file: a few times the size of the buffer, containing
// strings with zeroes and escaped quotes.
FILE *f = fopen(fname, "w");
for (int i = 0; i < BUFSIZE; ++i) {
fwrite(content, 1, sizeof(content) - 1, f);
}
fclose(f);
int count = 3 * BUFSIZE; // number of quoted strings written to file
// Initialize lexer state: all pointers are at the end of buffer.
Input in;
in.file = fopen(fname, "r");
in.cur = in.mar = in.tok = in.lim = in.buf + BUFSIZE;
in.eof = 0;
// Sentinel (at YYLIMIT pointer) is set to zero, which triggers YYFILL.
in.lim[0] = 0;
// Run the lexer.
assert(lex(in) == count);
// Cleanup: remove input file.
fclose(in.file);
remove(fname);
return 0;
}
YYFILL with padding
In the default case (when EOF rule is not used) YYFILL is a func-
tion-like primitive that accepts a single argument and does not return
any value. YYFILL invocation is triggered by condition (YYLIMIT - YY-
CURSOR) < n in C pointer API and YYLESSTHAN(n) in generic API. The ar-
gument passed to YYFILL is the minimal number of characters that must
be supplied. If it fails to do so, YYFILL must not return to the lexer
(for that reason it is best implemented as a macro that returns from
the calling function on failure). In case of a successful YYFILL invo-
cation the limit position must be set either to one after the last in-
put position in buffer, or to the end of YYMAXFILL padding (in case YY-
FILL has successfully read at least n characters, but not enough to
fill the entire buffer). The pictures below show the relative locations
of input positions in buffer before and after YYFILL invocation (YYMAX-
FILL padding on the second picture is marked with # symbols).
<-- shift --> <-- need -->
>-A------------B---------C-----D-------E---F--------G->
buffer token marker cursor limit
>-A------------B---------C-----D-------E---F--------G->
buffer, marker cursor limit
token
<-- shift --> <-- need -->
>-A------------B---------C-----D-------E-F (EOF)
buffer token marker cursor limit
>-A------------B---------C-----D-------E-F###############
buffer, marker cursor limit
token <- YYMAXFILL ->
Here is an example of a program that reads input file input.txt in
chunks of 4096 bytes and uses bounds-checking with padding.
// re2c $INPUT -o $OUTPUT
#include <assert.h>
#include <stdio.h>
#include <string.h>
/*!max:re2c*/
#define BUFSIZE (4096 - YYMAXFILL)
struct Input {
FILE *file;
char buf[BUFSIZE + YYMAXFILL], *lim, *cur, *tok;
bool eof;
};
static int fill(Input &in, size_t need) {
if (in.eof) return 1;
const size_t shift = in.tok - in.buf;
const size_t used = in.lim - in.tok;
// Error: lexeme too long. In real life could reallocate a larger buffer.
if (shift < need) return 2;
// Shift buffer contents (discard everything up to the current token).
memmove(in.buf, in.tok, used);
in.lim -= shift;
in.cur -= shift;
in.tok -= shift;
// Fill free space at the end of buffer with new data from file.
in.lim += fread(in.lim, 1, BUFSIZE - used, in.file);
// 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 < in.buf + BUFSIZE) {
in.eof = true;
memset(in.lim, 0, YYMAXFILL);
in.lim += YYMAXFILL;
}
return 0;
}
static int lex(Input &in) {
int count = 0;
for (;;) {
in.tok = in.cur;
/*!re2c
re2c:api:style = free-form;
re2c:define:YYCTYPE = char;
re2c:define:YYCURSOR = in.cur;
re2c:define:YYLIMIT = in.lim;
re2c:define:YYFILL = "if (fill(in, @@) != 0) return -1;";
str = ['] ([^'\\] | [\\][^])* ['];
[\x00] {
// Check that it is the sentinel, not some unexpected null.
return in.tok == in.lim - YYMAXFILL ? count : -1;
}
str { ++count; continue; }
[ ]+ { continue; }
* { return -1; }
*/
}
}
int main() {
const char *fname = "input";
const char content[] = "'qu\0tes' 'are' 'fine: \\'' ";
// Prepare input file: a few times the size of the buffer, containing
// strings with zeroes and escaped quotes.
FILE *f = fopen(fname, "w");
for (int i = 0; i < BUFSIZE; ++i) {
fwrite(content, 1, sizeof(content) - 1, f);
}
fclose(f);
int count = 3 * BUFSIZE; // number of quoted strings written to file
// Initialize lexer state: all pointers are at the end of buffer.
// This immediately triggers YYFILL, as the check `in.cur < in.lim` fails.
Input in;
in.file = fopen(fname, "r");
in.cur = in.tok = in.lim = in.buf + BUFSIZE;
in.eof = 0;
// Run the lexer.
assert(lex(in) == count);
// Cleanup: remove input file.
fclose(in.file);
remove(fname);
return 0;
}
MULTIPLE BLOCKS
Sometimes it is necessary to have multiple interrelated lexers (for ex-
ample, 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 lan-
guages 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.
// re2c $INPUT -o $OUTPUT -i
#include <stdint.h>
#include <limits.h>
#include <assert.h>
static const uint64_t ERROR = UINT64_MAX;
template<int BASE> static void add(uint64_t &u, char d) {
u = u * BASE + d;
if (u > UINT32_MAX) u = ERROR;
}
static uint64_t parse_u32(const char *s) {
const char *YYCURSOR = s, *YYMARKER;
uint64_t u = 0;
/*!re2c
re2c:yyfill:enable = 0;
re2c:define:YYCTYPE = char;
end = "\x00";
'0b' / [01] { goto bin; }
"0" { goto oct; }
"" / [1-9] { goto dec; }
'0x' / [0-9a-fA-F] { goto hex; }
* { return ERROR; }
*/
bin:
/*!re2c
end { return u; }
[01] { add<2>(u, YYCURSOR[-1] - '0'); goto bin; }
* { return ERROR; }
*/
oct:
/*!re2c
end { return u; }
[0-7] { add<8>(u, YYCURSOR[-1] - '0'); goto oct; }
* { return ERROR; }
*/
dec:
/*!re2c
end { return u; }
[0-9] { add<10>(u, YYCURSOR[-1] - '0'); goto dec; }
* { return ERROR; }
*/
hex:
/*!re2c
end { return u; }
[0-9] { add<16>(u, YYCURSOR[-1] - '0'); goto hex; }
[a-f] { add<16>(u, YYCURSOR[-1] - 'a' + 10); goto hex; }
[A-F] { add<16>(u, YYCURSOR[-1] - 'A' + 10); goto hex; }
* { return ERROR; }
*/
}
int main() {
assert(parse_u32("") == ERROR);
assert(parse_u32("1234567890") == 1234567890);
assert(parse_u32("0b1101") == 13);
assert(parse_u32("0x7Fe") == 2046);
assert(parse_u32("0644") == 420);
assert(parse_u32("9999999999") == ERROR);
return 0;
}
START CONDITIONS
Start conditions are enabled with --start-conditions option. They pro-
vide 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 condi-
tion or set it to the given number. Use /*!conditions:re2c*/ directive
or the --header option to generate numeric condition identifiers. Con-
figuration re2c:cond:enumprefix specifies the generated identifier pre-
fix.
In condition mode every rule must be prefixed with a list of comma-sep-
arated 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 ex-
ecutes 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 dedu-
plicate common code.
< > action
A rule that is merged to a special entry condition with num-
ber 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 num-
ber zero and name "0". It matches empty string, sets the cur-
rent condition to cond and executes the action.
< > :=> cond
A rule that is merged to a special entry condition with num-
ber zero and name "0". It matches empty string and immedi-
ately 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 im-
plemented 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 ac-
tions 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 yys-
tate 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, YYGET-
CONDITION/YYSETCONDITION are redundant if both storable states and con-
ditions 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*/.
// re2c $INPUT -o $OUTPUT -ci
#include <stdint.h>
#include <limits.h>
#include <assert.h>
static const uint64_t ERROR = UINT64_MAX;
/*!conditions:re2c*/
template<int BASE> static void add(uint64_t &u, char d) {
u = u * BASE + d;
if (u > UINT32_MAX) u = ERROR;
}
static uint64_t parse_u32(const char *s) {
const char *YYCURSOR = s, *YYMARKER;
int c = yycinit;
uint64_t u = 0;
/*!re2c
re2c:api:style = free-form;
re2c:define:YYCTYPE = char;
re2c:define:YYGETCONDITION = "c";
re2c:define:YYSETCONDITION = "c = @@;";
re2c:yyfill:enable = 0;
<*> * { return ERROR; }
<init> '0b' / [01] :=> bin
<init> "0" :=> oct
<init> "" / [1-9] :=> dec
<init> '0x' / [0-9a-fA-F] :=> hex
<bin, oct, dec, hex> "\x00" { return u; }
<bin> [01] { add<2>(u, YYCURSOR[-1] - '0'); goto yyc_bin; }
<oct> [0-7] { add<8>(u, YYCURSOR[-1] - '0'); goto yyc_oct; }
<dec> [0-9] { add<10>(u, YYCURSOR[-1] - '0'); goto yyc_dec; }
<hex> [0-9] { add<16>(u, YYCURSOR[-1] - '0'); goto yyc_hex; }
<hex> [a-f] { add<16>(u, YYCURSOR[-1] - 'a' + 10); goto yyc_hex; }
<hex> [A-F] { add<16>(u, YYCURSOR[-1] - 'A' + 10); goto yyc_hex; }
*/
}
int main() {
assert(parse_u32("") == ERROR);
assert(parse_u32("1234567890") == 1234567890);
assert(parse_u32("0b1101") == 13);
assert(parse_u32("0x7Fe") == 2046);
assert(parse_u32("0644") == 420);
assert(parse_u32("9999999999") == ERROR);
return 0;
}
STORABLE STATE
With --storable-state option re2c generates a lexer that can store its
current state, return to the caller, and later resume operations ex-
actly 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 per-
sistent 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 ulti-
mate end of input, and appending padding too early may cut off a par-
tially 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 con-
dition).
Here is an example of a "push" model lexer that simulates reading pack-
ets 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.
// re2c $INPUT -o $OUTPUT -f
#include <assert.h>
#include <stdio.h>
#include <string.h>
#define DEBUG 0
#define LOG(...) if (DEBUG) fprintf(stderr, __VA_ARGS__);
// Use a small buffer to cover the case when a lexeme doesn't fit.
// In real world use a larger buffer.
#define BUFSIZE 10
struct State {
FILE *file;
char buf[BUFSIZE + 1], *lim, *cur, *mar, *tok;
int state;
};
typedef enum {END, READY, WAITING, BAD_PACKET, BIG_PACKET} Status;
static Status fill(State &st) {
const size_t shift = st.tok - st.buf;
const size_t used = st.lim - st.tok;
const size_t free = BUFSIZE - used;
// Error: no space. In real life can reallocate a larger buffer.
if (free < 1) return BIG_PACKET;
// Shift buffer contents (discard already processed data).
memmove(st.buf, st.tok, used);
st.lim -= shift;
st.cur -= shift;
st.mar -= shift;
st.tok -= shift;
// Fill free space at the end of buffer with new data.
const size_t read = fread(st.lim, 1, free, st.file);
st.lim += read;
st.lim[0] = 0; // append sentinel symbol
return READY;
}
static Status lex(State &st, unsigned int *recv) {
char yych;
/*!getstate:re2c*/
for (;;) {
st.tok = st.cur;
/*!re2c
re2c:api:style = free-form;
re2c:define:YYCTYPE = "char";
re2c:define:YYCURSOR = "st.cur";
re2c:define:YYMARKER = "st.mar";
re2c:define:YYLIMIT = "st.lim";
re2c:define:YYGETSTATE = "st.state";
re2c:define:YYSETSTATE = "st.state = @@;";
re2c:define:YYFILL = "return WAITING;";
re2c:eof = 0;
packet = [a-z]+[;];
* { return BAD_PACKET; }
$ { return END; }
packet { *recv = *recv + 1; continue; }
*/
}
}
void test(const char **packets, Status expect) {
// Create a "socket" (open the same file for reading and writing).
const char *fname = "pipe";
FILE *fw = fopen(fname, "w");
FILE *fr = fopen(fname, "r");
setvbuf(fw, NULL, _IONBF, 0);
setvbuf(fr, NULL, _IONBF, 0);
// Initialize lexer state: `state` value is -1, all pointers are at the end
// of buffer.
State st;
st.file = fr;
st.cur = st.mar = st.tok = st.lim = st.buf + BUFSIZE;
// Sentinel (at YYLIMIT pointer) is set to zero, which triggers YYFILL.
st.lim[0] = 0;
st.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.
Status status;
unsigned int send = 0, recv = 0;
for (;;) {
status = lex(st, &recv);
if (status == END) {
LOG("done: got %u packets\n", recv);
break;
} else if (status == WAITING) {
LOG("waiting...\n");
if (*packets) {
LOG("sent packet %u\n", send);
fprintf(fw, "%s", *packets++);
++send;
}
status = fill(st);
LOG("queue: '%s'\n", st.buf);
if (status == BIG_PACKET) {
LOG("error: packet too big\n");
break;
}
assert(status == READY);
} else {
assert(status == BAD_PACKET);
LOG("error: ill-formed packet\n");
break;
}
}
// Check results.
assert(status == expect);
if (status == END) assert(recv == send);
// Cleanup: remove input file.
fclose(fw);
fclose(fr);
remove(fname);
}
int main() {
const char *packets1[] = {0};
const char *packets2[] = {"zero;", "one;", "two;", "three;", "four;", 0};
const char *packets3[] = {"zer0;", 0};
const char *packets4[] = {"looooooooooong;", 0};
test(packets1, END);
test(packets2, END);
test(packets3, BAD_PACKET);
test(packets4, BIG_PACKET);
return 0;
}
REUSABLE BLOCKS
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 direc-
tive 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 re-
cent one (whether named or unnamed). A use block can add named defini-
tions, 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 configura-
tions (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.
Example of a !use directive
// re2c $INPUT -o $OUTPUT
#include <assert.h>
// 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.
enum What { COLOR, FISH, DUNNO };
/*!rules:re2c:colors
* { assert(false); }
"red" | "salmon" | "magenta" { return COLOR; }
*/
/*!rules:re2c:fish
* { assert(false); }
"haddock" | "salmon" | "eel" { return FISH; }
*/
static What lex(const char *s) {
const char *YYCURSOR = s, *YYMARKER;
/*!re2c
re2c:yyfill:enable = 0;
re2c:define:YYCTYPE = char;
!use:fish;
!use:colors;
* { return DUNNO; } // overrides inherited '*' rules
*/
}
int main() {
assert(lex("salmon") == FISH);
assert(lex("what?") == DUNNO);
return 0;
}
Example of a /*!use:re2c ... */ block
// re2c $INPUT -o $OUTPUT --input-encoding utf8
#include <assert.h>
#include <stdint.h>
// 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;
"∀x ∃y" { return 0; }
* { return 1; }
*/
static int lex_utf8(const uint8_t *s) {
const uint8_t *YYCURSOR = s, *YYMARKER;
/*!use:re2c
re2c:define:YYCTYPE = uint8_t;
re2c:encoding:utf8 = 1;
*/
}
static int lex_utf32(const uint32_t *s) {
const uint32_t *YYCURSOR = s, *YYMARKER;
/*!use:re2c
re2c:define:YYCTYPE = uint32_t;
re2c:encoding:utf32 = 1;
*/
}
int main() {
static const uint8_t s8[] = // UTF-8
{ 0xe2, 0x88, 0x80, 0x78, 0x20, 0xe2, 0x88, 0x83, 0x79 };
static const uint32_t s32[] = // UTF32
{ 0x00002200, 0x00000078, 0x00000020, 0x00002203, 0x00000079 };
assert(lex_utf8(s8) == 0);
assert(lex_utf32(s32) == 0);
return 0;
}
SUBMATCH EXTRACTION
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 ex-
pression; 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 de-
note 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 Fi-
nite 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 mul-
tiple 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 de-
fined 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 YYM-
TAGP(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 se-
quence of tag values. The most naive and inefficient representation of
an m-tag is a list (array, vector) of tag values; a more efficient rep-
resentation is to store all m-tags in a prefix-tree represented as ar-
ray 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.
// re2c $INPUT -o $OUTPUT
#include <assert.h>
#include <stddef.h>
struct SemVer { int major, minor, patch; };
static int s2n(const char *s, const char *e) { // pre-parsed string to number
int n = 0;
for (; s < e; ++s) n = n * 10 + (*s - '0');
return n;
}
static bool lex(const char *str, SemVer &ver) {
const char *YYCURSOR = str, *YYMARKER;
// User-defined tag variables that are available in semantic action.
const char *t1, *t2, *t3, *t4, *t5;
// Autogenerated tag variables used by the lexer to track tag values.
/*!stags:re2c format = 'const char *@@;\n'; */
/*!re2c
re2c:yyfill:enable = 0;
re2c:define:YYCTYPE = char;
re2c:tags = 1;
num = [0-9]+;
@t1 num @t2 "." @t3 num @t4 ("." @t5 num)? [\x00] {
ver.major = s2n(t1, t2);
ver.minor = s2n(t3, t4);
ver.patch = t5 != NULL ? s2n(t5, YYCURSOR - 1) : 0;
return true;
}
* { return false; }
*/
}
int main() {
SemVer v;
assert(lex("23.34", v) && v.major == 23 && v.minor == 34 && v.patch == 0);
assert(lex("1.2.999", v) && v.major == 1 && v.minor == 2 && v.patch == 999);
assert(!lex("1.a", v));
return 0;
}
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.
// re2c $INPUT -o $OUTPUT --tags
#include <assert.h>
#include <stddef.h>
#include <stdio.h>
#include <string.h>
#include <vector>
#define BUFSIZE 4095
struct Input {
FILE *file;
char buf[BUFSIZE + 1], *lim, *cur, *mar, *tok;
// 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 = 'const char *@@;'; */
bool eof;
};
struct SemVer { int major, minor, patch; };
static bool operator==(const SemVer &x, const SemVer &y) {
return x.major == y.major && x.minor == y.minor && x.patch == y.patch;
}
static int s2n(const char *s, const char *e) { // pre-parsed string to number
int n = 0;
for (; s < e; ++s) n = n * 10 + (*s - '0');
return n;
}
static int fill(Input &in) {
if (in.eof) return 1;
const size_t shift = in.tok - in.buf;
const size_t used = in.lim - in.tok;
// Error: lexeme too long. In real life could reallocate a larger buffer.
if (shift < 1) return 2;
// Shift buffer contents (discard everything up to the current token).
memmove(in.buf, in.tok, used);
in.lim -= shift;
in.cur -= shift;
in.mar -= shift;
in.tok -= shift;
// Tag variables need to be shifted like other input positions. The check
// for non-NULL is only needed if some tags are nested inside of alternative
// or repetition, so that they can have NULL value.
/*!stags:re2c format = "if (in.@@) in.@@ -= shift;\n"; */
// Fill free space at the end of buffer with new data from file.
in.lim += fread(in.lim, 1, BUFSIZE - used, in.file);
in.lim[0] = 0;
in.eof = in.lim < in.buf + BUFSIZE;
return 0;
}
static bool lex(Input &in, std::vector<SemVer> &vers) {
// 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.
const char *t1, *t2, *t3, *t4;
for (;;) {
in.tok = in.cur;
/*!re2c
re2c:eof = 0;
re2c:api:style = free-form;
re2c:define:YYCTYPE = char;
re2c:define:YYCURSOR = in.cur;
re2c:define:YYMARKER = in.mar;
re2c:define:YYLIMIT = in.lim;
re2c:define:YYFILL = "fill(in) == 0";
re2c:tags:expression = "in.@@";
num = [0-9]+;
num @t1 "." @t2 num @t3 ("." @t4 num)? [\n] {
int major = s2n(in.tok, t1);
int minor = s2n(t2, t3);
int patch = t4 != NULL ? s2n(t4, in.cur - 1) : 0;
SemVer ver = {major, minor, patch};
vers.push_back(ver);
continue;
}
$ { return true; }
* { return false; }
*/}
}
int main() {
const char *fname = "input";
const SemVer semver = {1, 22, 333};
std::vector<SemVer> expect(BUFSIZE, semver), actual;
// Prepare input file (make sure it exceeds buffer size).
FILE *f = fopen(fname, "w");
for (int i = 0; i < BUFSIZE; ++i) fprintf(f, "1.22.333\n");
fclose(f);
// Reopen input file for reading.
f = fopen(fname, "r");
// Initialize lexer state: all pointers are at the end of buffer.
Input in;
in.file = f;
in.cur = in.mar = in.tok = in.lim = in.buf + BUFSIZE;
/*!stags:re2c format = "in.@@ = in.lim;\n"; */
in.eof = false;
// Sentinel (at YYLIMIT pointer) is set to zero, which triggers YYFILL.
*in.lim = 0;
// Run the lexer and check results.
assert(lex(in, actual) && expect == actual);
// Cleanup: remove input file.
fclose(f);
remove(fname);
return 0;
}
Here is an example of using POSIX capturing groups to parse semantic
versions.
// re2c $INPUT -o $OUTPUT
#include <assert.h>
#include <stddef.h>
// Maximum number of capturing groups among all rules.
/*!maxnmatch:re2c*/
struct SemVer { int major, minor, patch; };
static int s2n(const char *s, const char *e) { // pre-parsed string to number
int n = 0;
for (; s < e; ++s) n = n * 10 + (*s - '0');
return n;
}
static bool lex(const char *str, SemVer &ver) {
const char *YYCURSOR = str, *YYMARKER;
// Allocate memory for capturing parentheses (twice the number of groups).
const char *yypmatch[YYMAXNMATCH * 2];
size_t yynmatch;
// Autogenerated tag variables used by the lexer to track tag values.
/*!stags:re2c format = 'const char *@@;\n'; */
/*!re2c
re2c:yyfill:enable = 0;
re2c:define:YYCTYPE = char;
re2c:posix-captures = 1;
num = [0-9]+;
(num) "." (num) ("." num)? [\x00] {
// `yynmatch` is the number of capturing groups
assert(yynmatch == 4);
// Even `yypmatch` values are for opening parentheses, odd values
// are for closing parentheses, the first group is the whole match.
ver.major = s2n(yypmatch[2], yypmatch[3]);
ver.minor = s2n(yypmatch[4], yypmatch[5]);
ver.patch = yypmatch[6] ? s2n(yypmatch[6] + 1, yypmatch[7]) : 0;
return true;
}
* { return false; }
*/
}
int main() {
SemVer v;
assert(lex("23.34", v) && v.major == 23 && v.minor == 34 && v.patch == 0);
assert(lex("1.2.999", v) && v.major == 1 && v.minor == 2 && v.patch == 999);
assert(!lex("1.a", v));
return 0;
}
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.
// re2c $INPUT -o $OUTPUT
#include <assert.h>
#include <stddef.h>
#include <vector>
static const int MTAG_ROOT = -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).
struct Mtag {
const char *elem; // tag value
int pred; // index of the predecessor node or root
};
typedef std::vector<Mtag> MtagTrie;
typedef std::vector<int> Ver; // unbounded number of version components
static int s2n(const char *s, const char *e) { // pre-parsed string to number
int n = 0;
for (; s < e; ++s) n = n * 10 + (*s - '0');
return n;
}
// Append a single value to an m-tag history.
static void add_mtag(MtagTrie &trie, int &mtag, const char *value) {
Mtag m = {value, mtag};
mtag = (int)trie.size();
trie.push_back(m);
}
// Recursively unwind tag histories and collect version components.
static void unfold(const MtagTrie &trie, int x, int y, Ver &ver) {
// Reached the root of the m-tag tree, stop recursion.
if (x == MTAG_ROOT && y == MTAG_ROOT) return;
// Unwind history further.
unfold(trie, trie[x].pred, trie[y].pred, ver);
// Get tag values. Tag histories must have equal length.
assert(x != MTAG_ROOT && y != MTAG_ROOT);
const char *ex = trie[x].elem, *ey = trie[y].elem;
if (ex != NULL && ey != NULL) {
// Both tags are valid pointers, extract component.
ver.push_back(s2n(ex, ey));
} else {
// Both tags are NULL (this corresponds to zero repetitions).
assert(ex == NULL && ey == NULL);
}
}
static bool parse(const char *str, Ver &ver) {
const char *YYCURSOR = str, *YYMARKER;
MtagTrie mt;
// User-defined tag variables that are available in semantic action.
const char *t1, *t2;
int t3, t4;
// Autogenerated tag variables used by the lexer to track tag values.
/*!stags:re2c format = 'const char *@@ = NULL;'; */
/*!mtags:re2c format = 'int @@ = MTAG_ROOT;'; */
/*!re2c
re2c:api:style = free-form;
re2c:define:YYCTYPE = char;
re2c:define:YYSTAGP = "@@ = YYCURSOR;";
re2c:define:YYSTAGN = "@@ = NULL;";
re2c:define:YYMTAGP = "add_mtag(mt, @@, YYCURSOR);";
re2c:define:YYMTAGN = "add_mtag(mt, @@, NULL);";
re2c:yyfill:enable = 0;
re2c:tags = 1;
num = [0-9]+;
@t1 num @t2 ("." #t3 num #t4)* [\x00] {
ver.clear();
ver.push_back(s2n(t1, t2));
unfold(mt, t3, t4, ver);
return true;
}
* { return false; }
*/
}
int main() {
Ver v;
assert(parse("1", v) && v == Ver({1}));
assert(parse("1.2.3.4.5.6.7", v) && v == Ver({1, 2, 3, 4, 5, 6, 7}));
assert(!parse("1.2.", v));
return 0;
}
ENCODING SUPPORT
It is necessary to understand the difference between code points and
code units. A code point is a numeric identifier of a symbol. A code
unit is the smallest unit of storage in the encoded text. A single code
point may be represented with one or more code units. In a fixed-length
encoding all code points are represented with the same number of code
units. In a variable-length encoding code points may be represented
with a different number of code units. Note that the "any" rule [^]
matches any code point, but not necessarily any code unit (the only way
to match any code unit regardless of the encoding is the default rule
*). The generated lexer works with a stream of code units: yych stores
a code unit, and YYCTYPE is the code unit type. Regular expressions, on
the other hand, are specified in terms of code points. When re2c com-
piles regular expressions to automata it translates code points to code
units. This is generally not a simple mapping: in variable-length en-
codings 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 vari-
able-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 vari-
able-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 --encod-
ing-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.
// re2c $INPUT -o $OUTPUT -8 --case-ranges -i
#include <assert.h>
#include <stdint.h>
/*!include:re2c "unicode_categories.re" */
static int lex(const char *s) {
const char *YYCURSOR = s, *YYMARKER;
/*!re2c
re2c:define:YYCTYPE = 'unsigned char';
re2c:yyfill:enable = 0;
// Simplified "Unicode Identifier and Pattern Syntax"
// (see https://unicode.org/reports/tr31)
id_start = L | Nl | [$_];
id_continue = id_start | Mn | Mc | Nd | Pc | [\u200D\u05F3];
identifier = id_start id_continue*;
identifier { return 0; }
* { return 1; }
*/
}
int main() {
assert(lex("_Ыдентификатор") == 0);
return 0;
}
INCLUDE FILES
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 in-
cluded. 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. In-
clude 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 op-
tion 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.
Include file 1 (definitions.h)
typedef enum { OK, FAIL } Result;
/*!re2c
number = [1-9][0-9]*;
*/
Include file 2 (extra_rules.re.inc)
// floating-point numbers
frac = [0-9]* "." [0-9]+ | [0-9]+ ".";
exp = 'e' [+-]? [0-9]+;
float = frac exp? | [0-9]+ exp;
float { return OK; }
Input file
// re2c $INPUT -o $OUTPUT -i
#include <assert.h>
/*!include:re2c "definitions.h" */
Result lex(const char *s) {
const char *YYCURSOR = s, *YYMARKER;
/*!re2c
re2c:define:YYCTYPE = char;
re2c:yyfill:enable = 0;
* { return FAIL; }
number { return OK; }
!include "extra_rules.re.inc";
*/
}
int main() {
assert(lex("123") == OK);
assert(lex("123.4567") == OK);
return 0;
}
HEADER FILES
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 di-
rective 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). Auto-
generated header file may be needed in cases when re2c is used to gen-
erate definitions of constants, variables and structs that must be vis-
ible from other translation units.
Here is an example of generating a header file that contains definition
of the lexer state with tag variables (the number variables depends on
the regular grammar and is unknown to the programmer).
Input file
// re2c $INPUT -o $OUTPUT -i --header lexer/state.h
#include <assert.h>
#include <stddef.h>
#include "lexer/state.h" // the header is generated by re2c
/*!header:re2c:on*/
struct LexerState {
const char *str, *cur;
/*!stags:re2c format = "const char *@@;"; */
};
/*!header:re2c:off*/
long lex(LexerState& st) {
const char *t;
/*!re2c
re2c:header = "lexer/state.h";
re2c:yyfill:enable = 0;
re2c:define:YYCTYPE = char;
re2c:define:YYCURSOR = "st.cur";
re2c:tags = 1;
re2c:tags:expression = "st.@@";
[a]* @t [b]* { return t - st.str; }
*/
}
int main() {
const char *s = "ab";
LexerState st = { s, s /*!stags:re2c format = ", NULL"; */ };
assert(lex(st) == 1);
return 0;
}
Header file
/* Generated by re2c */
typedef struct {
const char *str, *cur, *mar;
const char *yyt1;
} LexerState;
SKELETON PROGRAMS
With the -S, --skeleton option, re2c ignores all non-re2c code and gen-
erates a self-contained C program that can be further compiled and exe-
cuted. The program consists of lexer code and input data. For each con-
structed 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 mini-
mized test cases (the original code may be large or have a lot of de-
pendencies).
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 gener-
ating 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 encod-
ings 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 infea-
sible) 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 transi-
tions and consumes constant amount of memory (re2c writes data to file
as soon as it is generated).
VISUALIZATION AND DEBUG
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 opti-
mizations and transformations. Earlier stages can be dumped with vari-
ous debug options, such as --dump-nfa, --dump-dfa-raw etc. (see the
full list of options).
SEE ALSO
You can find more information about re2c at the official website:
http://re2c.org. Similar programs are flex(1), lex(1), quex(-
http://quex.sourceforge.net).
AUTHORS
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.
RE2C(1)
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