Provided by: re2c_3.0-1_amd64 
NAME
re2c - generate fast lexical analyzers for C/C++, Go and Rust
SYNOPSIS
Note: This manual is for Rust, but it refers to re2c as the general program.
re2c [ OPTIONS ] [ WARNINGS ] INPUT
re2go [ OPTIONS ] [ WARNINGS ] INPUT
re2rust [ OPTIONS ] [ WARNINGS ] INPUT
Input can be either a file or - for stdin.
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:
// re2rust $INPUT -o $OUTPUT
fn lex(s: &[u8]) -> bool {
let mut cursor = 0;
/*!re2c
re2c:define:YYCTYPE = u8;
re2c:define:YYPEEK = "*s.get_unchecked(cursor)";
re2c:define:YYSKIP = "cursor += 1;";
re2c:yyfill:enable = 0;
number = [1-9][0-9]*;
number { return true; }
* { return false; }
*/
}
fn main() {
assert!(lex(b"1234\0"));
}
In the output everything between /*!re2c and */ has been replaced with the generated code:
/* Generated by re2c */
// re2rust $INPUT -o $OUTPUT
fn lex(s: &[u8]) -> bool {
let mut cursor = 0;
{
#[allow(unused_assignments)]
let mut yych : u8 = 0;
let mut yystate : usize = 0;
'yyl: loop {
match yystate {
0 => {
yych = unsafe {*s.get_unchecked(cursor)};
cursor += 1;
match yych {
0x31 ..= 0x39 => {
yystate = 2;
continue 'yyl;
}
_ => {
yystate = 1;
continue 'yyl;
}
}
}
1 => { return false; }
2 => {
yych = unsafe {*s.get_unchecked(cursor)};
match yych {
0x30 ..= 0x39 => {
cursor += 1;
yystate = 2;
continue 'yyl;
}
_ => {
yystate = 3;
continue 'yyl;
}
}
}
3 => { return true; }
_ => {
panic!("internal lexer error")
}
}
}
}
}
fn main() {
assert!(lex(b"1234\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, directives and rules. re2c
compiles regular expressions associated with each rule into a deterministic
finite automaton, encodes it in the form of conditional jumps in the target
language and replaces the block with the generated code. Names and
configurations defined in a global block are added to the global scope and
become visible to subsequent blocks. At the start of the program the global
scope is initialized with command-line options. The :<name> part is optional:
if specified, the name can be used to refer to the block in another part of the
program.
/*!local:re2c[:<name>] ... */
A local block is like a global block, but the names and configurations in it
have local scope (they do not affect other blocks).
/*!rules:re2c[:<name>] ... */
A rules block is like a local block, but it does not generate any code and is
meant to be reused in other blocks. This is a way of sharing code (more details
in the reusable blocks section).
There are also many auxiliary blocks; see section blocks and directives for a full list of
them. A block may contain the following kinds of statements:
<name> = <regular expression>;
A definition binds a name to a regular expression. Names may contain
alphanumeric characters and underscore. The regular expressions section gives an
overview of re2c syntax for regular expressions. Once defined, the name can be
used in other regular expressions and in rules. Recursion in named definitions
is not allowed, and each name should be defined before it is used. A block
inherits named definitions from the global scope. Redefining a name that exists
in the current scope is an error.
<configuration> = <value>;
A configuration allows one to change re2c behavior and customize the generated
code. For a full list of configurations supported by re2c see the configurations
section. Depending on a particular configuration, the value can be a keyword, a
nonnegative integer number or a one-line string which should be enclosed in
double or single quotes unless it consists of alphanumeric characters. A block
inherits configurations from the global scope and may redefine them or add new
ones. Configurations defined inside of a block affect the whole block, even if
they appear at the end of it.
<regular expression> { <code> }
A rule binds a regular expression to a semantic action (a block of code in the
target language). If the regular expression matches, the associated semantic
action is executed. If multiple rules match, the longest match takes precedence.
If multiple rules match the same string, the earliest one takes precedence.
There are two special rules: the default rule * and the end of input rule $. The
default rule should always be defined, it has the lowest priority regardless of
its place in the block, and it matches any code unit (not necessarily a valid
character, see the encoding support section). The end of input rule should be
defined if the corresponding method for handling the end of input is used. If
start conditions are used, rules have more complex syntax.
!<directive>;
A directive is one of the special predefined statements. Each directive has a
unique purpose. For example, the !use directive merges a rules block into the
current one (see the reusable blocks section), and the !include directive allows
one to include an outer file (see the include files section).
PROGRAM INTERFACE
The generated code interfaces with the outer program with the help of primitives --
symbolic names that can be defined as variables, functions or macros in the target
language (collectively referred to as the API). The definition of primitives is left for
the user: this gives them both freedom in customizing the lexer and responsibility to
understand how it works. Not all primitives have to be defined --- only those used by a
given program. The manual provides definitions for the most popular use cases. For a full
list of primitives and their meaning see the API primitives section.
There are two API flavors that define the set of primitives used by re2c:
Pointer API
This API is based on C pointer arithmetic. It was historically the first, and
for a long time the only one. It consists of pointer-like primitives YYCURSOR,
YYMARKER, YYCTXMARKER, YYLIMIT (which are normally defined as pointers of type
YYCTYPE*) and YYFILL. This API is enabled by default for C, and it cannot be
used with other backends that do not support pointer arithmetic.
Generic API
This API is more flexible. It consists generic operations and does not assume
any particular implementation. The primitives are YYPEEK, YYSKIP, YYBACKUP,
YYBACKUPCTX, YYSTAGP, YYSTAGN, YYMTAGP, YYMTAGN, YYRESTORE, YYRESTORECTX,
YYRESTORETAG, YYSHIFT, YYSHIFTSTAG, YYSHIFTMTAG, YYLESSTHAN and YYFILL. For the
C backend generic API is enabled with --api custom option or re2c:api = custom;
configuration; for Go and Rust it is enabled by default. Generic API was added
in version 0.14.
There are two API styles that determine the form in which the primitives should be
defined:
Free-form
Free-form style is enabled with configuration re2c:api:style = free-form;. It is
the default for Rust. In this style interface primitives should be defined as
free-form pieces of code with interpolated variables of the form @@{var} or
optionally just @@ if there is a single variable. The set of variables is
specific to each primitive. For example, if the input is a byte slice buffer:
&[u8], variables cursor, limit, marker and ctxmarker of type usize represent
input positions, and a constant NONE represents invalid position, then the API
can be defined as follows:
/*!re2c
re2c:define:YYPEEK = "*buffer.get_unchecked(cursor)";
re2c:define:YYSKIP = "cursor += 1;";
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} = NONE;";
re2c:define:YYSHIFT = "cursor = (cursor as isize + @@{shift}) as usize;";
re2c:define:YYSHIFTSTAG = "@@{tag} = (@@{tag} as isize + @@{shift}) as usize;";
*/
Function-like
Function-like style is enabled with configuration re2c:api:style = functions;.
In this style primitives should be defined as functions or macros with
parentheses, accepting the necessary arguments. This is well-suited for C
macros or Go closures, but it cannot be used with hygienic macros or functions
in Rust as the primitives need to access lexer context that is not passed as an
argument. It is possible to use closures, but the borrow-checker makes it hard
because different primitives read and modify the same parts of lexer context in
a series of interleaved invocations. As a workaround, one can use Rust Cell
type. For example, if the input is a byte slice buffer: &[u8] and cursor:
Cell::<usize> is the current input position, then YYPEEK and YYSKIP can be
defined as follows:
let YYPEEK = || unsafe { *buffer.get_unchecked(cursor.get()) };
let YYSKIP = || { cursor.set(cur.get() + 1); };
For YYFILL definition and instructions how to customize or disable end-of-input checks see
the handling the end of input and buffer refilling sections.
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 arithmetic (the default for C), and custom is
the generic API (the default for Go and Rust).
--bit-vectors -b
Optimize conditional jumps using bit masks. This option implies --nested-ifs.
--case-insensitive
Treat single-quoted and double-quoted strings as case-insensitive.
--case-inverted
Invert the meaning of single-quoted and double-quoted strings: treat single-quoted
strings as case-sensitive and double-quoted strings as case-insensitive.
--case-ranges
Collapse consecutive cases in a switch statements into a range of the form low ...
high. This syntax is a C/C++ language extension that is supported by compilers like
GCC, Clang and Tcc. The main advantage over using single cases is smaller generated
code and faster generation time, although for some compilers like Tcc it also
results in smaller binary size. This option is supported only for C.
--computed-gotos -g
Optimize conditional jumps using non-standard "computed goto" extension (which must
be supported by the compiler). re2c generates jump tables only in complex cases
with a lot of conditional branches. Complexity threshold can be configured with
cgoto:threshold configuration. This option implies --bit-vectors. It is supported
only for C.
--conditions --start-conditions -c
Enable support of Flex-like "conditions": multiple interrelated lexers within one
block. This is an alternative to manually specifying different re2c blocks
connected with goto or function calls.
--depfile FILE
Write dependency information to FILE in the form of a Makefile rule <output-file> :
<input-file> [include-file ...]. This allows one to track build dependencies in the
presence of include:re2c directives, so that updating include files triggers
regeneration of the output file. This option depends on the --output option.
--ebcdic --ecb -e
Generate a lexer that reads input in EBCDIC encoding. re2c assumes that the
character range is 0 -- 0xFF and character size is 1 byte.
--empty-class <match-empty | match-none | error>
Define the way re2c treats empty character classes. With match-empty (the default)
empty class matches empty input (which is illogical, but backwards-compatible).
With match-none empty class always fails to match. With error empty class raises a
compilation error.
--encoding-policy <fail | substitute | ignore>
Define the way re2c treats Unicode surrogates. With fail re2c aborts with an error
when a surrogate is encountered. With substitute re2c silently replaces surrogates
with the error code point 0xFFFD. With ignore (the default) re2c treats surrogates
as normal code points. The Unicode standard says that standalone surrogates are
invalid, but real-world libraries and programs behave in different ways.
--flex-syntax -F
Partial support for Flex syntax: in this mode named definitions don't need the
equal sign and the terminating semicolon, and when used they must be surrounded
with curly braces. Names without curly braces are treated as double-quoted strings.
--header --type-header -t HEADER
Generate a HEADER file. The contents of the file can be specified with directives
header:re2c:on and header:re2c:off. If conditions are used the header will have a
condition enum automatically appended to it (unless there is an explicit
conditions:re2c directive).
-I PATH
Add PATH to the list of locations which are used when searching for include files.
This option is useful in combination with include:re2c directive. re2c looks for
FILE in the directory of the parent file and in the include locations specified
with -I option.
--input-encoding <ascii | utf8>
Specify the way re2c parses regular expressions. With ascii (the default) re2c
handles input as ASCII-encoded: any sequence of code units is a sequence of
standalone 1-byte characters. With utf8 re2c handles input as UTF8-encoded and
recognizes multibyte characters.
--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. Individual states are
switch cases. The current state is stored in a variable yystate. Transitions
between states update yystate to the case label of the destination state and
continue to the head of the loop. This option is always enabled for Rust, as it has
no goto statement and cannot use the goto/label approach which is the default for C
and Go backends.
--nested-ifs -s
Use nested if statements instead of switch statements in conditional jumps. This
usually results in more efficient code with non-optimizing compilers.
--no-debug-info -i
Do not output line directives. This may be useful when the generated code is stored
in a version control system (to avoid huge autogenerated diffs on small changes).
This option is on by default for Rust, as it does not have line directives.
--no-generation-date
Suppress date output in the generated file.
--no-version
Suppress version output in the generated file.
--no-unsafe
Do not generate unsafe wrapper over YYPEEK (this option is specific to Rust). For
performance reasons YYPEEK should avoid bounds-checking, as the lexer already
performs end-of-input checks in a more efficient way. The user may choose to
provide a safe YYPEEK definition, or a definition that is unsafe only in release
builds, in which case the --no-unsafe option helps to avoid warnings about
redundant unsafe blocks.
--output -o OUTPUT
Specify the OUTPUT file.
--posix-captures -P
Enable submatch extraction with POSIX-style capturing groups.
--reusable -r
Deprecated since version 2.2 (reusable blocks are allowed by default now).
--skeleton -S
Ignore user-defined interface code and generate a self-contained "skeleton"
program. Additionally, generate input files with strings derived from the regular
grammar and compressed match results that are used to verify "skeleton" behavior on
all inputs. This option is useful for finding bugs in optimizations and code
generation. This option is supported only for C.
--storable-state -f
Generate a lexer which can store its inner state. This is useful in push-model
lexers which are stopped by an outer program when there is not enough input, and
then resumed when more input becomes available. In this mode users should
additionally define YYGETSTATE and YYSETSTATE primitives, and variables yych,
yyaccept and state should be part of the stored lexer state.
--tags -T
Enable submatch extraction with tags.
--ucs2 --wide-chars -w
Generate a lexer that reads UCS2-encoded input. re2c assumes that the character
range is 0 -- 0xFFFF and character size is 2 bytes. This option implies
--nested-ifs.
--utf8 --utf-8 -8
Generate a lexer that reads input in UTF-8 encoding. re2c assumes that the
character range is 0 -- 0x10FFFF and character size is 1 byte.
--utf16 --utf-16 -x
Generate a lexer that reads UTF16-encoded input. re2c assumes that the character
range is 0 -- 0x10FFFF and character size is 2 bytes. This option implies
--nested-ifs.
--utf32 --unicode -u
Generate a lexer that reads UTF32-encoded input. re2c assumes that the character
range is 0 -- 0x10FFFF and character size is 4 bytes. This option implies
--nested-ifs.
--verbose
Output a short message in case of success.
--vernum -V
Show version information in MMmmpp format (major, minor, patch).
--version -v
Show version information.
--single-pass -1
Deprecated. Does nothing (single pass is the default now).
--debug-output -d
Emit YYDEBUG invocations in the generated code. This is useful to trace lexer
execution.
--dump-adfa
Debug option: output DFA after tunneling (in .dot format).
--dump-cfg
Debug option: output control flow graph of tag variables (in .dot format).
--dump-closure-stats
Debug option: output statistics on the number of states in closure.
--dump-dfa-det
Debug option: output DFA immediately after determinization (in .dot format).
--dump-dfa-min
Debug option: output DFA after minimization (in .dot format).
--dump-dfa-tagopt
Debug option: output DFA after tag optimizations (in .dot format).
--dump-dfa-tree
Debug option: output DFA under construction with states represented as tag history
trees (in .dot format).
--dump-dfa-raw
Debug option: output DFA under construction with expanded state-sets (in .dot
format).
--dump-interf
Debug option: output interference table produced by liveness analysis of tag
variables.
--dump-nfa
Debug option: output NFA (in .dot format).
--emit-dot -D
Instead of normal output generate lexer graph in .dot format. The output can be
converted to an image with the help of Graphviz (e.g. something like dot -Tpng
-odfa.png dfa.dot).
--dfa-minimization <moore | table>
Internal option: DFA minimization algorithm used by re2c. The moore option is the
Moore algorithm (it is the default). The table option is the "table filling"
algorithm. Both algorithms should produce the same DFA up to states relabeling;
table filling is simpler and much slower and serves as a reference implementation.
--eager-skip
Internal option: make the generated lexer advance the input position eagerly --
immediately after reading the input symbol. This changes the default behavior when
the input position is advanced lazily -- after transition to the next state. 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 semantics: gor1 (the default) stands for
Goldberg-Radzik algorithm, and gtop stands for "global topological order"
algorithm.
--posix-prectable <complex | naive>
Internal option: specify the algorithm used to compute POSIX precedence table. The
complex algorithm computes precedence table in one traversal of tag history tree
and has quadratic complexity in the number of TNFA states; it is the default. The
naive algorithm has worst-case cubic complexity in the number of TNFA states, but
it is much simpler than complex and may be slightly faster in non-pathological
cases.
--stadfa
Internal option: 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 offset. For tags that are under alternative or
repetition it is also necessary to check if the base tag has a no-match value (in
that case fixed tag should also be set to no-match, disregarding the offset). For
tags in top-level concatenation the check is not needed, because they always match.
WARNINGS
Warnings can be invividually enabled, disabled and turned into an error.
-W Turn on all warnings.
-Werror
Turn warnings into errors. Note that this option alone doesn't turn on any
warnings; it only affects those warnings that have been turned on so far or will be
turned on later.
-W<warning>
Turn on warning.
-Wno-<warning>
Turn off warning.
-Werror-<warning>
Turn on warning and treat it as an error (this implies -W<warning>).
-Wno-error-<warning>
Don't treat this particular warning as an error. This doesn't turn off the warning
itself.
-Wcondition-order
Warn if the generated program makes implicit assumptions about condition numbering.
One should use either the ---header option or the conditions:re2c directive to
generate a mapping of condition names to numbers and then use the autogenerated
condition names.
-Wempty-character-class
Warn if a regular expression contains an empty character class. Trying to match an
empty character class makes no sense: it should always fail. However, for
backwards compatibility reasons re2c permits empty character classes and treats
them as empty strings. Use the --empty-class option to change the default behavior.
-Wmatch-empty-string
Warn if a rule is nullable (matches an empty string). If the lexer runs in a loop
and the empty match is unintentional, the lexer may unexpectedly hang in an
infinite loop.
-Wswapped-range
Warn if the lower bound of a range is greater than its upper bound. The default
behavior is to silently swap the range bounds.
-Wundefined-control-flow
Warn if some input strings cause undefined control flow in the lexer (the faulty
patterns are reported). This is a dangerous and common mistake. It can be easily
fixed by adding the default rule * which has the lowest priority, matches any code
unit, and always consumes a single code unit.
-Wunreachable-rules
Warn about rules that are shadowed by other rules and will never match.
-Wuseless-escape
Warn if a symbol is escaped when it shouldn't be. By default, re2c silently
ignores such escapes, but this may as well indicate a typo or an error in the
escape sequence.
-Wnondeterministic-tags
Warn if a tag has n-th degree of nondeterminism, where n is greater than 1.
-Wsentinel-in-midrule
Warn if the sentinel symbol occurs in the middle of a rule --- this may cause reads
past the end of buffer, crashes or memory corruption in the generated lexer. This
warning is only applicable if the sentinel method of checking for the end of input
is used. It is set to an error if re2c:sentinel configuration is used.
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 specified.
/*!local:re2c[:<name>] ... */
A local re2c block with an optional name. Unlike global blocks, definitions and
configurations inside of a local block are not added into the global scope. In all
other respects local blocks are the same as global blocks.
/*!rules:re2c[:<name>] ... */
A reusable block with an optional name. Rules blocks have the same structure as
local or global blocks, but they do not produce any code and they can be reused
multiple times in other blocks with the help of a !use:<name>; directive or a
/*!use:re2c[:<name>] ... */ block. A rules block on its own does not add any
definitions into the global scope. The code for it is generated at the point of
use. Prior to re2c version 2.2 rules blocks required -r --reusable option.
/*!use:re2c[:<name>] ... */
A use block that references a previously defined rules block. If the name is
specified, re2c looks for a rules blocks with this name. Otherwise the most recent
rules block is used (either a named or an unnamed one). A use block can add
definitions, configurations and rules of its own, which are added to those of the
referenced rules block. Prior to re2c version 2.2 use blocks required -r --reusable
option.
!use:<name>;
An in-block use directive that merges a previously defined rules block with the
specified name into the current block. Named definitions, configurations and rules
of the referenced block are added to the current ones. Conflicts between
overlapping rules and configurations are resolved in the usual way: the first rule
takes priority, and the latest configuration overrides the preceding ones. One
exception is the special rules *, $ and <!> for which a block-local definition
always takes priority. A use directive can be placed anywhere inside of a block,
and multiple use directives are allowed.
/*!max:re2c[:<name1>[:<name2>...]] ... */
A directive that generates YYMAXFILL definition. An optional list of block names
specifies which blocks should be included when computing YYMAXFILL value (if the
list is empty, all blocks are included). By default the generated code is a
macro-definition for C (#define YYMAXFILL <n>), or a global variable for Go (var
YYMAXFILL int = <n>). It can be customized with an optional configuration format
that specifies a template string where @@{max} (or @@ for short) is replaced with
the numeric value of YYMAXFILL.
/*!maxnmatch:re2c[:<name1>[:<name2>...]] ... */
A directive that generates YYMAXNMATCH definition (it requires -P --posix-captures
option). An optional list of block names specifies which blocks should be included
when computing YYMAXNMATCH value (if the list is empty, all blocks are included).
By default the generated code is a macro-definition for C (#define YYMAXNMATCH
<n>), or a global variable for Go (var YYMAXNMATCH int = <n>). It can be customized
with an optional configuration format that specifies a template string where
@@{max} (or @@ for short) is replaced with the numeric value of YYMAXNMATCH.
/*!stags:re2c[:<name1>[:<name2>...]] ... */, /*!mtags:re2c[:<name1>[:<name2>...]] ... */
Directives that specify a template piece of code that is expanded for each
s-tag/m-tag variable generated by re2c. An optional list of block names specifies
which blocks should be included when computing the set of tag variables (if the
list is empty, all blocks are included). There are two optional configurations:
format and separator. Configuration format specifies a template string where
@@(tag} (or @@ for short) is replaced with the name of each tag variable.
Configuration separator specifies a piece of code used to join the generated format
pieces for different tag variables.
/*!getstate:re2c[:<name1>[:<name2>...]] ... */
A directive that generates conditional dispatch on the lexer state (it requires
--storable-state option). An optional list of block names specifies which blocks
should be included in the state dispatch. The default transition goes to the start
label of the first block on the list. If the list is empty, all blocks are
included, and the default transition goes to the first block in the file that has a
start label. This directive is incompatible with the --loop-switch option and
Rust, as it requires cross-block transitions that are unsupported without the goto
statement.
/*!conditions:re2c[:<name1>[:<name2>...]] ... */, /*!types:re2c... */
A directive that generates condition enumeration (it requires --conditions option).
An optional list of block names specifies which blocks should be included when
computing the set of conditions (if the list is empty, all blocks are included).
By default the generated code is an enumeration YYCONDTYPE. It can be customized
with optional configurations format and separator. Configuration format specifies
a template string where @@(cond} (or @@ for short) is replaced with the name of
each condition, and @@{num} is replaced with a numeric index of that condition.
Configuration separator specifies a piece of code used to join the generated format
pieces for different conditions.
/*!include:re2c <file> */
This directive allows one to include <file>, which must be a double-quoted file
path. The contents of the file are literally substituted in place of the directive,
in the same way as #include works in C/C++. This directive can be used together
with the --depfile option to generate build system dependencies on the included
files.
!include <file>;
This directive is the same as /*!include:re2c <file> */, except that it should be
used inside of a re2c block.
/*!header:re2c:on*/
This directive marks the start of header file. Everything after it and up to the
following /*!header:re2c:off*/ directive is processed by re2c and written to the
header file specified with -t --type-header option.
/*!header:re2c:off*/
This directive marks the end of header file started with /*!header:re2c:on*/.
/*!ignore:re2c ... */
A block which contents are ignored and removed from the output file.
%{ ... %}
A global re2c block in the --flex-support mode. This is deprecated and exists for
backward compatibility.
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 features and so on. All the necessary primitives should be defined by
the user in the form of macros, functions, variables, free-form pieces of code, or any
other suitable form. re2c does not (and cannot) check the definitions, so if anything is
missing or defined incorrectly the generated code will not compile.
YYCTYPE
The type of the input characters (code units). For ASCII, EBCDIC and UTF-8
encodings it should be 1-byte unsigned integer. For UTF-16 or UCS-2 it should be
2-byte unsigned integer. For UTF-32 it should be 4-byte unsigned integer.
YYCURSOR
A pointer-like l-value that stores the current input position (usually a pointer of
type YYCTYPE*). Initially YYCURSOR should point to the first input character. It is
advanced by the generated code. When a rule matches, YYCURSOR points to the
position after the last matched character. It is used only in C pointer API.
YYLIMIT
A pointer-like r-value that stores the end of input position (usually a pointer of
type YYCTYPE*). Initially YYLIMIT should point to the position after the last
available input character. It is not changed by the generated code. The lexer
compares YYCURSOR to YYLIMIT in order to determine if there are enough input
characters left. YYLIMIT is used only in C pointer API.
YYMARKER
A pointer-like l-value (usually a pointer of type YYCTYPE*) that stores the
position of the latest matched rule. It is used to restore the YYCURSOR position if
the longer match fails and the lexer needs to rollback. Initialization is not
needed. YYMARKER is used only in C pointer API.
YYCTXMARKER
A pointer-like l-value that stores the position of the trailing context (usually a
pointer of type YYCTYPE*). No initialization is needed. It is used only in C
pointer API, and only with the lookahead operator /.
YYFILL A generic API primitive with one argument len. YYFILL should provide at least len
more input characters or fail. If re2c:eof is used, then len is always 1 and
YYFILL should always return to the calling function; zero return value indicates
success. If re2c:eof is not used, then YYFILL return value is ignored and it
should not return on failure. The maximum value of len is YYMAXFILL. The
definition of YYFILL can be either function-like or free-form depending on the API
style (see re2c:api:style and re2c:define:YYFILL:naked).
YYMAXFILL
An integral constant equal to the maximum value of the argument to YYFILL. It can
be generated with /*!max:re2c*/ directive.
YYLESSTHAN
A generic API primitive with one argument len. It should be defined as an r-value
of boolean type that equals true if and only if there are less than len input
characters left. The definition can be either function-like or free-form depending
on the API style (see re2c:api:style).
YYPEEK A generic API primitive with no arguments. It should be defined as an r-value of
type YYCTYPE that is equal to the character at the current input position. The
definition can be either function-like or free-form depending on the API style (see
re2c:api:style).
YYSKIP A generic API primitive with no arguments. YYSKIP should advance the current input
position by one character. The definition can be either function-like or free-form
depending on the API style (see re2c:api:style).
YYBACKUP
A generic API primitive with no arguments. YYBACKUP should save the current input
position, which is later restored with YYRESTORE. The definition should be either
function-like or free-form depending on the API style (see re2c:api:style).
YYRESTORE
A generic API primitive with no arguments. YYRESTORE should restore the current
input position to the value saved by YYBACKUP. The definition should be either
function-like or free-form depending on the API style (see re2c:api:style).
YYBACKUPCTX
A generic API primitive with zero arguments. YYBACKUPCTX should save the current
input position as the position of the trailing context, which is later restored by
YYRESTORECTX. The definition should be either function-like or free-form depending
on the API style (see re2c:api:style).
YYRESTORECTX
A generic API primitive with no arguments. YYRESTORECTX should restore the
trailing context position saved with YYBACKUPCTX. The definition should be either
function-like or free-form depending on the API style (see re2c:api:style).
YYRESTORETAG
A generic API primitive with one argument tag. YYRESTORETAG should restore the
trailing context position to the value of tag. The definition should be either
function-like or free-form depending on the API style (see re2c:api:style).
YYSTAGP
A generic API primitive with one argument tag, where tag can be a pointer or an
offset (see submatch extraction section for details). YYSTAGP should set tag to
the current input position. The definition should be either function-like or
free-form depending on the API style (see re2c:api:style).
YYSTAGN
A generic API primitive with one argument tag, where tag can be a pointer or an
offset (see submatch extraction section for details). YYSTAGN should to set tag to
a value that represents non-existent input position. The definition should be
either function-like or free-form depending on the API style (see re2c:api:style).
YYMTAGP
A generic API primitive with one argument tag. YYMTAGP should append the current
position to the submatch history of tag (see the submatch extraction section for
details.) The definition should be either function-like or free-form depending on
the API style (see re2c:api:style).
YYMTAGN
A generic API primitive with one argument tag. YYMTAGN should append a value that
represents non-existent input position position to the submatch history of tag (see
the submatch extraction section for details.) The definition can be either
function-like or free-form depending on the API style (see re2c:api:style).
YYSHIFT
A generic API primitive with one argument shift. YYSHIFT should shift the current
input position by shift characters (the shift value may be negative). The
definition can be either function-like or free-form depending on the API style (see
re2c:api:style).
YYSHIFTSTAG
A generic API primitive with two arguments, tag and shift. YYSHIFTSTAG should
shift tag by shift characters (the shift value may be negative). The definition
can be either function-like or free-form depending on the API style (see
re2c:api:style).
YYSHIFTMTAG
A generic API primitive with two arguments, tag and shift. YYSHIFTMTAG should
shift the latest value in the history of tag by shift characters (the shift value
may be negative). The definition should be either function-like or free-form
depending on the API style (see re2c:api:style).
YYMAXNMATCH
An integral constant equal to the maximal number of POSIX capturing groups in a
rule. It is generated with /*!maxnmatch:re2c*/ directive.
YYCONDTYPE
The type of the condition enum. It should be generated either with the
/*!types:re2c*/ directive or the -t --type-header option.
YYGETCONDITION
An API primitive with zero arguments. It should be defined as an r-value of type
YYCONDTYPE that is equal to the current condition identifier. The definition can be
either function-like or free-form depending on the API style (see re2c:api:style
and re2c:define:YYGETCONDITION:naked).
YYSETCONDITION
An API primitive with one argument cond. The meaning of YYSETCONDITION is to set
the current condition identifier to cond. The definition should be either
function-like or free-form depending on the API style (see re2c:api:style and
re2c:define:YYSETCONDITION@cond).
YYGETSTATE
An API primitive with zero arguments. It should be defined as an r-value of
integer type that is equal to the current lexer state. Should be initialized to -1.
The definition can be either function-like or free-form depending on the API style
(see re2c:api:style and re2c:define:YYGETSTATE:naked).
YYSETSTATE
An API primitive with one argument state. The meaning of YYSETSTATE is to set the
current lexer state to state. The definition should be either function-like or
free-form depending on the API style (see re2c:api:style and
re2c:define:YYSETSTATE@state).
YYDEBUG
A debug API primitive with two arguments. It can be used to debug the generated
code (with -d --debug-output option). YYDEBUG should return no value and accept two
arguments: state (either a DFA state index or -1) and symbol (the current input
symbol).
yych An l-value of type YYCTYPE that stores the current input character. User
definition is necessary only with -f --storable-state option.
yyaccept
An l-value of unsigned integral type that stores the number of the latest matched
rule. User definition is necessary only with -f --storable-state option.
yynmatch
An l-value of unsigned integral type that stores the number of POSIX capturing
groups in the matched rule. Used only with -P --posix-captures option.
yypmatch
An array of l-values that are used to hold the tag values corresponding to the
capturing parentheses in the matching rule. Array length must be at least yynmatch
* 2 (usually YYMAXNMATCH * 2 is a good choice). Used only with -P --posix-captures
option.
CONFIGURATIONS
re2c:api, re2c:flags:input
Same as the --api option.
re2c:api:sigil
Specify the marker ("sigil") that is used for argument placeholders in the API
primitives. The default is @@. A placeholder starts with sigil followed by the
argument name in curly braces. For example, if sigil is set to $, then placeholders
will have the form ${name}. Single-argument APIs may use shorthand notation without
the name in braces. This option can be overridden by options for individual API
primitives, e.g. re2c:define:YYFILL@len for YYFILL.
re2c:api:style
Specify API style. Possible values are functions (the default for C) and free-form
(the default for Go and Rust). In functions style API primitives are generated
with an argument list in parentheses following the name of the primitive. The
arguments are provided only for autogenerated parameters (such as the number of
characters passed to YYFILL), but not for the general lexer context, so the
primitives behave more like macros in C/C++ or closures in Go and Rust. In
free-form style API primitives do not have a fixed form: they should be defined as
strings containing free-form pieces of code with interpolated variables of the form
@@{var} or @@ (they correspond to arguments in function-like style). This
configuration may be overridden for individual API primitives, see for example
re2c:define:YYFILL:naked configuration for YYFILL.
re2c:bit-vectors, re2c:flags:bit-vectors, re2c:flags:b
Same as the --bit-vectors option, but can be configured on per-block basis.
re2c:case-insensitive, re2c:flags:case-insensitive
Same as the --case-insensitive option, but can be configured on per-block basis.
re2c:case-inverted, re2c:flags:case-inverted
Same as the --case-inverted option, but can be configured on per-block basis.
re2c:case-ranges, re2c:flags:case-ranges
Same as the --case-ranges option, but can be configured on per-block basis.
re2c:computed-gotos, re2c:flags:computed-gotos, re2c:flags:g
Same as the --computed-gotos option, but can be configured on per-block basis.
re2c:computed-gotos:threshold, re2c:cgoto:threshold
If computed goto is used, this configuration specifies the complexity threshold
that triggers the generation of jump tables instead of nested if statements and
bitmaps. The default value is 9.
re2c:cond:goto
Specifies a piece of code used for the autogenerated shortcut rules :=> in
conditions. The default is goto @@;. The @@ placeholder is substituted with
condition name (see configurations re2c:api:sigil and re2c:cond:goto@cond).
re2c:cond:goto@cond
Specifies the sigil used for argument substitution in re2c:cond:goto definition.
The default value is @@. Overrides the more generic re2c:api:sigil configuration.
re2c:cond:divider
Defines the divider for condition blocks. The default value is /*
*********************************** */. Placeholders are substituted with
condition name (see re2c:api;sigil and re2c:cond:divider@cond).
re2c:cond:divider@cond
Specifies the sigil used for argument substitution in re2c:cond:divider definition.
The default is @@. Overrides the more generic re2c:api:sigil configuration.
re2c:cond:prefix, re2c:condprefix
Specifies the prefix used for condition labels. The default is yyc_.
re2c:cond:enumprefix, re2c:condenumprefix
Specifies the prefix used for condition identifiers. The default is yyc.
re2c:debug-output, re2c:flags:debug-output, re2c:flags:d
Same as the --debug-output option, but can be configured on per-block basis.
re2c:define:YYBACKUP
Defines generic API primitive YYBACKUP (see the API primitives section).
re2c:define:YYBACKUPCTX
Defines generic API primitive YYBACKUPCTX (see the API primitives section).
re2c:define:YYCONDTYPE
Defines YYCONDTYPE (see the API primitives section).
re2c:define:YYCTYPE
Defines YYCTYPE (see the API primitives section).
re2c:define:YYCTXMARKER
Defines API primitive YYCTXMARKER (see the API primitives section).
re2c:define:YYCURSOR
Defines API primitive YYCURSOR (see the API primitives section).
re2c:define:YYDEBUG
Defines API primitive YYDEBUG (see the API primitives section).
re2c:define:YYFILL
Defines API primitive YYFILL (see the API primitives section).
re2c:define:YYFILL@len
Specifies the sigil used for argument substitution in YYFILL definition. Defaults
to @@. Overrides the more generic re2c:api:sigil configuration.
re2c:define:YYFILL:naked
Overrides the more generic re2c:api:style configuration for YYFILL. Zero value
corresponds to free-form API style.
re2c:define:YYGETCONDITION
Defines API primitive YYGETCONDITION (see the API primitives section).
re2c:define:YYGETCONDITION:naked
Overrides the more generic re2c:api:style configuration for YYGETCONDITION. Zero
value corresponds to free-form API style.
re2c:define:YYGETSTATE
Defines API primitive YYGETSTATE (see the API primitives section).
re2c:define:YYGETSTATE:naked
Overrides the more generic re2c:api:style configuration for YYGETSTATE. Zero value
corresponds to free-form API style.
re2c:define:YYLESSTHAN
Defines generic API primitive YYLESSTHAN (see the API primitives section).
re2c:define:YYLIMIT
Defines API primitive YYLIMIT (see the API primitives section).
re2c:define:YYMARKER
Defines API primitive YYMARKER (see the API primitives section).
re2c:define:YYMTAGN
Defines generic API primitive YYMTAGN (see the API primitives section).
re2c:define:YYMTAGP
Defines generic API primitive YYMTAGP (see the API primitives section).
re2c:define:YYPEEK
Defines generic API primitive YYPEEK (see the API primitives section).
re2c:define:YYRESTORE
Defines generic API primitive YYRESTORE (see the API primitives section).
re2c:define:YYRESTORECTX
Defines generic API primitive YYRESTORECTX (see the API primitives section).
re2c:define:YYRESTORETAG
Defines generic API primitive YYRESTORETAG (see the API primitives section).
re2c:define:YYSETCONDITION
Defines API primitive YYSETCONDITION (see the API primitives section).
re2c:define:YYSETCONDITION@cond
Specifies the sigil used for argument substitution in YYSETCONDITION definition.
The default value is @@. Overrides the more generic re2c:api:sigil configuration.
re2c:define:YYSETCONDITION:naked
Overrides the more generic re2c:api:style configuration for YYSETCONDITION. Zero
value corresponds to free-form API style.
re2c:define:YYSETSTATE
Defines API primitive YYSETSTATE (see the API primitives section).
re2c:define:YYSETSTATE@state
Specifies the sigil used for argument substitution in YYSETSTATE definition. The
default value is @@. Overrides the more generic re2c:api:sigil configuration.
re2c:define:YYSETSTATE:naked
Overrides the more generic re2c:api:style configuration for YYSETSTATE. Zero value
corresponds to free-form API style.
re2c:define:YYSKIP
Defines generic API primitive YYSKIP (see the API primitives section).
re2c:define:YYSHIFT
Defines generic API primitive YYSHIFT (see the API primitives section).
re2c:define:YYSHIFTMTAG
Defines generic API primitive YYSHIFTMTAG (see the API primitives section).
re2c:define:YYSHIFTSTAG
Defines generic API primitive YYSHIFTSTAG (see the API primitives section).
re2c:define:YYSTAGN
Defines generic API primitive YYSTAGN (see the API primitives section).
re2c:define:YYSTAGP
Defines generic API primitive YYSTAGP (see the API primitives section).
re2c:empty-class, re2c:flags:empty-class
Same as the --empty-class option, but can be configured on per-block basis.
re2c:encoding:ebcdic, re2c:flags:ecb, re2c:flags:e
Same as the --ebcdic option, but can be configured on per-block basis.
re2c:encoding:ucs2, re2c:flags:wide-chars, re2c:flags:w
Same as the --ucs2 option, but can be configured on per-block basis.
re2c:encoding:utf8, re2c:flags:utf-8, re2c:flags:8
Same as the --utf8 option, but can be configured on per-block basis.
re2c:encoding:utf16, re2c:flags:utf-16, re2c:flags:x
Same as the --utf16 option, but can be configured on per-block basis.
re2c:encoding:utf32, re2c:flags:unicode, re2c:flags:u
Same as the --utf32 option, but can be configured on per-block basis.
re2c:encoding-policy, re2c:flags:encoding-policy
Same as the --encoding-policy option, but can be configured on per-block basis.
re2c:eof
Specifies the sentinel symbol used with the end-of-input rule $. The default value
is -1 ($ rule is not used). Other possible values include all valid code units.
Only decimal numbers are recognized.
re2c:header, re2c:flags:type-header, re2c:flags:t
Specifies the name of the generated header file relative to the directory of the
output file. Same as the --header option except that the file path is relative.
re2c:indent:string
Specifies the string used for indentation. The default is a single tab character
"\t". Indent string should contain whitespace characters only. To disable
indentation entirely, set this configuration to an empty string.
re2c:indent:top
Specifies the minimum amount of indentation to use. The default value is zero. The
value should be a non-negative integer number.
re2c:label:prefix, re2c:labelprefix
Specifies the prefix used for DFA state labels. The default is yy.
re2c:label:start, re2c:startlabel
Controls the generation of a block start label. The default value is zero, which
means that the start label is generated only if it is used. An integer value
greater than zero forces the generation of start label even if it is unused by the
lexer. A string value also forces start label generation and sets the label name to
the specified string. This configuration applies only to the current block (it is
reset to default for the next block).
re2c:label:yyFillLabel
Specifies the prefix of YYFILL labels used with re2c:eof and in storable state
mode.
re2c:label:yyloop
Specifies the name of the label marking the start of the lexer loop with
--loop-switch option. The default is yyloop.
re2c:label:yyNext
Specifies the name of the optional label that follows YYGETSTATE switch in storable
state mode (enabled with re2c:state:nextlabel). The default is yyNext.
re2c:lookahead, re2c:flags:lookahead
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 inconvenient, for example if tag
variables are defined as fields in a struct. All occurrences of @@{tag} or @@ are
replaced with the actual tag name. For example, re2c:tags:expression = "s.@@";
results in expressions of the form s.yyt<N> in the generated code. See also
re2c:api:sigil configuration.
re2c:tags:prefix
Specifies the prefix for tag variable names. The default is yyt.
re2c:sentinel
Specifies the sentinel symbol used for the end-of-input checks (when bounds checks
are disabled with re2c:yyfill:enable = 0; and re2c:eof is not set). This
configuration does not affect code generation: its purpose is to verify that the
sentinel is not allowed in the middle of a rule, and ensure that the lexer won't
read past the end of buffer. The default value is -1` (in that case re2c assumes
that the sentinel is zero, which is the most common case). Only decimal numbers are
recognized.
re2c:state:abort
If set to a positive integer value, changes the default case in YYGETSTATE switch:
by default it aborts the program, and an explicit -1 case contains transition to
the start of the block.
re2c:state:nextlabel
Controls if the YYGETSTATE switch is followed by an yyNext label (the default value
is zero, which corresponds to no label). Alternatively one can use
re2c:label:start to generate a specific start label, or an explicit getstate:re2c
directive to generate the YYGETSTATE switch separately from the lexer block.
re2c:unsafe, re2c:flags:unsafe
Same as the --no-unsafe option, but can be configured on per-block basis. If set
to zero, it suppresses the generation of unsafe wrappers around YYPEEK. The default
is non-zero (wrappers are generated). This configuration is specific to Rust.
re2c:variable:yyaccept
Specifies the name of the yyaccept variable (see the API primitives section).
re2c:variable:yybm
Specifies the name of the yybm variable (used for bitmaps).
re2c:variable:yybm:hex, re2c:yybm:hex
If set to nonzero, bitmaps for the --bit-vectors option are generated in
hexadecimal format. The default is zero (bitmaps are in decimal format).
re2c:variable:yych
Specifies the name of the yych variable (see the API primitives section).
re2c:variable:yych:emit, re2c:yych:emit
If set to zero, yych definition is not generated. The default is non-zero.
re2c:variable:yych:conversion, re2c:yych:conversion
If set to non-zero, re2c automatically generates a conversion to YYCTYPE every time
yych is read. The default is to zero (no conversion).
re2c:variable:yyctable
Specifies the name of the yyctable variable (the jump table generated for
YYGETCONDITION switch with --computed-gotos option).
re2c:variable:yytarget
Specifies the name of the yytarget variable.
re2c:variable:yystable
Deprecated.
re2c:variable:yystate
Specifies the name of the yystate variable (used with the --loop-switch option to
store the current DFA state).
re2c:yyfill:check
If set to zero, suppresses the generation of pre-YYFILL check for the number of
input characters (the YYLESSTHAN definition in generic API and the YYLIMIT-based
comparison in C pointer API). The default is non-zero (generate the check).
re2c:yyfill:enable
If set to zero, suppresses the generation of YYFILL (together with the check). This
should be used when the whole input fits into one piece of memory (there is no need
for buffering) and the end-of-input checks do not rely on the YYFILL checks (e.g.
if a sentinel character is used). Use warnings (-W option) and re2c:sentinel
configuration to verify that the generated lexer cannot read past the end of input.
The default is non-zero (YYFILL is enabled).
re2c:yyfill:parameter
If set to zero, suppresses the generation of parameter passed to YYFILL. The
parameter is the minimum number of characters that must be supplied. Defaults to
non-zero (the parameter is generated). This configuration can be overridden with
re2c:define:YYFILL:naked or re2c:api:style.
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 hexadecimal 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 automatically stops. The third condition, end of
input, is different: it may happen in any state, and the lexer should be able to handle
it. Checking for the end of input interrupts the normal lexer workflow and adds
conditional branches to the generated program, therefore it is necessary to minimize the
number of such checks. re2c supports a few different methods for handling the end of
input. Which one to use depends on the complexity of regular expressions, the need for
buffering, performance considerations and other factors. Here is a list of methods:
• Sentinel. This method eliminates the need for the end of input checks altogether. It is
simple and efficient, but limited to the case when there is a natural "sentinel"
character that can never occur in valid input. This character may still occur in invalid
input, but it should not be allowed by the regular expressions, except perhaps as the
last character of a rule. The sentinel is appended at the end of input and serves as a
stop signal: when the lexer reads this character, it is either a syntax error or the end
of input. In both cases the lexer should stop. This method is used if YYFILL is disabled
with re2c:yyfill:enable = 0; and re2c:eof has the default value -1.
• Sentinel with bounds checks. This method is generic: it allows one to handle any input
without restrictions on the regular expressions. The idea is to reduce the number of end
of input checks by performing them only on certain characters. Similar to the "sentinel"
method, one of the characters is chosen as a "sentinel" and appended at the end of
input. However, there is no restriction on where the sentinel may occur (in fact, any
character can be chosen for a sentinel). When the lexer reads this character, it
additionally performs a bounds check. If the current position is within bounds, the
lexer resumes matching and handles the sentinel as a regular character. Otherwise it
invokes YYFILL (unless it is disabled). If more input is supplied, the lexer will
rematch the last character and continue as if the sentinel wasn't there. Otherwise it
must be the real end of input, and the lexer stops. This method is used when re2c:eof
has non-negative value (it should be set to the numeric value of the sentinel). YYFILL
is optional.
• Bounds checks with padding. This method is generic, and it may be faster than the
"sentinel with bounds checks" method, but it is also more complex. The idea is to
partition DFA states into strongly connected components (SCCs) and generate a single
check per SCC for enough characters to cover the longest non-looping path in this SCC.
This reduces the number of checks, but there is a problem with short lexemes at the end
of input, as the check requires enough characters to cover the longest lexeme. This can
be fixed by padding the input with a few fake characters that do not form a valid lexeme
suffix (so that the lexer cannot match them). The length of padding should be YYMAXFILL,
generated with /*!max:re2c*/. If there is not enough input, the lexer invokes YYFILL
which should supply at least the required number of characters or not return. This
method is used if YYFILL is enabled and re2c:eof is -1 (this is the default
configuration).
• Custom checks. Generic API allows one to override basic operations like reading a
character, which makes it possible to include the end-of-input checks as part of them.
This approach is error-prone and should be used with caution. To use a custom method,
enable generic API with --api custom or re2c:api = custom; and disable default bounds
checks with re2c:yyfill:enable = 0; or re2c:yyfill:check = 0;.
The following subsections contain an example of each method.
Sentinel
This example uses a sentinel character to handle the end of input. The program counts
space-separated words in a null-terminated string. The sentinel is null: it is the last
character of each input string, and it is not allowed in the middle of a lexeme by any of
the rules (in particular, it is not included in character ranges where it is easy to
overlook). If a null occurs in the middle of a string, it is a syntax error and the lexer
will match default rule *, but it won't read past the end of input or crash (use
-Wsentinel-in-midrule warning and re2c:sentinel configuration to verify this).
Configuration re2c:yyfill:enable = 0; suppresses the generation of bounds checks and
YYFILL invocations.
// re2rust $INPUT -o $OUTPUT
// Expect a null-terminated string.
fn lex(s: &[u8]) -> isize {
let mut cur = 0;
let mut count = 0;
'lex: loop {/*!re2c
re2c:define:YYCTYPE = u8;
re2c:define:YYPEEK = "*s.get_unchecked(cur)";
re2c:define:YYSKIP = "cur += 1;";
re2c:yyfill:enable = 0;
* { return -1; }
[\x00] { return count; }
[a-z]+ { count += 1; continue 'lex; }
[ ]+ { continue 'lex; }
*/}
}
fn main() {
assert_eq!(lex(b"\x00"), 0);
assert_eq!(lex(b"one two three\x00"), 3);
assert_eq!(lex(b"f0ur\x00"), -1);
}
Sentinel with bounds checks
This example uses sentinel with bounds checks to handle the end of input (this method was
added in version 1.2). The program counts space-separated single-quoted strings. The
sentinel character is null, which is specified with re2c:eof = 0; configuration. As in the
sentinel method, null is the last character of each input string, but it is allowed in the
middle of a rule (for example, 'aaa\0aa'\0 is valid input, but 'aaa\0 is a syntax error).
Bounds checks are generated in each state that matches an input character, but they are
scoped to the branch that handles null. Bounds checks are of the form YYLIMIT <= YYCURSOR
or YYLESSTHAN(1) with generic API. If the check condition is true, lexer has reached the
end of input and should stop (YYFILL is disabled with re2c:yyfill:enable = 0; as the input
fits into one buffer, see the YYFILL with sentinel section for an example that uses
YYFILL). Reaching the end of input opens three possibilities: if the lexer is in the
initial state it will match the end-of-input rule $, otherwise it may fallback to a
previously matched rule (including default rule *) or go to a default state, causing
-Wundefined-control-flow.
// re2rust $INPUT -o $OUTPUT
// Expect a null-terminated string.
fn lex(s: &[u8]) -> isize {
let (mut cur, mut mar) = (0, 0);
let lim = s.len() - 1; // null-terminator not included
let mut count = 0;
'lex: loop {/*!re2c
re2c:define:YYCTYPE = u8;
re2c:define:YYPEEK = "*s.get_unchecked(cur)";
re2c:define:YYSKIP = "cur += 1;";
re2c:define:YYBACKUP = "mar = cur;";
re2c:define:YYRESTORE = "cur = mar;";
re2c:define:YYLESSTHAN = "cur >= lim";
re2c:yyfill:enable = 0;
re2c:eof = 0;
str = ['] ([^'\\] | [\\][^])* ['];
* { return -1; }
$ { return count; }
str { count += 1; continue 'lex; }
[ ]+ { continue 'lex; }
*/}
}
fn main() {
assert_eq!(lex(b"\0"), 0);
assert_eq!(lex(b"'qu\0tes' 'are' 'fine: \\'' \0"), 3);
assert_eq!(lex(b"'unterminated\\'\0"), -1);
}
Bounds checks with padding
This example uses bounds checks with padding to handle the end of input (this method is
enabled by default). The program counts space-separated single-quoted strings. There is a
padding of YYMAXFILL null characters appended at the end of input, where YYMAXFILL value
is autogenerated with /*!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 suffix (in this example
padding should not contain single quotes, as they may be mistaken for a suffix of a
single-quoted string). There is a "stop" rule that matches the first padding character
(null) and terminates the lexer (note that it checks if null is at the beginning of
padding, otherwise it is a syntax error). Bounds checks are generated only in some states
that are determined by the strongly connected components of the underlying automaton.
Checks have the form (YYLIMIT - YYCURSOR) < n or YYLESSTHAN(n) with generic API, where n
is the minimum number of characters that are needed for the lexer to proceed (it also
means that the next bounds check will occur in at most n characters). If the check
condition is true, the lexer has reached the end of input and will invoke YYFILL(n) that
should either supply at least n input characters or not return. In this example YYFILL
always fails and terminates the lexer with an error (which is fine because the input fits
into one buffer). See the YYFILL with padding section for an example that refills the
input buffer with YYFILL.
// re2rust $INPUT -o $OUTPUT
/*!max:re2c*/
fn lex(s: &[u8]) -> isize {
let mut count = 0;
let mut cur = 0;
let lim = s.len() + YYMAXFILL;
// Copy string to a buffer and add YYMAXFILL zero padding.
let mut buf = Vec::with_capacity(lim);
buf.extend(s.iter());
buf.extend(vec![0; YYMAXFILL]);
'lex: loop {/*!re2c
re2c:define:YYCTYPE = u8;
re2c:define:YYPEEK = "*buf.get_unchecked(cur)";
re2c:define:YYSKIP = "cur += 1;";
re2c:define:YYFILL = "return -1;";
re2c:define:YYLESSTHAN = "cur + @@ > lim";
str = ['] ([^'\\] | [\\][^])* ['];
[\x00] {
// Check that it is the sentinel, not some unexpected null.
return if cur == s.len() + 1 { count } else { -1 }
}
str { count += 1; continue 'lex; }
[ ]+ { continue 'lex; }
* { return -1; }
*/}
}
fn main() {
assert_eq!(lex(b""), 0);
assert_eq!(lex(b"'qu\0tes' 'are' 'fine: \\'' "), 3);
assert_eq!(lex(b"'unterminated\\'"), -1);
assert_eq!(lex(b"'unexpected \0 null"), -1);
}
Custom checks
This example uses a custom end-of-input handling method based on generic API. The program
counts space-separated single-quoted strings. It is the same as the sentinel 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 absence 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 condition fails, YYPEEK returns the real character, otherwise it
returns a fake sentinel character.
// re2rust $INPUT -o $OUTPUT
// Expect a string without terminating null.
fn lex(s: &[u8]) -> isize {
let (mut cur, mut mar) = (0, 0);
let lim = s.len();
let mut count = 0;
'lex: loop {/*!re2c
re2c:define:YYCTYPE = u8;
re2c:define:YYPEEK = "if cur < lim {*s.get_unchecked(cur)} else {0}";
re2c:define:YYSKIP = "cur += 1;";
re2c:define:YYBACKUP = "mar = cur;";
re2c:define:YYRESTORE = "cur = mar;";
re2c:define:YYLESSTHAN = "cur >= lim";
re2c:yyfill:enable = 0;
re2c:eof = 0;
str = ['] ([^'\\] | [\\][^])* ['];
* { return -1; }
$ { return count; }
str { count += 1; continue 'lex; }
[ ]+ { continue 'lex; }
*/}
}
fn main() {
assert_eq!(lex(b""), 0);
assert_eq!(lex(b"'qu\0tes' 'are' 'fine: \\'' "), 3);
assert_eq!(lex(b"'unterminated\\'"), -1);
}
BUFFER REFILLING
The need for buffering arises when the input cannot be mapped in memory all at once:
either it is too large, or it comes in a streaming fashion (like reading from a socket).
The usual technique in such cases is to allocate a fixed-sized memory buffer and process
input in chunks that fit into the buffer. When the current chunk is processed, it is moved
out and new data is moved in. In practice it is somewhat more complex, because lexer state
consists not of a single input position, but a set of interrelated positions:
• cursor: the next input character to be read (YYCURSOR in C pointer API or YYSKIP/YYPEEK
in generic API)
• limit: the position after the last available input character (YYLIMIT in C pointer API,
implicitly handled by YYLESSTHAN in generic API)
• marker: the position of the most recent match, if any (YYMARKER in default API or
YYBACKUP/YYRESTORE in generic API)
• token: the start of the current lexeme (implicit in re2c API, as it is not needed for
the normal lexer operation and can be defined and updated by the user)
• context marker: the position of the trailing context (YYCTXMARKER in C pointer API or
YYBACKUPCTX/YYRESTORECTX in generic API)
• tag variables: submatch positions (defined with /*!stags:re2c*/ and /*!mtags:re2c*/
directives and YYSTAGP/YYSTAGN/YYMTAGP/YYMTAGN in generic API)
Not all these are used in every case, but if used, they must be updated by YYFILL. All
active positions are contained in the segment between token and cursor, therefore
everything between buffer start and token can be discarded, the segment from token and up
to limit should be moved to the beginning of buffer, and the free space at the end of
buffer should be filled with new data. In order to avoid frequent YYFILL calls it is best
to fill in as many input characters as possible (even though fewer characters might
suffice to resume the lexer). The details of YYFILL implementation are slightly different
depending on which EOF handling method is used: the case of EOF rule is somewhat simpler
than the case of bounds-checking with padding. Also note that if -f --storable-state
option is used, YYFILL has slightly different semantics (described in the section about
storable state).
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.
// re2rust $INPUT -o $OUTPUT
use std::fs::File;
use std::io::{Read, Write};
const BUFSIZE: usize = 4096;
struct State {
file: File,
buf: [u8; BUFSIZE],
lim: usize,
cur: usize,
mar: usize,
tok: usize,
eof: bool,
}
#[derive(PartialEq)]
enum Fill { Ok, Eof, LongLexeme }
fn fill(st: &mut State) -> Fill {
if st.eof { return Fill::Eof; }
// Error: lexeme too long. In real life could reallocate a larger buffer.
if st.tok < 1 { return Fill::LongLexeme; }
// Shift buffer contents (discard everything up to the current token).
st.buf.copy_within(st.tok..st.lim, 0);
st.lim -= st.tok;
st.cur -= st.tok;
st.mar = st.mar.overflowing_sub(st.tok).0; // may underflow if marker is unused
st.tok = 0;
// Fill free space at the end of buffer with new data from file.
match st.file.read(&mut st.buf[st.lim..BUFSIZE - 1]) { // -1 for sentinel
Ok(n) => {
st.lim += n;
st.eof = n == 0; // end of file
st.buf[st.lim] = 0; // append sentinel
}
Err(why) => panic!("cannot read from file: {}", why)
}
return Fill::Ok;
}
fn lex(st: &mut State) -> isize {
let mut count: isize = 0;
'lex: loop {
st.tok = st.cur;
/*!re2c
re2c:define:YYCTYPE = u8;
re2c:define:YYPEEK = "*st.buf.get_unchecked(st.cur)";
re2c:define:YYSKIP = "st.cur += 1;";
re2c:define:YYBACKUP = "st.mar = st.cur;";
re2c:define:YYRESTORE = "st.cur = st.mar;";
re2c:define:YYLESSTHAN = "st.cur >= st.lim";
re2c:define:YYFILL = "fill(st) == Fill::Ok";
re2c:eof = 0;
str = ['] ([^'\\] | [\\][^])* ['];
* { return -1; }
$ { return count; }
str { count += 1; continue 'lex; }
[ ]+ { continue 'lex; }
*/}
}
fn main() {
let fname = "input";
let content = b"'qu\0tes' 'are' 'fine: \\'' ";
// Prepare input file: a few times the size of the buffer, containing
// strings with zeroes and escaped quotes.
match File::create(fname) {
Err(why) => panic!("cannot open {}: {}", fname, why),
Ok(mut file) => match file.write_all(&content.repeat(BUFSIZE)) {
Err(why) => panic!("cannot write to {}: {}", fname, why),
Ok(_) => {}
}
};
let count = 3 * BUFSIZE; // number of quoted strings written to file
// Reopen input file for reading.
let file = match File::open(fname) {
Err(why) => panic!("cannot read file {}: {}", fname, why),
Ok(file) => file,
};
// Initialize lexer state: all offsets are at the end of buffer.
let lim = BUFSIZE - 1;
let mut st = State {
file: file,
// Sentinel (at `lim` offset) is set to null, which triggers YYFILL.
buf: [0; BUFSIZE],
lim: lim,
cur: lim,
mar: lim,
tok: lim,
eof: false,
};
// Run the lexer.
assert_eq!(lex(&mut st), count as isize);
// Cleanup: remove input file.
match std::fs::remove_file(fname) {
Err(why) => panic!("cannot remove {}: {}", fname, why),
Ok(_) => {}
}
}
YYFILL with padding
In the default case (when EOF rule is not used) YYFILL is a function-like primitive that
accepts a single argument and does not return any value. YYFILL invocation is triggered
by condition (YYLIMIT - YYCURSOR) < n in C pointer API and YYLESSTHAN(n) in generic API.
The argument passed to YYFILL is the minimal number of characters that must be supplied.
If it fails to do so, YYFILL must not return to the lexer (for that reason it is best
implemented as a macro that returns from the calling function on failure). In case of a
successful YYFILL invocation the limit position must be set either to one after the last
input position in buffer, or to the end of YYMAXFILL padding (in case YYFILL has
successfully read at least n characters, but not enough to fill the entire buffer). The
pictures below show the relative locations of input positions in buffer before and after
YYFILL invocation (YYMAXFILL padding on the second picture is marked with # symbols).
<-- shift --> <-- need -->
>-A------------B---------C-----D-------E---F--------G->
buffer token marker cursor limit
>-A------------B---------C-----D-------E---F--------G->
buffer, marker cursor limit
token
<-- shift --> <-- need -->
>-A------------B---------C-----D-------E-F (EOF)
buffer token marker cursor limit
>-A------------B---------C-----D-------E-F###############
buffer, marker cursor limit
token <- YYMAXFILL ->
Here is an example of a program that reads input file input.txt in chunks of 4096 bytes
and uses bounds-checking with padding.
// re2rust $INPUT -o $OUTPUT
use std::fs::File;
use std::io::{Read, Write};
/*!max:re2c*/
const BUFSIZE: usize = 4096;
struct State {
file: File,
buf: [u8; BUFSIZE],
lim: usize,
cur: usize,
mar: usize,
tok: usize,
eof: bool,
}
#[derive(PartialEq)]
enum Fill { Ok, Eof, LongLexeme }
fn fill(st: &mut State, need: usize) -> Fill {
if st.eof { return Fill::Eof; }
// Error: lexeme too long. In real life can reallocate a larger buffer.
if st.tok < need { return Fill::LongLexeme; }
// Shift buffer contents (discard everything up to the current token).
st.buf.copy_within(st.tok..st.lim, 0);
st.lim -= st.tok;
st.cur -= st.tok;
st.mar = st.mar.overflowing_sub(st.tok).0; // underflows if marker is unused
st.tok = 0;
// Fill free space at the end of buffer with new data from file.
let n = match st.file.read(&mut st.buf[st.lim..BUFSIZE - YYMAXFILL]) {
Ok(n) => n,
Err(why) => panic!("cannot read from file: {}", why)
};
st.lim += n;
// If read zero characters, this is end of input => add zero padding
// so that the lexer can access characters at the end of buffer.
if n == 0 {
st.eof = true;
for i in 0..YYMAXFILL { st.buf[st.lim + i] = 0; }
st.lim += YYMAXFILL;
}
return Fill::Ok;
}
fn lex(st: &mut State) -> isize {
let mut count: isize = 0;
'lex: loop {
st.tok = st.cur;
/*!re2c
re2c:define:YYCTYPE = u8;
re2c:define:YYPEEK = "*st.buf.get_unchecked(st.cur)";
re2c:define:YYSKIP = "st.cur += 1;";
re2c:define:YYBACKUP = "st.mar = st.cur;";
re2c:define:YYRESTORE = "st.cur = st.mar;";
re2c:define:YYLESSTHAN = "st.lim - st.cur < @@";
re2c:define:YYFILL = "if fill(st, @@) != Fill::Ok { return -1; }";
str = ['] ([^'\\] | [\\][^])* ['];
[\x00] {
// Check that it is the sentinel, not some unexpected null.
return if st.tok == st.lim - YYMAXFILL { count } else { -1 }
}
str { count += 1; continue 'lex; }
[ ]+ { continue 'lex; }
* { return -1; }
*/}
}
fn main() {
let fname = "input";
let content = b"'qu\0tes' 'are' 'fine: \\'' ";
// Prepare input file: a few times the size of the buffer, containing
// strings with zeroes and escaped quotes.
match File::create(fname) {
Err(why) => panic!("cannot open {}: {}", fname, why),
Ok(mut file) => match file.write_all(&content.repeat(BUFSIZE)) {
Err(why) => panic!("cannot write to {}: {}", fname, why),
Ok(_) => {}
}
};
let count = 3 * BUFSIZE; // number of quoted strings written to file
// Reopen input file for reading.
let file = match File::open(fname) {
Err(why) => panic!("cannot read file {}: {}", fname, why),
Ok(file) => file,
};
// Initialize lexer state: all offsets are at the end of buffer.
// This immediately triggers YYFILL, as the YYLESSTHAN condition is true.
let lim = BUFSIZE - YYMAXFILL;
let mut st = State {
file: file,
buf: [0; BUFSIZE],
lim: lim,
cur: lim,
mar: lim,
tok: lim,
eof: false,
};
// Run the lexer.
assert_eq!(lex(&mut st), count as isize);
// Cleanup: remove input file.
match std::fs::remove_file(fname) {
Err(why) => panic!("cannot remove {}: {}", fname, why),
Ok(_) => {}
}
}
MULTIPLE BLOCKS
Sometimes it is necessary to have multiple interrelated lexers (for example, if there is a
high-level state machine that transitions between lexer modes). This can be implemented
using multiple connected re2c blocks. Another option is to use start conditions.
The implementation of connections between blocks depends on the target language. In
languages that have goto statement (such as C/C++ and Go) one can have all blocks in one
function, each of them prefixed with a label. Transition from one block to another is a
simple goto. In languages that do not have goto (such as Rust) it is necessary to use a
loop with a switch on a state variable, similar to the yystate loop/switch generated by
re2c, or else wrap each block in a function and use function calls.
The example below uses multiple blocks to parse binary, octal, decimal and hexadecimal
numbers. Each base has its own block. The initial block determines base and dispatches to
other blocks. Common configurations are defined in a separate block at the beginning of
the program; they are inherited by the other blocks.
// re2rust $INPUT -o $OUTPUT
// Store u32 number in u64 during parsing to simplify overflow hadling.
struct State<'a> {
str: &'a [u8],
cur: usize,
mar: usize,
num: u64,
}
/*!re2c // Common re2c definitions shared between all functions.
re2c:yyfill:enable = 0;
re2c:define:YYCTYPE = u8;
re2c:define:YYPEEK = "*st.str.get_unchecked(st.cur)";
re2c:define:YYSKIP = "st.cur += 1;";
re2c:define:YYBACKUP = "st.mar = st.cur;";
re2c:define:YYRESTORE = "st.cur = st.mar;";
re2c:define:YYSHIFT = "st.cur = (st.cur as isize + @@) as usize;";
*/
const ERROR: u64 = std::u32::MAX as u64 + 1; // overflow
macro_rules! maybe { // Convert the number from u64 to optional u32.
($n:expr) => { if $n < ERROR { Some($n as u32) } else { None } }
}
// Add digit with the given base, checking for overflow.
fn add(st: &mut State, offs: u8, base: u64) {
let digit = unsafe { st.str.get_unchecked(st.cur - 1) } - offs;
st.num = std::cmp::min(st.num * base + digit as u64, ERROR);
}
fn parse_u32(s: & [u8]) -> Option<u32> {
let mut st = State {str: s, cur: 0, mar: 0, num: 0};
/*!re2c
'0b' / [01] { return parse_bin(&mut st); }
"0" { return parse_oct(&mut st); }
"" / [1-9] { return parse_dec(&mut st); }
'0x' / [0-9a-fA-F] { return parse_hex(&mut st); }
* { return None; }
*/
}
fn parse_bin(st: &mut State) -> Option<u32> {
'bin: loop {/*!re2c
[01] { add(st, 48, 2); continue 'bin; }
* { return maybe!(st.num); }
*/}
}
fn parse_oct(st: &mut State) -> Option<u32> {
'oct: loop {/*!re2c
[0-7] { add(st, 48, 8); continue 'oct; }
* { return maybe!(st.num); }
*/}
}
fn parse_dec(st: &mut State) -> Option<u32> {
'dec: loop {/*!re2c
[0-9] { add(st, 48, 10); continue 'dec; }
* { return maybe!(st.num); }
*/}
}
fn parse_hex(st: &mut State) -> Option<u32> {
'hex: loop {/*!re2c
[0-9] { add(st, 48, 16); continue 'hex; }
[a-f] { add(st, 87, 16); continue 'hex; }
[A-F] { add(st, 55, 16); continue 'hex; }
* { return maybe!(st.num); }
*/}
}
fn main() {
assert_eq!(parse_u32(b"\0"), None);
assert_eq!(parse_u32(b"1234567890\0"), Some(1234567890));
assert_eq!(parse_u32(b"0b1101\0"), Some(13));
assert_eq!(parse_u32(b"0x7Fe\0"), Some(2046));
assert_eq!(parse_u32(b"0644\0"), Some(420));
assert_eq!(parse_u32(b"9999999999\0"), None);
}
START CONDITIONS
Start conditions are enabled with --start-conditions option. They provide a way to encode
multiple interrelated automata within the same re2c block.
Each condition corresponds to a single automaton and has a unique name specified by the
user and a unique internal number defined by re2c. The numbers are used to switch between
conditions: the generated code uses YYGETCONDITION and YYSETCONDITION primitives to get
the current condition or set it to the given number. Use /*!conditions:re2c*/ directive or
the --header option to generate numeric condition identifiers. Configuration
re2c:cond:enumprefix specifies the generated identifier prefix.
In condition mode every rule must be prefixed with a list of comma-separated condition
names in angle brackets, or a wildcard <*> to denote all conditions. The rule syntax is
extended as follows:
< cond-list > regexp action
A rule that is merged to every condition on the cond-list. It matches regexp
and executes the associated action.
< cond-list > regexp => cond action
A rule that is merged to every condition on the cond-list. It matches regexp,
sets the current condition to cond and executes the associated action.
< cond-list > regexp :=> cond
A rule that is merged to every condition on the cond-list. It matches regexp
and immediately transitions to cond (there is no semantic action).
<! cond-list > action
The action is prepended to semantic actions of all rules for every condition on
the cond-list. This may be used to deduplicate common code.
< > action
A rule that is merged to a special entry condition with number zero and name
"0". It matches empty string and executes the action.
< > => cond action
A rule that is merged to a special entry condition with number zero and name
"0". It matches empty string, sets the current condition to cond and executes
the action.
< > :=> cond
A rule that is merged to a special entry condition with number zero and name
"0". It matches empty string and immediately transitions to cond.
The code re2c generates for conditions depends on whether re2c uses goto/label approach or
loop/switch approach to encode the automata.
In languages that have goto statement (such as C/C++ and Go) conditions are naturally
implemented as blocks of code prefixed with labels of the form yyc_<cond>, where cond is a
condition name (label prefix can be changed with re2c:cond:prefix). Transitions between
conditions are implemented using goto and condition labels. Before all conditions re2c
generates an initial switch on YYGETSTATE that jumps to the start state of the current
condition. The shortcut rules :=> bypass the initial switch and jump directly to the
specified condition (re2c:cond:goto can be used to change the default behavior). The rules
with semantic actions do not automatically jump to the next condition; this should be done
by the user-defined action code.
In languages that do not have goto (such as Rust) re2c reuses the yystate variable to
store condition numbers. Each condition gets a numeric identifier equal to the number of
its start state, and a switch between conditions is no different than a switch between DFA
states of a single condition. There is no need for a separate initial condition switch.
(Since the same approach is used to implement storable states,
YYGETCONDITION/YYSETCONDITION are redundant if both storable states and conditions are
used).
The program below uses start conditions to parse binary, octal, decimal and hexadecimal
numbers. There is a single block where each base has its own condition, and the initial
condition is connected to all of them. User-defined variable cond stores the current
condition number; it is initialized to the number of the initial condition generated with
/*!conditions:re2c*/.
// re2rust $INPUT -o $OUTPUT -c
/*!conditions:re2c*/
const ERROR: u64 = std::u32::MAX as u64 + 1; // overflow
// Add digit with the given base, checking for overflow.
fn add(num: &mut u64, str: &[u8], cur: usize, offs: u8, base: u64) {
let digit = unsafe { str.get_unchecked(cur - 1) } - offs;
*num = std::cmp::min(*num * base + digit as u64, ERROR);
}
fn parse_u32(str: &[u8]) -> Option<u32> {
let (mut cur, mut mar) = (0, 0);
let mut cond = YYC_INIT;
let mut num = 0u64; // Store number in u64 to simplify overflow checks.
'lex: loop {/*!re2c
re2c:define:YYCTYPE = u8;
re2c:define:YYPEEK = "*str.get_unchecked(cur)";
re2c:define:YYSKIP = "cur += 1;";
re2c:define:YYBACKUP = "mar = cur;";
re2c:define:YYRESTORE = "cur = mar;";
re2c:define:YYSHIFT = "cur = (cur as isize + @@) as usize;";
re2c:define:YYGETCONDITION = "cond";
re2c:define:YYSETCONDITION = "cond = @@;";
re2c:yyfill:enable = 0;
<INIT> '0b' / [01] :=> BIN
<INIT> "0" :=> OCT
<INIT> "" / [1-9] :=> DEC
<INIT> '0x' / [0-9a-fA-F] :=> HEX
<INIT> * { return None; }
<BIN> [01] { add(&mut num, str, cur, 48, 2); continue 'lex; }
<OCT> [0-7] { add(&mut num, str, cur, 48, 8); continue 'lex; }
<DEC> [0-9] { add(&mut num, str, cur, 48, 10); continue 'lex; }
<HEX> [0-9] { add(&mut num, str, cur, 48, 16); continue 'lex; }
<HEX> [a-f] { add(&mut num, str, cur, 87, 16); continue 'lex; }
<HEX> [A-F] { add(&mut num, str, cur, 55, 16); continue 'lex; }
<BIN, OCT, DEC, HEX> * {
return if num < ERROR { Some(num as u32) } else { None };
}
*/}
}
fn main() {
assert_eq!(parse_u32(b"\0"), None);
assert_eq!(parse_u32(b"1234567890\0"), Some(1234567890));
assert_eq!(parse_u32(b"0b1101\0"), Some(13));
assert_eq!(parse_u32(b"0x7Fe\0"), Some(2046));
assert_eq!(parse_u32(b"0644\0"), Some(420));
assert_eq!(parse_u32(b"9999999999\0"), None);
}
STORABLE STATE
With --storable-state option re2c generates a lexer that can store its current state,
return to the caller, and later resume operations exactly where it left off. The default
mode of operation in re2c is a "pull" model, in which the lexer "pulls" more input
whenever it needs it. This may be unacceptable in cases when the input becomes available
piece by piece (for example, if the lexer is invoked by the parser, or if the lexer
program communicates via a socket protocol with some other program that must wait for a
reply from the lexer before it transmits the next message). Storable state feature is
intended exactly for such cases: it allows one to generate lexers that work in a "push"
model. When the lexer needs more input, it stores its state and returns to the caller.
Later, when more input becomes available, the caller resumes the lexer exactly where it
stopped. There are a few changes necessary compared to the "pull" model:
• Define YYSETSTATE() and YYGETSTATE(state) primitives.
• Define yych, yyaccept (if used) and state variables as a part of persistent lexer state.
The state variable should be initialized to -1.
• YYFILL should return to the outer program instead of trying to supply more input. Return
code should indicate that lexer needs more input.
• The outer program should recognize situations when lexer needs more input and respond
appropriately.
• Optionally use getstate:re2c to generate YYGETSTATE switch detached from the main lexer.
This only works for languages that have goto (not in --loop-switch mode).
• Use re2c:eof and the sentinel with bounds checks method to handle the end of input.
Padding-based method may not work because it is unclear when to append padding: the
current end of input may not be the ultimate end of input, and appending padding too
early may cut off a partially read greedy lexeme. Furthermore, due to high-level
program logic getting more input may depend on processing the lexeme at the end of
buffer (which already is blocked due to the end-of-input condition).
Here is an example of a "push" model lexer that simulates reading packets from a socket.
The lexer loops until it encounters the end of input and returns to the calling function.
The calling function provides more input by "sending" the next packet and resumes lexing.
This process stops when all the packets have been sent, or when there is an error.
// re2rust $INPUT -o $OUTPUT -f
use std::fs::File;
use std::io::{Read, Write};
const DEBUG: bool = false;
macro_rules! log {
($($fmt:expr)? $(, $args:expr)*) => {
if DEBUG { println!($($fmt)? $(, $args)*) }
}
}
// Use a small buffer to cover the case when a lexeme doesn't fit.
// In real world use a larger buffer.
const BUFSIZE: usize = 10;
struct State {
file: File,
buf: [u8; BUFSIZE],
lim: usize,
cur: usize,
mar: usize,
tok: usize,
state: isize,
}
#[derive(Debug, PartialEq)]
enum Status {End, Ready, Waiting, BadPacket, BigPacket}
fn fill(st: &mut State) -> Status {
// Error: lexeme too long. In real life can reallocate a larger buffer.
if st.tok < 1 { return Status::BigPacket; }
// Shift buffer contents (discard everything up to the current lexeme).
st.buf.copy_within(st.tok..st.lim, 0);
st.lim -= st.tok;
st.cur -= st.tok;
st.mar = st.mar.overflowing_sub(st.tok).0; // underflows if marker is unused
st.tok = 0;
// Fill free space at the end of buffer with new data.
match st.file.read(&mut st.buf[st.lim..BUFSIZE - 1]) { // -1 for sentinel
Ok(n) => {
st.lim += n;
st.buf[st.lim] = 0; // append sentinel symbol
},
Err(why) => panic!("cannot read from file: {}", why)
}
return Status::Ready;
}
fn lex(st: &mut State, recv: &mut usize) -> Status {
let mut yych;
'lex: loop {
st.tok = st.cur;
/*!re2c
re2c:eof = 0;
re2c:define:YYCTYPE = "u8";
re2c:define:YYPEEK = "*st.buf.get_unchecked(st.cur)";
re2c:define:YYSKIP = "st.cur += 1;";
re2c:define:YYBACKUP = "st.mar = st.cur;";
re2c:define:YYRESTORE = "st.cur = st.mar;";
re2c:define:YYLESSTHAN = "st.cur >= st.lim";
re2c:define:YYGETSTATE = "st.state";
re2c:define:YYSETSTATE = "st.state = @@;";
re2c:define:YYFILL = "return Status::Waiting;";
packet = [a-z]+[;];
* { return Status::BadPacket; }
$ { return Status::End; }
packet { *recv += 1; continue 'lex; }
*/}
}
fn test(packets: Vec<&[u8]>, expect: Status) {
// Create a "socket" (open the same file for reading and writing).
let fname = "pipe";
let mut fw: File = match File::create(fname) {
Err(why) => panic!("cannot open {}: {}", fname, why),
Ok(file) => file,
};
let fr: File = match File::open(fname) {
Err(why) => panic!("cannot read file {}: {}", fname, why),
Ok(file) => file,
};
// Initialize lexer state: `state` value is -1, all offsets are at the end
// of buffer, the character at `lim` offset is the sentinel (null).
let lim = BUFSIZE - 1;
let mut state = State {
file: fr,
// Sentinel (at `lim` offset) is set to null, which triggers YYFILL.
buf: [0; BUFSIZE],
cur: lim,
mar: lim,
tok: lim,
lim: lim,
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.
let mut status;
let mut send = 0;
let mut recv = 0;
loop {
status = lex(&mut state, &mut recv);
if status == Status::End {
log!("done: got {} packets", recv);
break;
} else if status == Status::Waiting {
log!("waiting...");
if send < packets.len() {
log!("sent packet {}", send);
match fw.write_all(packets[send]) {
Err(why) => panic!("cannot write to {}: {}", fname, why),
Ok(_) => send += 1,
}
}
status = fill(&mut state);
log!("queue: '{}'", String::from_utf8_lossy(&state.buf));
if status == Status::BigPacket {
log!("error: packet too big");
break;
}
assert_eq!(status, Status::Ready);
} else {
assert_eq!(status, Status::BadPacket);
log!("error: ill-formed packet");
break;
}
}
// Check results.
assert_eq!(status, expect);
if status == Status::End { assert_eq!(recv, send); }
// Cleanup: remove input file.
match std::fs::remove_file(fname) {
Err(why) => panic!("cannot remove {}: {}", fname, why),
Ok(_) => {}
}
}
fn main() {
test(vec![], Status::End);
test(vec![b"zero;", b"one;", b"two;", b"three;", b"four;"], Status::End);
test(vec![b"zer0;"], Status::BadPacket);
test(vec![b"goooooooooogle;"], Status::BigPacket);
}
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
directive inside of another block. The code for a rules block is generated at every point
of use.
Use blocks are defined with /*!use:re2c[:<name>] ... */. The <name> is optional; if not
specified, the associated rules block is the most recent one (whether named or unnamed). A
use block can add named definitions, configurations and rules of its own. An important
use case for use blocks is a lexer that supports multiple input encodings: the same rules
block is reused multiple times with encoding-specific configurations (see the example
below).
In-block use directive !use:<name>; can be used from inside of a re2c block. It merges the
referenced block <name> into the current one. If some of the merged rules and
configurations overlap with the previously defined ones, conflicts are resolved in the
usual way: the earliest rule takes priority, and latest configuration overrides preceding
ones. One exception are the special rules *, $ and (in condition mode) <!>, for which a
block-local definition overrides any inherited ones. Use directive allows one to combine
different re2c blocks together in one block (see the example below).
Named blocks and in-block use directive were added in re2c version 2.2. Since that
version reusable blocks are allowed by default (no special option is needed). Before
version 2.2 reuse mode was enabled with -r --reusable option. Before version 1.2 reusable
blocks could not be mixed with normal blocks.
Example of a !use directive
// re2rust $INPUT -o $OUTPUT
// 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.
#[derive(Debug, PartialEq)]
enum Ans { Color, Fish, Dunno }
/*!rules:re2c:colors
* { panic!("ah"); }
"red" | "salmon" | "magenta" { return Ans::Color; }
*/
/*!rules:re2c:fish
* { panic!("oh"); }
"haddock" | "salmon" | "eel" { return Ans::Fish; }
*/
fn lex(str: &[u8]) -> Ans {
let (mut cur, mut mar) = (0, 0);
/*!re2c
re2c:yyfill:enable = 0;
re2c:define:YYCTYPE = u8;
re2c:define:YYPEEK = "*str.get_unchecked(cur)";
re2c:define:YYSKIP = "cur += 1;";
re2c:define:YYBACKUP = "mar = cur;";
re2c:define:YYRESTORE = "cur = mar;";
!use:fish;
!use:colors;
* { return Ans::Dunno; } // overrides inherited '*' rules
*/
}
fn main() {
assert_eq!(lex(b"salmon"), Ans::Fish);
assert_eq!(lex(b"what?"), Ans::Dunno);
}
Example of a /*!use:re2c ... */ block
// re2rust $INPUT -o $OUTPUT --input-encoding utf8
// This example supports multiple input encodings: UTF-8 and UTF-32.
// Both lexers are generated from the same rules block, and the use
// blocks add only encoding-specific configurations.
/*!rules:re2c
re2c:yyfill:enable = 0;
re2c:define:YYPEEK = "*str.get_unchecked(cur)";
re2c:define:YYSKIP = "cur += 1;";
re2c:define:YYBACKUP = "mar = cur;";
re2c:define:YYRESTORE = "cur = mar;";
"∀x ∃y" { return Some(cur); }
* { return None; }
*/
fn lex_utf8(str: &[u8]) -> Option<usize> {
let (mut cur, mut mar) = (0, 0);
/*!use:re2c
re2c:encoding:utf8 = 1;
re2c:define:YYCTYPE = u8;
*/
}
fn lex_utf32(str: &[u32]) -> Option<usize> {
let (mut cur, mut mar) = (0, 0);
/*!use:re2c
re2c:encoding:utf32 = 1;
re2c:define:YYCTYPE = u32;
*/
}
fn main() {
let s8 = vec![0xe2, 0x88, 0x80, 0x78, 0x20, 0xe2, 0x88, 0x83, 0x79];
assert_eq!(lex_utf8(&s8), Some(s8.len()));
let s32 = vec![0x2200, 0x78, 0x20, 0x2203, 0x79];
assert_eq!(lex_utf32(&s32), Some(s32.len()));
}
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 expression; semantically they are just position markers. Tags
of the form @stag are called s-tags: they denote a single submatch value (the last input
position where this tag matched). Tags of the form #mtag are called m-tags: they denote
multiple submatch values (the whole history of repetitions of this tag). All tags should
be defined by the user as variables with the corresponding names. With standalone tags
re2c uses leftmost greedy disambiguation: submatch positions correspond to the leftmost
matching path through the regular expression.
The second option is -P --posix-captures: it enables POSIX-compliant capturing groups. In
this mode parentheses in regular expressions denote the beginning and the end of capturing
groups; the whole regular expression is group number zero. The number of groups for the
matching rule is stored in a variable yynmatch, and submatch results are stored in
yypmatch array. Both yynmatch and yypmatch should be defined by the user, and yypmatch
size must be at least [yynmatch * 2]. re2c provides a directive /*!maxnmatch:re2c*/ that
defines YYMAXNMATCH: a constant equal to the maximal value of yynmatch among all rules.
Note that re2c implements POSIX-compliant disambiguation: each subexpression matches as
long as possible, and subexpressions that start earlier in regular expression have
priority over those starting later. Capturing groups are translated into s-tags under the
hood, therefore we use the word "tag" to describe them as well.
With both -P --posix-captures and T --tags options re2c uses efficient submatch extraction
algorithm described in the Tagged Deterministic Finite Automata with Lookahead paper. The
overhead on submatch extraction in the generated lexer grows with the number of tags ---
if this number is moderate, the overhead is barely noticeable. In the lexer tags are
implemented using a number of tag variables generated by re2c. There is no one-to-one
correspondence between tag variables and tags: a single variable may be reused for
different tags, and one tag may require multiple variables to hold all its ambiguous
values. Eventually ambiguity is resolved, and only one final variable per tag survives.
When a rule matches, all its tags are set to the values of the corresponding tag
variables. The exact number of tag variables is unknown to the user; this number is
determined by re2c. However, tag variables should be defined by the user as a part of the
lexer state and updated by YYFILL, therefore re2c provides directives /*!stags:re2c*/ and
/*!mtags:re2c*/ that can be used to declare, initialize and manipulate tag variables.
These directives have two optional configurations: format = "@@"; (specifies the template
where @@ is substituted with the name of each tag variable), and separator = "";
(specifies the piece of code used to join the generated pieces for different tag
variables).
S-tags support the following operations:
• save input position to an s-tag: t = YYCURSOR with C pointer API or a user-defined
operation YYSTAGP(t) with generic API
• save default value to an s-tag: t = NULL with C pointer API or a user-defined operation
YYSTAGN(t) with generic API
• copy one s-tag to another: t1 = t2
M-tags support the following operations:
• append input position to an m-tag: a user-defined operation YYMTAGP(t) with both default
and generic API
• append default value to an m-tag: a user-defined operation YYMTAGN(t) with both default
and generic API
• copy one m-tag to another: t1 = t2
S-tags can be implemented as scalar values (pointers or offsets). M-tags need a more
complex representation, as they need to store a sequence of tag values. The most naive and
inefficient representation of an m-tag is a list (array, vector) of tag values; a more
efficient representation is to store all m-tags in a prefix-tree represented as array of
nodes (v, p), where v is tag value and p is a pointer to parent node.
Here is a simple example of using s-tags to parse semantic versions consisting of three
numeric components: major, minor, patch (the latter is optional). See below for a more
complex example that uses YYFILL.
// re2rust $INPUT -o $OUTPUT
#[derive(Debug, PartialEq)]
struct SemVer(u32, u32, u32); // version: (major, minor, patch)
fn s2n(str: &[u8]) -> u32 { // convert a pre-parsed string to a number
let mut n = 0;
for i in str { n = n * 10 + *i as u32 - 48; }
return n;
}
fn parse(str: &[u8]) -> Option<SemVer> {
let (mut cur, mut mar) = (0, 0);
// User-defined tag variables that are available in semantic action.
let (t1, mut t2, t3, t4, t5);
// Autogenerated tag variables used by the lexer to track tag values.
const NONE: usize = std::usize::MAX;
/*!stags:re2c format = 'let mut @@{tag} = NONE;'; */
/*!re2c
re2c:define:YYCTYPE = u8;
re2c:define:YYPEEK = "*str.get_unchecked(cur)";
re2c:define:YYSKIP = "cur += 1;";
re2c:define:YYBACKUP = "mar = cur;";
re2c:define:YYRESTORE = "cur = mar;";
re2c:define:YYSTAGP = "@@{tag} = cur;";
re2c:define:YYSTAGN = "@@{tag} = NONE;";
re2c:define:YYSHIFTSTAG = "@@{tag} -= -@@{shift}isize as usize;";
re2c:yyfill:enable = 0;
re2c:tags = 1;
num = [0-9]+;
@t1 num @t2 "." @t3 num @t4 ("." @t5 num)? [\x00] {
let major = s2n(&str[t1..t2]);
let minor = s2n(&str[t3..t4]);
let patch = if t5 != NONE {s2n(&str[t5..cur - 1])} else {0};
return Some(SemVer(major, minor, patch));
}
* { return None; }
*/
}
fn main() {
assert_eq!(parse(b"23.34\0"), Some(SemVer(23, 34, 0)));
assert_eq!(parse(b"1.2.99999\0"), Some(SemVer(1, 2, 99999)));
assert_eq!(parse(b"1.a\0"), None);
}
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.
// re2rust $INPUT -o $OUTPUT
use std::fs::File;
use std::io::{Read, Write};
const BUFSIZE: usize = 4096;
const NONE: usize = std::usize::MAX;
struct State {
file: File,
buf: [u8; BUFSIZE],
lim: usize,
cur: usize,
mar: usize,
tok: usize,
// Tag variables must be part of the lexer state passed to YYFILL.
// They don't correspond to tags and should be autogenerated by re2c.
/*!stags:re2c format = "@@: usize,\n"; */
eof: bool,
}
#[derive(PartialEq)]
enum Fill { Ok, Eof, LongLexeme }
#[derive(Debug, PartialEq)]
struct SemVer(u32, u32, u32); // version: (major, minor, patch)
fn s2n(str: &[u8]) -> u32 { // convert a pre-parsed string to a number
let mut n = 0;
for i in str { n = n * 10 + *i as u32 - 48; }
return n;
}
macro_rules! shift { // ignore overflow, marker and tags may not be set yet
($x:expr, $y:expr) => { $x = $x.overflowing_sub($y).0 }
}
fn fill(st: &mut State) -> Fill {
if st.eof { return Fill::Eof; }
// Error: lexeme too long. In real life could reallocate a larger buffer.
if st.tok < 1 { return Fill::LongLexeme; }
// Shift buffer contents (discard everything up to the current token).
st.buf.copy_within(st.tok..st.lim, 0);
st.lim -= st.tok;
st.cur -= st.tok;
shift!(st.mar, st.tok);
// Tag variables need to be shifted like other input positions. The check
// for NONE is only needed if some tags are nested inside of alternative or
// repetition, so that they can have NONE value.
/*!stags:re2c format = "if st.@@ != NONE { shift!(st.@@, st.tok); }\n"; */
st.tok = 0;
// Fill free space at the end of buffer with new data from file.
match st.file.read(&mut st.buf[st.lim..BUFSIZE - 1]) {
Ok(n) => {
st.lim += n;
st.eof = n == 0;
st.buf[st.lim] = 0;
}
Err(why) => panic!("cannot read from file: {}", why)
}
return Fill::Ok;
}
fn parse(st: &mut State) -> Option<Vec::<SemVer>> {
let mut vers = Vec::new();
// 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.
let (mut t1, mut t2, mut t3, mut t4);
'parse: loop {
st.tok = st.cur;
/*!re2c
re2c:eof = 0;
re2c:define:YYCTYPE = u8;
re2c:define:YYPEEK = "*st.buf.get_unchecked(st.cur)";
re2c:define:YYSKIP = "st.cur += 1;";
re2c:define:YYBACKUP = "st.mar = st.cur;";
re2c:define:YYRESTORE = "st.cur = st.mar;";
re2c:define:YYSTAGP = "@@{tag} = st.cur;";
re2c:define:YYSTAGN = "@@{tag} = NONE;";
re2c:define:YYSHIFTSTAG = "@@{tag} -= -@@{shift}isize as usize;";
re2c:define:YYLESSTHAN = "st.cur >= st.lim";
re2c:define:YYFILL = "fill(st) == Fill::Ok";
re2c:tags = 1;
re2c:tags:expression = "st.@@";
num = [0-9]+;
num @t1 "." @t2 num @t3 ("." @t4 num)? [\n] {
let major = s2n(&st.buf[st.tok..t1]);
let minor = s2n(&st.buf[t2..t3]);
let patch = if t4 != NONE {s2n(&st.buf[t4..st.cur - 1])} else {0};
vers.push(SemVer(major, minor, patch));
continue 'parse;
}
$ { return Some(vers); }
* { return None; }
*/
}
}
fn main() {
let fname = "input";
let verstr = b"1.22.333\n";
let expect = (0..BUFSIZE).map(|_| SemVer(1, 22, 333)).collect();
// Prepare input file (make sure it exceeds buffer size).
match File::create(fname) {
Err(why) => panic!("cannot open {}: {}", fname, why),
Ok(mut file) => match file.write_all(&verstr.repeat(BUFSIZE)) {
Err(why) => panic!("cannot write to {}: {}", fname, why),
Ok(_) => {}
}
};
// Reopen input file for reading.
let file = match File::open(fname) {
Err(why) => panic!("cannot read file {}: {}", fname, why),
Ok(file) => file,
};
// Initialize lexer state.
let lim = BUFSIZE - 1;
let mut st = State {
file: file,
buf: [0; BUFSIZE], // sentinel is set to zero, which triggers YYFILL
lim: lim,
cur: lim,
mar: lim,
tok: lim,
/*!stags:re2c format = "@@: NONE,\n"; */
eof: false,
};
// Run the lexer and check results.
assert_eq!(parse(&mut st), Some(expect));
// Cleanup: remove input file.
match std::fs::remove_file(fname) {
Err(why) => panic!("cannot remove {}: {}", fname, why),
Ok(_) => {}
}
}
Here is an example of using POSIX capturing groups to parse semantic versions.
// re2rust $INPUT -o $OUTPUT
// Maximum number of capturing groups among all rules.
/*!maxnmatch:re2c*/
#[derive(Debug, PartialEq)]
struct SemVer(u32, u32, u32); // version: (major, minor, patch)
fn s2n(str: &[u8]) -> u32 { // convert a pre-parsed string to a number
let mut n = 0;
for i in str { n = n * 10 + *i as u32 - 48; }
return n;
}
fn parse(str: &[u8]) -> Option<SemVer> {
let (mut cur, mut mar) = (0, 0);
// Allocate memory for capturing parentheses (twice the number of groups).
let yynmatch: usize;
let mut yypmatch = [0; YYMAXNMATCH*2];
// Autogenerated tag variables used by the lexer to track tag values.
const NONE: usize = std::usize::MAX;
/*!stags:re2c format = 'let mut @@{tag} = NONE;'; */
/*!re2c
re2c:define:YYCTYPE = u8;
re2c:define:YYPEEK = "*str.get_unchecked(cur)";
re2c:define:YYSKIP = "cur += 1;";
re2c:define:YYBACKUP = "mar = cur;";
re2c:define:YYRESTORE = "cur = mar;";
re2c:define:YYSTAGP = "@@{tag} = cur;";
re2c:define:YYSTAGN = "@@{tag} = NONE;";
re2c:define:YYSHIFTSTAG = "@@{tag} -= -@@{shift}isize as usize;";
re2c:yyfill:enable = 0;
re2c:posix-captures = 1;
num = [0-9]+;
(num) "." (num) ("." num)? [\x00] {
// `yynmatch` is the number of capturing groups
assert_eq!(yynmatch, 4);
// Even `yypmatch` values are for opening parentheses, odd values
// are for closing parentheses, the first group is the whole match.
let major = s2n(&str[yypmatch[2]..yypmatch[3]]);
let minor = s2n(&str[yypmatch[4]..yypmatch[5]]);
let patch = if yypmatch[6] == NONE {0}
else {s2n(&str[yypmatch[6] + 1..yypmatch[7]])};
return Some(SemVer(major, minor, patch));
}
* { return None; }
*/
}
fn main() {
assert_eq!(parse(b"23.34\0"), Some(SemVer(23, 34, 0)));
assert_eq!(parse(b"1.2.99999\0"), Some(SemVer(1, 2, 99999)));
assert_eq!(parse(b"1.a\0"), None);
}
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.
// re2rust $INPUT -o $OUTPUT
const NONE: usize = std::usize::MAX;
const MTAG_ROOT: usize = NONE - 1;
// An m-tag tree is a way to store histories with an O(1) copy operation.
// Histories naturally form a tree, as they have common start and fork at some
// point. The tree is stored as an array of pairs (tag value, link to parent).
// An m-tag is represented with a single link in the tree (array index).
type MtagTrie = Vec::<MtagElem>;
struct MtagElem {
elem: usize, // tag value
pred: usize, // index of the predecessor node or root
}
// Append a single value to an m-tag history.
fn add_mtag(trie: &mut MtagTrie, mtag: usize, value: usize) -> usize {
trie.push(MtagElem{elem: value, pred: mtag});
return trie.len() - 1;
}
// Recursively unwind tag histories and collect version components.
fn unwind(trie: &MtagTrie, x: usize, y: usize, str: &[u8], ver: &mut Ver) {
// Reached the root of the m-tag tree, stop recursion.
if x == MTAG_ROOT && y == MTAG_ROOT { return; }
// Unwind history further.
unwind(trie, trie[x].pred, trie[y].pred, str, ver);
// Get tag values. Tag histories must have equal length.
assert!(x != MTAG_ROOT && y != MTAG_ROOT);
let (ex, ey) = (trie[x].elem, trie[y].elem);
if ex != NONE && ey != NONE {
// Both tags are valid string indices, extract component.
ver.push(s2n(&str[ex..ey]));
} else {
// Both tags are NONE (this corresponds to zero repetitions).
assert!(ex == NONE && ey == NONE);
}
}
type Ver = Vec::<u32>; // unbounded number of version components
fn s2n(str: &[u8]) -> u32 { // convert a pre-parsed string to a number
let mut n = 0;
for i in str { n = n * 10 + *i as u32 - 48; }
return n;
}
fn parse(str: &[u8]) -> Option<Ver> {
let (mut cur, mut mar) = (0, 0);
let mut mt: MtagTrie = Vec::new();
// User-defined tag variables that are available in semantic action.
let (t1, t2, t3, t4);
// Autogenerated tag variables used by the lexer to track tag values.
/*!stags:re2c format = 'let mut @@ = NONE;'; */
/*!mtags:re2c format = 'let mut @@ = MTAG_ROOT;'; */
/*!re2c
re2c:define:YYCTYPE = u8;
re2c:define:YYPEEK = "*str.get_unchecked(cur)";
re2c:define:YYSKIP = "cur += 1;";
re2c:define:YYBACKUP = "mar = cur;";
re2c:define:YYRESTORE = "cur = mar;";
re2c:define:YYSTAGP = "@@ = cur;";
re2c:define:YYSTAGN = "@@ = NONE;";
re2c:define:YYMTAGP = "@@ = add_mtag(&mut mt, @@, cur);";
re2c:define:YYMTAGN = "@@ = add_mtag(&mut mt, @@, NONE);";
re2c:yyfill:enable = 0;
re2c:tags = 1;
num = [0-9]+;
@t1 num @t2 ("." #t3 num #t4)* [\x00] {
let mut ver: Ver = Vec::new();
ver.push(s2n(&str[t1..t2]));
unwind(&mt, t3, t4, str, &mut ver);
return Some(ver);
}
* { return None; }
*/
}
fn main() {
assert_eq!(parse(b"1\0"), Some(vec![1]));
assert_eq!(parse(b"1.2.3.4.5.6.7\0"), Some(vec![1, 2, 3, 4, 5, 6, 7]));
assert_eq!(parse(b"1.2.\0"), None);
}
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 compiles regular expressions to automata it translates
code points to code units. This is generally not a simple mapping: in variable-length
encodings a single code point range may get translated to a complex code unit graph. The
following encodings are supported:
• ASCII (enabled by default). It is a fixed-length encoding with code space [0-255] and
1-byte code points and code units.
• EBCDIC (enabled with --ebcdic or re2c:encoding:ebcdic). It is a fixed-length encoding
with code space [0-255] and 1-byte code points and code units.
• UCS2 (enabled with --ucs2 or re2c:encoding:ucs2). It is a fixed-length encoding with
code space [0-0xFFFF] and 2-byte code points and code units.
• UTF8 (enabled with --utf8 or re2c:encoding:utf8). It is a variable-length Unicode
encoding. Code unit size is 1 byte. Code points are represented with 1 -- 4 code units.
• UTF16 (enabled with --utf16 or re2c:encoding:utf16). It is a variable-length Unicode
encoding. Code unit size is 2 bytes. Code points are represented with 1 -- 2 code units.
• UTF32 (enabled with --utf32 or re2c:encoding:utf32). It is a fixed-length Unicode
encoding with code space [0-0x10FFFF] and 4-byte code points and code units.
Include file include/unicode_categories.re provides re2c definitions for the standard
Unicode categories.
Option --input-encoding specifies source file encoding, which can be used to enable
Unicode literals in regular expressions. For example --input-encoding utf8 tells re2c that
the source file is in UTF8 (it differs from --utf8 which sets input text encoding). Option
--encoding-policy specifies the way re2c handles Unicode surrogates (code points in range
[0xD800-0xDFFF]).
Below is an example of a lexer for UTF8 encoded Unicode identifiers.
// re2rust $INPUT -o $OUTPUT --utf8
/*!include:re2c "unicode_categories.re" */
fn lex(str: &[u8]) -> bool {
let (mut cur, mut mar) = (0, 0);
/*!re2c
re2c:define:YYCTYPE = u8;
re2c:define:YYPEEK = "*str.get_unchecked(cur)";
re2c:define:YYSKIP = "cur += 1;";
re2c:define:YYBACKUP = "mar = cur;";
re2c:define:YYRESTORE = "cur = mar;";
re2c:yyfill:enable = 0;
// Simplified "Unicode Identifier and Pattern Syntax"
// (see https://unicode.org/reports/tr31)
id_start = L | Nl | [$_];
id_continue = id_start | Mn | Mc | Nd | Pc | [\u200D\u05F3];
identifier = id_start id_continue*;
identifier { return true; }
* { return false; }
*/
}
fn main() {
assert!(lex("_Ыдентификатор\0".as_bytes()));
}
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 included. The first form should be used
outside of re2c blocks, and the second form allows one to include a file in the middle of
a re2c block. re2c looks for included files in the directory of the including file and in
include locations, which can be specified with -I option. Include directives in re2c work
in the same way as C/C++ #include: the contents of FILE are copy-pasted verbatim in place
of the directive. Include files may have further includes of their own. Use --depfile
option to track build dependencies of the output file on include files. re2c provides
some predefined include files that can be found in the include/ subdirectory of the
project. These files contain definitions that can be useful to other projects (such as
Unicode categories) and form something like a standard library for re2c. Below is an
example of using include directive.
Include file 1 (definitions.rs)
#[derive(Debug, PartialEq)]
enum Num { Int, Float, NaN }
/*!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 Num::Float; }
Input file
// re2rust $INPUT -o $OUTPUT
/*!include:re2c "definitions.rs" */
fn lex(str: &[u8]) -> Num {
let mut cur = 0;
let mut mar = 0;
/*!re2c
re2c:yyfill:enable = 0;
re2c:define:YYCTYPE = u8;
re2c:define:YYPEEK = "*str.get_unchecked(cur)";
re2c:define:YYSKIP = "cur += 1;";
re2c:define:YYBACKUP = "mar = cur;";
re2c:define:YYRESTORE = "cur = mar;";
* { return Num::NaN; }
number { return Num::Int; }
!include "extra_rules.re.inc";
*/
}
fn main() {
assert_eq!(lex(b"123\0"), Num::Int);
assert_eq!(lex(b"123.4567\0"), Num::Float);
}
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 directive marks the beginning of header file, and the
second directive marks the end of it. Everything between these directives is processed by
re2c, and the generated code is written to the file specified by the -t --type-header
option (or stdout if this option was not used). Autogenerated header file may be needed in
cases when re2c is used to generate definitions of constants, variables and structs that
must be visible from other translation units.
Here is an example of generating a header file that contains definition of the lexer state
with tag variables (the number variables depends on the regular grammar and is unknown to
the programmer).
Input file
// re2rust $INPUT -o $OUTPUT --header lexer/state.rs
mod lexer;
use lexer::state::State; // the module is generated by re2c
/*!header:re2c:on*/
pub struct State<'a> {
pub str: &'a [u8],
pub cur: usize,
/*!stags:re2c format = "pub @@: usize,"; */
}
/*!header:re2c:off*/
fn lex(st: &mut State) -> usize {
let t: usize;
/*!re2c
re2c:header = "lexer/state.rs";
re2c:yyfill:enable = 0;
re2c:define:YYCTYPE = "u8";
re2c:define:YYPEEK = "*st.str.get_unchecked(st.cur)";
re2c:define:YYSKIP = "st.cur += 1;";
re2c:define:YYSTAGP = "@@ = st.cur;";
re2c:tags = 1;
re2c:tags:expression = "st.@@";
[a]* @t [b]* { return t; }
*/
}
fn main() {
let mut st = State {
str: b"ab\0",
cur: 0,
/*!stags:re2c format = "@@: 0,"; */
};
assert_eq!(lex(&mut st), 1);
}
Header file
/* Generated by re2c */
pub struct State<'a> {
pub str: &'a [u8],
pub cur: usize,
pub yyt1: usize,
}
SKELETON PROGRAMS
With the -S, --skeleton option, re2c ignores all non-re2c code and generates a
self-contained C program that can be further compiled and executed. The program consists
of lexer code and input data. For each constructed DFA (block or condition) re2c generates
a standalone lexer and two files: an .input file with strings derived from the DFA and a
.keys file with expected match results. The program runs each lexer on the corresponding
.input file and compares results with the expectations. Skeleton programs are very useful
for a number of reasons:
• They can check correctness of various re2c optimizations (the data is generated early in
the process, before any DFA transformations have taken place).
• Generating a set of input data with good coverage may be useful for both testing and
benchmarking.
• Generating self-contained executable programs allows one to get minimized test cases
(the original code may be large or have a lot of dependencies).
The difficulty with generating input data is that for all but the most trivial cases the
number of possible input strings is too large (even if the string length is limited). re2c
solves this difficulty by generating sufficiently many strings to cover almost all DFA
transitions. It uses the following algorithm. First, it constructs a skeleton of the DFA.
For encodings with 1-byte code unit size (such as ASCII, UTF-8 and EBCDIC) skeleton is
just an exact copy of the original DFA. For encodings with multibyte code units skeleton
is a copy of DFA with certain transitions omitted: namely, re2c takes at most 256 code
units for each disjoint continuous range that corresponds to a DFA transition. The chosen
values are evenly distributed and include range bounds. Instead of trying to cover all
possible paths in the skeleton (which is infeasible) re2c generates sufficiently many
paths to cover all skeleton transitions, and thus trigger the corresponding conditional
jumps in the lexer. The algorithm implementation is limited by ~1Gb of transitions and
consumes constant amount of memory (re2c writes data to file as soon as it is generated).
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 optimizations and transformations. Earlier stages can be dumped with
various debug options, such as --dump-nfa, --dump-dfa-raw etc. (see the full list of
options).
SEE ALSO
You can find more information about re2c at the official website: http://re2c.org.
Similar programs are flex(1), lex(1), quex(http://quex.sourceforge.net).
AUTHORS
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)