PERLREAPI(1) Perl Programmers Reference Guide PERLREAPI(1)
NAME
perlreapi - perl regular expression plugin interface
DESCRIPTION
As of Perl 5.9.5 there is a new interface for plugging and using other regular expression engines than the
default one.
Each engine is supposed to provide access to a constant structure of the following format:
typedef struct regexp_engine {
REGEXP* (*comp) (pTHX_ const SV * const pattern, const U32 flags);
I32 (*exec) (pTHX_ REGEXP * const rx, char* stringarg, char* strend,
char* strbeg, I32 minend, SV* screamer,
void* data, U32 flags);
char* (*intuit) (pTHX_ REGEXP * const rx, SV *sv, char *strpos,
char *strend, U32 flags,
struct re_scream_pos_data_s *data);
SV* (*checkstr) (pTHX_ REGEXP * const rx);
void (*free) (pTHX_ REGEXP * const rx);
void (*numbered_buff_FETCH) (pTHX_ REGEXP * const rx, const I32 paren,
SV * const sv);
void (*numbered_buff_STORE) (pTHX_ REGEXP * const rx, const I32 paren,
SV const * const value);
I32 (*numbered_buff_LENGTH) (pTHX_ REGEXP * const rx, const SV * const sv,
const I32 paren);
SV* (*named_buff) (pTHX_ REGEXP * const rx, SV * const key,
SV * const value, U32 flags);
SV* (*named_buff_iter) (pTHX_ REGEXP * const rx, const SV * const lastkey,
const U32 flags);
SV* (*qr_package)(pTHX_ REGEXP * const rx);
#ifdef USE_ITHREADS
void* (*dupe) (pTHX_ REGEXP * const rx, CLONE_PARAMS *param);
#endif
When a regexp is compiled, its "engine" field is then set to point at the appropriate structure, so that when
it needs to be used Perl can find the right routines to do so.
In order to install a new regexp handler, $^H{regcomp} is set to an integer which (when casted appropriately)
resolves to one of these structures. When compiling, the "comp" method is executed, and the resulting regexp
structure's engine field is expected to point back at the same structure.
The pTHX_ symbol in the definition is a macro used by perl under threading to provide an extra argument to the
routine holding a pointer back to the interpreter that is executing the regexp. So under threading all
routines get an extra argument.
Callbacks
comp
REGEXP* comp(pTHX_ const SV * const pattern, const U32 flags);
Compile the pattern stored in "pattern" using the given "flags" and return a pointer to a prepared "REGEXP"
structure that can perform the match. See "The REGEXP structure" below for an explanation of the individual
fields in the REGEXP struct.
The "pattern" parameter is the scalar that was used as the pattern. previous versions of perl would pass two
"char*" indicating the start and end of the stringified pattern, the following snippet can be used to get the
old parameters:
The "eogc" flags are stripped out before being passed to the comp routine. The regex engine does not need to
know whether any of these are set as those flags should only affect what perl does with the pattern and its
match variables, not how it gets compiled and executed.
By the time the comp callback is called, some of these flags have already had effect (noted below where
applicable). However most of their effect occurs after the comp callback has run in routines that read the
"rx->extflags" field which it populates.
In general the flags should be preserved in "rx->extflags" after compilation, although the regex engine might
want to add or delete some of them to invoke or disable some special behavior in perl. The flags along with
any special behavior they cause are documented below:
The pattern modifiers:
"/m" - RXf_PMf_MULTILINE
If this is in "rx->extflags" it will be passed to "Perl_fbm_instr" by "pp_split" which will treat the
subject string as a multi-line string.
"/s" - RXf_PMf_SINGLELINE
"/i" - RXf_PMf_FOLD
"/x" - RXf_PMf_EXTENDED
If present on a regex "#" comments will be handled differently by the tokenizer in some cases.
TODO: Document those cases.
"/p" - RXf_PMf_KEEPCOPY
TODO: Document this
Character set
The character set semantics are determined by an enum that is contained in this field. This is still
experimental and subject to change, but the current interface returns the rules by use of the in-line
function "get_regex_charset(const U32 flags)". The only currently documented value returned from it is
REGEX_LOCALE_CHARSET, which is set if "use locale" is in effect. If present in "rx->extflags", "split"
will use the locale dependent definition of whitespace when RXf_SKIPWHITE or RXf_WHITE is in effect. ASCII
whitespace is defined as per isSPACE, and by the internal macros "is_utf8_space" under UTF-8, and
"isSPACE_LC" under "use locale".
Additional flags:
RXf_UTF8
Set if the pattern is SvUTF8(), set by Perl_pmruntime.
A regex engine may want to set or disable this flag during compilation. The perl engine for instance may
upgrade non-UTF-8 strings to UTF-8 if the pattern includes constructs such as "\x{...}" that can only
match Unicode values.
RXf_SPLIT
If "split" is invoked as "split ' '" or with no arguments (which really means "split(' ', $_)", see
split), perl will set this flag. The regex engine can then check for it and set the SKIPWHITE and WHITE
extflags. To do this the perl engine does:
if (flags & RXf_SPLIT && r->prelen == 1 && r->precomp[0] == ' ')
r->extflags |= (RXf_SKIPWHITE|RXf_WHITE);
Perl's engine sets this if the pattern is "/^/" ("plen == 1 && *exp == '^'"), even under "/^/s", see
split. Of course a different regex engine might want to use the same optimizations with a different
syntax.
RXf_WHITE
Tells the split operator to split the target string on whitespace without invoking the regex engine. The
definition of whitespace varies depending on whether the target string is a UTF-8 string and on whether
RXf_PMf_LOCALE is set.
Perl's engine sets this flag if the pattern is "\s+".
RXf_NULL
Tells the split operator to split the target string on characters. The definition of character varies
depending on whether the target string is a UTF-8 string.
Perl's engine sets this flag on empty patterns, this optimization makes "split //" much faster than it
would otherwise be. It's even faster than "unpack".
exec
I32 exec(pTHX_ REGEXP * const rx,
char *stringarg, char* strend, char* strbeg,
I32 minend, SV* screamer,
void* data, U32 flags);
Execute a regexp.
intuit
char* intuit(pTHX_ REGEXP * const rx,
SV *sv, char *strpos, char *strend,
const U32 flags, struct re_scream_pos_data_s *data);
Find the start position where a regex match should be attempted, or possibly whether the regex engine should
not be run because the pattern can't match. This is called as appropriate by the core depending on the values
of the extflags member of the regexp structure.
checkstr
SV* checkstr(pTHX_ REGEXP * const rx);
Return a SV containing a string that must appear in the pattern. Used by "split" for optimising matches.
free
void free(pTHX_ REGEXP * const rx);
Called by perl when it is freeing a regexp pattern so that the engine can release any resources pointed to by
the "pprivate" member of the regexp structure. This is only responsible for freeing private data; perl will
handle releasing anything else contained in the regexp structure.
Numbered capture callbacks
Called to get/set the value of "$`", "$'", $& and their named equivalents, ${^PREMATCH}, ${^POSTMATCH} and
$^{MATCH}, as well as the numbered capture groups ($1, $2, ...).
The "paren" parameter will be "-2" for "$`", "-1" for "$'", 0 for $&, 1 for $1 and so forth.
This callback is where perl untaints its own capture variables under taint mode (see perlsec). See the
"Perl_reg_numbered_buff_fetch" function in regcomp.c for how to untaint capture variables if that's something
you'd like your engine to do as well.
numbered_buff_STORE
void (*numbered_buff_STORE) (pTHX_ REGEXP * const rx, const I32 paren,
SV const * const value);
Set the value of a numbered capture variable. "value" is the scalar that is to be used as the new value. It's
up to the engine to make sure this is used as the new value (or reject it).
Example:
if ("ook" =~ /(o*)/) {
# 'paren' will be '1' and 'value' will be 'ee'
$1 =~ tr/o/e/;
}
Perl's own engine will croak on any attempt to modify the capture variables, to do this in another engine use
the following callback (copied from "Perl_reg_numbered_buff_store"):
void
Example_reg_numbered_buff_store(pTHX_ REGEXP * const rx, const I32 paren,
SV const * const value)
{
PERL_UNUSED_ARG(rx);
PERL_UNUSED_ARG(paren);
PERL_UNUSED_ARG(value);
if (!PL_localizing)
Perl_croak(aTHX_ PL_no_modify);
}
Actually perl will not always croak in a statement that looks like it would modify a numbered capture
variable. This is because the STORE callback will not be called if perl can determine that it doesn't have to
modify the value. This is exactly how tied variables behave in the same situation:
package CaptureVar;
use base 'Tie::Scalar';
sub TIESCALAR { bless [] }
sub FETCH { undef }
sub STORE { die "This doesn't get called" }
package main;
tie my $sv => "CaptureVar";
$sv =~ y/a/b/;
Because $sv is "undef" when the "y///" operator is applied to it the transliteration won't actually execute
and the program won't "die". This is different to how 5.8 and earlier versions behaved since the capture
variables were READONLY variables then, now they'll just die when assigned to in the default engine.
I32 len = t1 - s1;
This is a little bit more complex in the case of UTF-8, see what "Perl_reg_numbered_buff_length" does with
is_utf8_string_loclen.
Named capture callbacks
Called to get/set the value of "%+" and "%-" as well as by some utility functions in re.
There are two callbacks, "named_buff" is called in all the cases the FETCH, STORE, DELETE, CLEAR, EXISTS and
SCALAR Tie::Hash callbacks would be on changes to "%+" and "%-" and "named_buff_iter" in the same cases as
FIRSTKEY and NEXTKEY.
The "flags" parameter can be used to determine which of these operations the callbacks should respond to, the
following flags are currently defined:
Which Tie::Hash operation is being performed from the Perl level on "%+" or "%+", if any:
RXapif_FETCH
RXapif_STORE
RXapif_DELETE
RXapif_CLEAR
RXapif_EXISTS
RXapif_SCALAR
RXapif_FIRSTKEY
RXapif_NEXTKEY
Whether "%+" or "%-" is being operated on, if any.
RXapif_ONE /* %+ */
RXapif_ALL /* %- */
Whether this is being called as "re::regname", "re::regnames" or "re::regnames_count", if any. The first two
will be combined with "RXapif_ONE" or "RXapif_ALL".
RXapif_REGNAME
RXapif_REGNAMES
RXapif_REGNAMES_COUNT
Internally "%+" and "%-" are implemented with a real tied interface via Tie::Hash::NamedCapture. The methods
in that package will call back into these functions. However the usage of Tie::Hash::NamedCapture for this
purpose might change in future releases. For instance this might be implemented by magic instead (would need
an extension to mgvtbl).
named_buff
SV* (*named_buff) (pTHX_ REGEXP * const rx, SV * const key,
SV * const value, U32 flags);
named_buff_iter
SV* (*named_buff_iter) (pTHX_ REGEXP * const rx, const SV * const lastkey,
const U32 flags);
qr_package
Example_qr_package(pTHX_ REGEXP * const rx)
{
PERL_UNUSED_ARG(rx);
return newSVpvs("re::engine::Example");
}
Any method calls on an object created with "qr//" will be dispatched to the package as a normal object.
use re::engine::Example;
my $re = qr//;
$re->meth; # dispatched to re::engine::Example::meth()
To retrieve the "REGEXP" object from the scalar in an XS function use the "SvRX" macro, see "REGEXP Functions"
in perlapi.
void meth(SV * rv)
PPCODE:
REGEXP * re = SvRX(sv);
dupe
void* dupe(pTHX_ REGEXP * const rx, CLONE_PARAMS *param);
On threaded builds a regexp may need to be duplicated so that the pattern can be used by multiple threads.
This routine is expected to handle the duplication of any private data pointed to by the "pprivate" member of
the regexp structure. It will be called with the preconstructed new regexp structure as an argument, the
"pprivate" member will point at the old private structure, and it is this routine's responsibility to
construct a copy and return a pointer to it (which perl will then use to overwrite the field as passed to this
routine.)
This allows the engine to dupe its private data but also if necessary modify the final structure if it really
must.
On unthreaded builds this field doesn't exist.
The REGEXP structure
The REGEXP struct is defined in regexp.h. All regex engines must be able to correctly build such a structure
in their "comp" routine.
The REGEXP structure contains all the data that perl needs to be aware of to properly work with the regular
expression. It includes data about optimisations that perl can use to determine if the regex engine should
really be used, and various other control info that is needed to properly execute patterns in various contexts
such as is the pattern anchored in some way, or what flags were used during the compile, or whether the
program contains special constructs that perl needs to be aware of.
In addition it contains two fields that are intended for the private use of the regex engine that compiled the
pattern. These are the "intflags" and "pprivate" members. "pprivate" is a void pointer to an arbitrary
structure whose use and management is the responsibility of the compiling engine. perl will never modify
either of these values.
typedef struct regexp {
/* what engine created this regexp? */
const struct regexp_engine* engine;
/* what re is this a lightweight copy of? */
U32 nparens; /* number of capture groups */
/* private engine specific data */
U32 intflags; /* Engine Specific Internal flags */
void *pprivate; /* Data private to the regex engine which
created this object. */
/* Data about the last/current match. These are modified during matching*/
U32 lastparen; /* last open paren matched */
U32 lastcloseparen; /* last close paren matched */
regexp_paren_pair *swap; /* Swap copy of *offs */
regexp_paren_pair *offs; /* Array of offsets for (@-) and (@+) */
char *subbeg; /* saved or original string so \digit works forever. */
SV_SAVED_COPY /* If non-NULL, SV which is COW from original */
I32 sublen; /* Length of string pointed by subbeg */
/* Information about the match that isn't often used */
I32 prelen; /* length of precomp */
const char *precomp; /* pre-compilation regular expression */
char *wrapped; /* wrapped version of the pattern */
I32 wraplen; /* length of wrapped */
I32 seen_evals; /* number of eval groups in the pattern - for security checks */
HV *paren_names; /* Optional hash of paren names */
/* Refcount of this regexp */
I32 refcnt; /* Refcount of this regexp */
} regexp;
The fields are discussed in more detail below:
"engine"
This field points at a regexp_engine structure which contains pointers to the subroutines that are to be used
for performing a match. It is the compiling routine's responsibility to populate this field before returning
the regexp object.
Internally this is set to "NULL" unless a custom engine is specified in $^H{regcomp}, perl's own set of
callbacks can be accessed in the struct pointed to by "RE_ENGINE_PTR".
"mother_re"
TODO, see http://www.mail-archive.com/[email protected]/msg17328.html <http://www.mail-
archive.com/[email protected]/msg17328.html>
"extflags"
This will be used by perl to see what flags the regexp was compiled with, this will normally be set to the
value of the flags parameter by the comp callback. See the comp documentation for valid flags.
"minlen" "minlenret"
The minimum string length required for the pattern to match. This is used to prune the search space by not
bothering to match any closer to the end of a string than would allow a match. For instance there is no point
in even starting the regex engine if the minlen is 10 but the string is only 5 characters long. There is no
way that the pattern can match.
Left offset from pos() to start match at.
"substrs"
Substring data about strings that must appear in the final match. This is currently only used internally by
perl's engine for but might be used in the future for all engines for optimisations.
"nparens", "lastparen", and "lastcloseparen"
These fields are used to keep track of how many paren groups could be matched in the pattern, which was the
last open paren to be entered, and which was the last close paren to be entered.
"intflags"
The engine's private copy of the flags the pattern was compiled with. Usually this is the same as "extflags"
unless the engine chose to modify one of them.
"pprivate"
A void* pointing to an engine-defined data structure. The perl engine uses the "regexp_internal" structure
(see "Base Structures" in perlreguts) but a custom engine should use something else.
"swap"
Unused. Left in for compatibility with perl 5.10.0.
"offs"
A "regexp_paren_pair" structure which defines offsets into the string being matched which correspond to the $&
and $1, $2 etc. captures, the "regexp_paren_pair" struct is defined as follows:
typedef struct regexp_paren_pair {
I32 start;
I32 end;
} regexp_paren_pair;
If "->offs[num].start" or "->offs[num].end" is "-1" then that capture group did not match.
"->offs[0].start/end" represents $& (or "${^MATCH" under "//p") and "->offs[paren].end" matches $$paren where
$paren = 1>.
"precomp" "prelen"
Used for optimisations. "precomp" holds a copy of the pattern that was compiled and "prelen" its length. When
a new pattern is to be compiled (such as inside a loop) the internal "regcomp" operator checks whether the
last compiled "REGEXP"'s "precomp" and "prelen" are equivalent to the new one, and if so uses the old pattern
instead of compiling a new one.
The relevant snippet from "Perl_pp_regcomp":
if (!re || !re->precomp || re->prelen != (I32)len ||
memNE(re->precomp, t, len))
/* Compile a new pattern */
"paren_names"
This is a hash used internally to track named capture groups and their offsets. The keys are the names of the
buffers the values are dualvars, with the IV slot holding the number of buffers with the given name and the pv
being an embedded array of I32. The values may also be contained independently in the data array in cases
where named backreferences are used.
"substrs"
Holds information on the longest string that must occur at a fixed offset from the start of the pattern, and
to have "qr//" stringify to the supplied pattern, note that this will create undesired patterns in cases such
as:
my $x = qr/a|b/; # "a|b"
my $y = qr/c/i; # "c"
my $z = qr/$x$y/; # "a|bc"
There's no solution for this problem other than making the custom engine understand a construct like "(?:)".
"seen_evals"
This stores the number of eval groups in the pattern. This is used for security purposes when embedding
compiled regexes into larger patterns with "qr//".
"refcnt"
The number of times the structure is referenced. When this falls to 0 the regexp is automatically freed by a
call to pregfree. This should be set to 1 in each engine's "comp" routine.
HISTORY
Originally part of perlreguts.
AUTHORS
Originally written by Yves Orton, expanded by var Arnfjoer` Bjarmason.
LICENSE
Copyright 2006 Yves Orton and 2007 var Arnfjoer` Bjarmason.
This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.
perl v5.16.3 2013-03-04 PERLREAPI(1)
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