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:


    STRLEN plen;
    char*  exp = SvPV(pattern, plen);
    char* xend = exp + plen;

Since any scalar can be passed as a pattern it’s possible to implement an engine that does something with an array (

"ook" =~ [ qw/ eek
hlagh / ]
) or with the non-stringified form of a compiled regular expression (
"ook" =~ qr/eek/
). perl’s own engine will always stringify everything using the snippet above but that doesn’t mean other engines have to.

The

flags
parameter is a bitfield which indicates which of the
msixp
flags the regex was compiled with. It also contains additional info such as whether
use locale
is in effect.

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
Additional flags:
RXf_PMf_LOCALE Set if
use locale
is in effect. If present in
rx->extflags
split
will use the locale dependent definition of whitespace under when RXf_SKIPWHITE or RXf_WHITE are in effect. Under ASCII whitespace is defined as per isSPACE, and by the internal macros
is_utf8_space
under UTF-8 and
isSPACE_LC
under
use
locale
.
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);
These flags can be set during compilation to enable optimizations in the

split
operator.
RXf_SKIPWHITE If the flag is present in
rx->extflags
split
will delete whitespace from the start of the subject string before it’s operated on. What is considered whitespace depends on whether the subject is a UTF-8 string and whether the
RXf_PMf_LOCALE
flag is set.

If RXf_WHITE is set in addition to this flag

split
will behave like
split " "
under the perl engine.
RXf_START_ONLY Tells the split operator to split the target string on newlines (
\n
) without invoking the regex engine.

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 buffers (
$1
,
$2
, ...).

The

paren
parameter will be
-2
for
$`
,
-1
for
$
,
0
for
$&
,
1
for
$1
and so forth.

The names have been chosen by analogy with Tie::Scalar methods names with an additional LENGTH callback for efficiency. However named capture variables are currently not tied internally but implemented via magic.

numbered_buff_FETCH


    void numbered_buff_FETCH(pTHX_ REGEXP * const rx, const I32 paren,
                             SV * const sv);

Fetch a specified numbered capture.

sv
should be set to the scalar to return, the scalar is passed as an argument rather than being returned from the function because when it’s called perl already has a scalar to store the value, creating another one would be redundant. The scalar can be set with
sv_setsv
,
sv_setpvn
and friends, see perlapi.

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 doesnt get called" }

    package main;

    tie my $sv => "CatptureVar";
    $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.

numbered_buff_LENGTH


    I32 numbered_buff_LENGTH (pTHX_ REGEXP * const rx, const SV * const sv,
                              const I32 paren);

Get the

length
of a capture variable. There’s a special callback for this so that perl doesn’t have to do a FETCH and run
length
on the result, since the length is (in perl’s case) known from an offset stored in
<rx-
offs> this is much more efficient:


    I32 s1  = rx->offs[paren].start;
    I32 s2  = rx->offs[paren].end;
    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



    SV* qr_package(pTHX_ REGEXP * const rx);

The package the qr// magic object is blessed into (as seen by

ref
qr//
). It is recommended that engines change this to their package name for identification regardless of whether they implement methods on the object.

The package this method returns should also have the internal

Regexp
package in its
@ISA
.
qr//-
isa(Regexp)> should always be true regardless of what engine is being used.

Example implementation might be:


    SV*
    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? */
        struct regexp* mother_re;

        /* Information about the match that the perl core uses to manage things */
        U32 extflags;   /* Flags used both externally and internally */
        I32 minlen;     /* mininum possible length of string to match */
        I32 minlenret;  /* mininum possible length of $& */
        U32 gofs;       /* chars left of pos that we search from */

        /* substring data about strings that must appear
           in the final match, used for optimisations */
        struct reg_substr_data *substrs;

        U32 nparens;  /* number of capture buffers */

        /* 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 isnt 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:

\f(CWengine\fP


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
.

\f(CWmother_re\fP


TODO, see <perl5-changes@perl.org/msg17328.html">http://www.mail-archive.com/perl5-changes@perl.org/msg17328.html>

\f(CWextflags\fP


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.

\f(CWminlen\fP \f(CWminlenret\fP


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.

minlenret
is the minimum length of the string that would be found in $& after a match.

The difference between

minlen
and
minlenret
can be seen in the following pattern:


    /ns(?=\d)/

where the

minlen
would be 3 but
minlenret
would only be 2 as the \d is required to match but is not actually included in the matched content. This distinction is particularly important as the substitution logic uses the
minlenret
to tell whether it can do in-place substitution which can result in considerable speedup.

\f(CWgofs\fP


Left offset from pos() to start match at.

\f(CWsubstrs\fP


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.

\f(CWnparens\fP, \f(CWlasparen\fP, and \f(CWlastcloseparen\fP


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.

\f(CWintflags\fP


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.

\f(CWpprivate\fP


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.

\f(CWswap\fP


TODO: document

\f(CWoffs\fP


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 buffer did not match.
->offs[0].start/end
represents
$&
(or
${^MATCH
under
//p
) and
->offs[paren].end
matches
$$paren
where
$paren 
= 1>.

\f(CWprecomp\fP \f(CWprelen\fP


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 */

\f(CWparen_names\fP


This is a hash used internally to track named capture buffers 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.

\f(CWsubstrs\fP


Holds information on the longest string that must occur at a fixed offset from the start of the pattern, and the longest string that must occur at a floating offset from the start of the pattern. Used to do Fast-Boyer-Moore searches on the string to find out if its worth using the regex engine at all, and if so where in the string to search.

\f(CWsubbeg\fP \f(CWsublen\fP \f(CWsaved_copy\fP


Used during execution phase for managing search and replace patterns.

\f(CWwrapped\fP \f(CWwraplen\fP


Stores the string

qr//
stringifies to. The perl engine for example stores
(?-xism:eek)
in the case of
qr/eek/
.

When using a custom engine that doesn’t support the

(?:)
construct for inline modifiers, it’s probably best 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

(?:)
.

\f(CWseen_evals\fP


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//
.

\f(CWrefcnt\fP


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 Ævar Arnfjoer- Bjarmason.

LICENSE


Copyright 2006 Yves Orton and 2007 Ævar Arnfjoer- Bjarmason.

This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

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