perlreapi - Perl regular expression plugin interface
As of Perl 5.9.5 there is a new interface for plugging and using regular expression engines other 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,
- SSize_t minend, SV* sv,
- void* data, U32 flags);
- char* (*intuit) (pTHX_
- REGEXP * const rx, SV *sv,
- const char * const strbeg,
- 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
- REGEXP* (*op_comp) (...);
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.
- 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:
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
msixpn
flags the regex was compiled with. It also contains
additional info, such as if 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 if 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
The character set rules 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:
This flag was removed in perl 5.18.0. split ' '
is now special-cased
solely in the parser. RXf_SPLIT is still #defined, so you can test for it.
This is how it used to work:
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.
This flag was removed in perl 5.18.0. It is still #defined, so you can set it, but doing so will have no effect. This is how it used to work:
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 if the subject is a
UTF-8 string and if 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.
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.
Tells the split operator to split the target string on whitespace without invoking the regex engine. The definition of whitespace varies depending on if the target string is a UTF-8 string and on if RXf_PMf_LOCALE is set.
Perl's engine sets this flag if the pattern is \s+.
Tells the split operator to split the target string on characters. The definition of character varies depending on if 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.
Added in perl 5.18.0, this flag indicates that a regular expression might
perform an operation that would interfere with inplace substitution. For
instance it might contain lookbehind, or assign to non-magical variables
(such as $REGMARK and $REGERROR) during matching. s/// will skip
certain optimisations when this is set.
- I32 exec(pTHX_ REGEXP * const rx,
- char *stringarg, char* strend, char* strbeg,
- SSize_t minend, SV* sv,
- void* data, U32 flags);
Execute a regexp. The arguments are
The regular expression to execute.
This is the SV to be matched against. Note that the
actual char array to be matched against is supplied by the arguments
described below; the SV is just used to determine UTF8ness, pos() etc.
Pointer to the physical start of the string.
Pointer to the character following the physical end of the string (i.e.
the \0
, if any).
Pointer to the position in the string where matching should start; it might
not be equal to strbeg
(for example in a later iteration of /.../g
).
Minimum length of string (measured in bytes from stringarg
) that must
match; if the engine reaches the end of the match but hasn't reached this
position in the string, it should fail.
Optimisation data; subject to change.
Optimisation flags; subject to change.
- char* intuit(pTHX_
- REGEXP * const rx,
- SV *sv,
- const char * const strbeg,
- 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 if 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.
Arguments:
- rx: the regex to match against
- sv: the SV being matched: only used for utf8 flag; the string
- itself is accessed via the pointers below. Note that on
- something like an overloaded SV, SvPOK(sv) may be false
- and the string pointers may point to something unrelated to
- the SV itself.
- strbeg: real beginning of string
- strpos: the point in the string at which to begin matching
- strend: pointer to the byte following the last char of the string
- flags currently unused; set to 0
- data: currently unused; set to NULL
- SV* checkstr(pTHX_ REGEXP * const rx);
Return a SV containing a string that must appear in the pattern. Used
by split for optimising matches.
- 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.
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 1
for $1
, 2
for $2
and so
forth, and have these symbolic values for the special variables:
- ${^PREMATCH} RX_BUFF_IDX_CARET_PREMATCH
- ${^POSTMATCH} RX_BUFF_IDX_CARET_POSTMATCH
- ${^MATCH} RX_BUFF_IDX_CARET_FULLMATCH
- $` RX_BUFF_IDX_PREMATCH
- $' RX_BUFF_IDX_POSTMATCH
- $& RX_BUFF_IDX_FULLMATCH
Note that in Perl 5.17.3 and earlier, the last three constants were also used for the caret variants of the variables.
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.
- 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.
- 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:
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 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.
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
If %+
or %-
is being operated on, if any.
- RXapif_ONE /* %+ */
- RXapif_ALL /* %- */
If 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).
- 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);
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 if 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.
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);
- 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.
This is private to the Perl core and subject to change. Should be left null.
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 if the pattern anchored in some way, or what flags were used during the compile, or if 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 number of chars in */
- string to match */
- I32 minlenret; /* mininum possible number of chars in $& */
- U32 gofs; /* chars left of pos that we search from */
- /* substring data about strings that must appear
- in the final match, used for optimisations */
- struct reg_substr_data *substrs;
- U32 nparens; /* number of capture groups */
- /* private engine specific data */
- U32 intflags; /* Engine Specific Internal flags */
- void *pprivate; /* Data private to the regex engine which
- created this object. */
- /* Data about the last/current match. These are modified during
- * matching*/
- U32 lastparen; /* highest close paren matched ($+) */
- U32 lastcloseparen; /* last close paren matched ($^N) */
- 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 */
- I32 suboffset; /* byte offset of subbeg from logical start of
- str */
- I32 subcoffset; /* suboffset equiv, but in chars (for @-/@+) */
- /* 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/perl5-changes@perl.org/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 (in characters) 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 (in characters) 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 if it can do in-place substitutions (these can
result in considerable speed-up).
gofs
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, 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 if 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 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.
subbeg
sublen
saved_copy
suboffset
subcoffset
Used during the execution phase for managing search and replace patterns,
and for providing the text for $&
, $1
etc. subbeg
points to a
buffer (either the original string, or a copy in the case of
RX_MATCH_COPIED(rx)
), and sublen
is the length of the buffer. The
RX_OFFS
start and end indices index into this buffer.
In the presence of the REXEC_COPY_STR
flag, but with the addition of
the REXEC_COPY_SKIP_PRE
or REXEC_COPY_SKIP_POST
flags, an engine
can choose not to copy the full buffer (although it must still do so in
the presence of RXf_PMf_KEEPCOPY
or the relevant bits being set in
PL_sawampersand
). In this case, it may set suboffset
to indicate the
number of bytes from the logical start of the buffer to the physical start
(i.e. subbeg
). It should also set subcoffset
, the number of
characters in the offset. The latter is needed to support @-
and @+
which work in characters, not bytes.
wrapped
wraplen
Stores the string qr// stringifies to. The Perl engine for example
stores (?^: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:
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.
Originally part of perlreguts.
Originally written by Yves Orton, expanded by Ævar Arnfjörð Bjarmason.
Copyright 2006 Yves Orton and 2007 Ævar Arnfjörð Bjarmason.
This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.