PERLREAPI(1)
💖 PERLREAPI(1) 💖
fresh out of the Perl Programmers Reference Guide 🦋✨
🎀 NAME 🎀
perlreapi — the Perl regular expression plugin interface 💅
🌈 DESCRIPTION 🌈
okay so bestie, as of Perl 5.9.5 there is a whole brand new interface for plugging and using regular expression engines that aren't the default one. iconic. we love to see it 🫶
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) (...);So 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. slay ✨
And like, 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. it's giving circular but make it fashion 💅
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. no cap, every single one 😭
🦋 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. Peep "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. do what you want bestie 🫶
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. not me double-checking that twice 💀
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. (the docs said "later bestie" and left it at that 🙈)
"/p"-RXf_PMf_KEEPCOPY
TODO: Document this
🌸 Character set 🌸
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 💎
RXf_SPLIT— This flag was removed in perl 5.18.0."split ' '"is now special-cased solely in the parser.RXf_SPLITis 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. 🔥
RXf_SKIPWHITE— 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. 💅
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 if the target string is a UTF-8 string and on ifRXf_PMf_LOCALEis 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 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". 🔥 (ate, honestly.)
RXf_NO_INPLACE_SUBST— 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$REGMARKand$REGERROR) during matching."s///"will skip certain optimisations when this is set. ⚠️
💅 exec
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:
rx— The regular expression to execute. 👑sv— 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. 🌈strbeg— Pointer to the physical start of the string.strend— Pointer to the character following the physical end of the string (i.e. the"\0", if any).stringarg— 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"). 🎀minend— 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. 💀data— Optimisation data; subject to change. (vibe check pending 🙈)flags— Optimisation flags; subject to change. (also pending 🙈)
💅 intuit
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);okay so this callback is literally about finding the start position where a regex match should even be attempted, or maybe it's telling Perl “bestie, don't even bother running the engine because this pattern can't match anything.” The core calls it whenever it feels like it, depending on what's in the extflags member of the regexp structure. 💅
Arguments, no cap:
rx: the regex to match againstsv: 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 stringstrpos: the point in the string at which to begin matchingstrend: pointer to the byte following the last char of the stringflags: currently unused; set to 0data: currently unused; set to NULL
checkstr
SV* checkstr(pTHX_ REGEXP * const rx);Returns an SV containing a string that must appear in the pattern. split uses this for optimising matches. slay.
free
void free(pTHX_ REGEXP * const rx);Perl calls this when it's freeing a regexp pattern so 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. periodt.
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 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_FULLMATCHNote 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. ✨
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 parent '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.
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_NEXTKEYIf %+ 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_COUNTInternally %+ 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 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.
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.
op_comp
This is private to the Perl core and subject to change. Should be left null.
✨ 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 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. 👑
/* copied from: regexp.h */
typedef struct regexp {
/*----------------------------------------------------------------------
* Fields required for compatibility with SV types
*/
_XPV_HEAD;
/*----------------------------------------------------------------------
* Operational fields
*/
const struct regexp_engine* engine; /* what engine created this regexp? */
REGEXP *mother_re; /* what re is this a lightweight copy of? */
HV *paren_names; /* Optional hash of paren names */
/*----------------------------------------------------------------------
* Information about the match that the perl core uses to manage things
*/
/* see comment in regcomp_internal.h about branch reset to understand
the distinction between physical and logical capture buffers */
U32 nparens; /* physical number of capture buffers */
U32 logical_nparens; /* logical_number of capture buffers */
I32 *logical_to_parno; /* map logical parno to first physical */
I32 *parno_to_logical; /* map every physical parno to logical */
I32 *parno_to_logical_next; /* map every physical parno to the next
physical with the same logical id */
SSize_t maxlen; /* maximum possible number of chars in string to match */
SSize_t minlen; /* minimum possible number of chars in string to match */
SSize_t minlenret; /* minimum possible number of chars in $& */
STRLEN gofs; /* chars left of pos that we search from */
/* substring data about strings that must appear in
* the final match, used for optimisations */
struct reg_substr_data *substrs;
/* private engine specific data */
void *pprivate; /* Data private to the regex engine which
* created this object. */
U32 extflags; /* Flags used both externally and internally */
U32 intflags; /* Engine Specific Internal flags */
/*----------------------------------------------------------------------
* Data about the last/current match. These are modified during matching
*/
U32 lastparen; /* highest close paren matched ($+) */
U32 lastcloseparen; /* last close paren matched ($^N) */
regexp_paren_pair *offs; /* Array of offsets for (@-) and (@+) */
char **recurse_locinput; /* used to detect infinite recursion, XXX: move to internal */
/*---------------------------------------------------------------------- */
/* offset from wrapped to the start of precomp */
PERL_BITFIELD32 pre_prefix:4;
/* original flags used to compile the pattern, may differ from
* extflags in various ways */
PERL_BITFIELD32 compflags:9;
/*---------------------------------------------------------------------- */
char *subbeg; /* saved or original string so \digit works forever. */
SV_SAVED_COPY /* If non-NULL, SV which is COW from original */
SSize_t sublen; /* Length of string pointed by subbeg */
SSize_t suboffset; /* byte offset of subbeg from logical start of str */
SSize_t subcoffset; /* suboffset equiv, but in chars (for @-/@+) */
/*----------------------------------------------------------------------
* More Operational fields
*/
CV *qr_anoncv; /* the anon sub wrapped round qr/(?{..})/ */
} regexp;okay so almost every field in there is reached through a macro prefixed RX_ or RXp_ — you basically never poke the struct directly, you go through the macros like a civilised person. ✨
Now let's go field by field. Grab a snack, this is the ✨lore✨ part.
engine 🔧
This one points at a regexp_engine struct, which is basically a lil bundle of function pointers to the subroutines that do the actual matching. And whose job is it to fill this in? the compiling routine's — it has to populate this field before it hands you back the regexp object. No half-finished structs, bestie. 💅
Internally this is set to NULL unless you specified a custom engine in $^H{regcomp} — and in that case Perl's own set of callbacks can be found in the struct that RE_ENGINE_PTR points to. 🫶
mother_re 👑
A pointer to another struct regexp that this one was derived from. Why does this exist? Because qr// objects mean the exact same regexp pattern can be out here living in different contexts at the same time, and as long as match status information is stored in the structure (there are plans to change this eventually, no cap), we need to support multiple copies of the structure being in use simultaneously. 💀
Here's the split that matters: the fields related to the regexp program itself are copied from the mother_re and owned by the mother_re, whereas the match state variables are owned by the struct itself. Two owners, one happy family. 🎀
extflags 🚩
Perl uses this to see what flags the regexp was compiled with. Normally it just gets set to the value of the flags parameter by the comp callback. For the valid flags, go read the comp documentation (that's the comp callback, which we already covered earlier). 📋
minlen minlenret 📏
minlen is the minimum string length (in characters) required for the pattern to match. Perl uses it to prune the search space — like, there is genuinely no point bothering to match any closer to the end of a string than would still 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. Zero. Zilch. the math is not mathing. 😭
minlenret is the minimum length (in characters) of the string that would be found in $& after a match.
The difference between minlen and minlenret shows up real clear in this pattern:
/ns(?=\d)/where the minlen would be 3 but the minlenret would only be 2 — because the \d is required to match but is not actually included in the matched content. 🫠
And this distinction is particularly important because the substitution logic uses the minlenret to tell if it can do in-place substitutions (which can result in considerable speed-up). So like, this lil number is doing a lot of heavy lifting behind the scenes. slay. ✨
gofs 📍
Left offset from pos() to start match at. That's it, that's the field. 💅
substrs 🔍
Substring data about strings that must appear in the final match. Currently this is only used internally by Perl's engine — but it might be used in the future by all engines, for optimisations. foreshadowing? maybe so. 👀
nparens, logical_nparens 🧮
These two keep track of the number of physical and logical paren capture groups in the pattern — and yes, those two numbers can differ, specifically if the pattern includes the branch reset construct (?| ... | ... ). 🌈
Example: the pattern /(?|(foo)|(bar))/ contains two physical capture buffers, but only one logical capture buffer. Two physical, one logical. It's giving illusion of choice. 💀
Most of the internals logic in the regex engine uses the physical capture buffer ids, but the user-exposed logic uses the logical capture buffer ids. The next section has the data structures that let you map from one to the other. 🔁
logical_to_parno, parno_to_logical, parno_to_logical_next 🗺️
These three exist purely to facilitate mapping between logical and physical capture buffer numbers. Here's the vibe of each:
logical_to_parno— an array whose Kth element contains the lowest physical capture buffer id for the Kth logical capture buffer.parno_to_logical— an array whose Kth element contains the logical capture buffer associated with the Kth physical capture buffer.parno_to_logical_next— an array whose Kth element contains the next physical capture buffer with the same logical id, or0if there is none.
⚠️ Note: all three of these arrays are ONLY populated when the pattern includes the use of the branch reset concept. Patterns which do not use branch-reset effectively have a 1:1 mapping between logical and physical, so there is simply no need for this meta-data. Don't go looking for it, it's not there. 🙈
The following table gives an example of how this works:
Pattern /(a) (?| (b) (c) (d) | (e) (f) | (g) ) (h)/
Logical: $1 $2 $3 $4 $2 $3 $2 $5
Physical: 1 2 3 4 5 6 7 8
Next: 0 5 6 0 7 0 0 0Also, note that the 0th element of any of these arrays is not used, because it represents the "entire pattern". 🙅♀️
lastparen, and lastcloseparen 🥇
These two keep track of:
- which was the highest paren to be closed (see
$+in perlvar) - which was the most recent paren to be closed (see
$^Nin perlvar)
Highest vs. most recent. Not the same thing, bestie, don't mix them up. ✨
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, in which case, surprise, they differ. 👀
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. Your engine, your struct, your rules. 💅
offs 🎯
A regexp_paren_pair structure which defines offsets into the string being matched, corresponding to the $& and $1, $2 etc. captures. The regexp_paren_pair struct is defined like this:
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. 😬
And ->offs[0].start/end represents $& (or ${^MATCH} under /p), and ->offs[paren].end matches $$paren where $paren = 1>. 🫶
RX_PRECOMP RX_PRELEN ⚡
Used for optimisations! RX_PRECOMP holds a copy of the pattern that was compiled, and RX_PRELEN holds its length. 📝
So when a new pattern is to be compiled (such as inside a loop — you know the one), the internal regcomp operator checks if the last compiled REGEXP's RX_PRECOMP and RX_PRELEN are equivalent to the new one, and if so, it just uses the old pattern instead of compiling a whole new one. Free real estate. 🔥
In older perls these two macros were actually fields in the structure with the names precomp and prelen respectively. History lesson, you're welcome. 🎀
paren_names 🏷️
This is a hash used internally to track named capture groups and their offsets. The keys are the names of the buffers, and the values are dualvars — 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 🧵
Okay and here's substrs again (yes, it gets two entries, it's that important) — it 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. 🌊
What's it for? Doing Fast-Boyer-Moore searches on the string, to find out if it's even worth using the regex engine at all — and if it is, where in the string to search. Basically it's the "should we even bother" check, and honestly? relatable. 💅
🫧 "subbeg" "sublen" "saved_copy" "suboffset" "subcoffset" 🫧
okay so these five are your search-and-replace era besties 💖 they get used during the execution phase, and they're the ones serving up the actual text for $&, $1 and the whole numbered-capture squad.
subbeg— points at a buffer. that buffer is either the original string, or a copy when you're in theRX_MATCH_COPIED(rx_sv)situation 📋sublen— literally just the length of that buffer, no cap.
the RX_OFFS_START(rx_sv,n) and RX_OFFS_END(rx_sv,n) macros index into this buffer — and so does the data structure that RX_OFFSp(rx_sv) hands you back, but do not touch that one directly, bestie, it's giving "off limits" 🚫
now, when the REXEC_COPY_STR flag is in the chat but REXEC_COPY_SKIP_PRE or REXEC_COPY_SKIP_POST are tagging along too? an engine can choose not to copy the full buffer. however — and this is the important part, no skipping — it must still do the full copy if RXf_PMf_KEEPCOPY is present, or if the relevant bits are set in PL_sawampersand. no exceptions, periodt 💅
when it does skip, the engine can set suboffset to show how many bytes sit between the logical start of the buffer and the physical start (i.e. subbeg). and it should also set subcoffset — that's the number of characters in the offset. why do we need both? because @- and @+ work in characters, not bytes, and they would simply not function without that character count 🌸
🎀 "RX_WRAPPED" "RX_WRAPLEN" 🎀
these macros are your keys to the string that a qr// stringifies into. for example, the Perl engine stores (?^:eek) when you write qr/eek/ ✨
now listen — if you're building a custom engine that does not support the (?:) construct for inline modifiers, it's probably best to have qr// just stringify to the pattern you were supplied. but heads up, that creates some truly cursed patterns in cases like:
my $x = qr/a|b/; # "a|b"
my $y = qr/c/i; # "c"
my $z = qr/$x$y/; # "a|bc"there's genuinely no solution to this problem other than making your custom engine understand a construct like (?:). that's it. that's the only way. 😭
👑 RX_REFCNT() 👑
the number of times the structure is being referenced. when it drops all the way down to 0, the regexp gets automatically freed via a call to pregfree 🕊️
you should set this to 1 in each engine's comp routine. and btw, in older perls this was an actual member in the struct called refcnt — but in more modern perls, where the regexp structure got unified with the SV structure, this is now just an alias to SvREFCNT(). glow up fr fr 🌈
📜 HISTORY 📜
Originally part of perlreguts.
✍️ AUTHORS ✍️
Originally written by Yves Orton, expanded by Ævar Arnfjörð Bjarmason.
⚖️ LICENSE ⚖️
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.
and that's the whole page, bestie 🫶 this was perl v5.42.3, dated 2026-08-03 — iconic documentation, ate and left no crumbs, catch you in the next manpage 💖✨
📦 where this page comes from
- fedora 43: perl-doc 5.42.3-524.fc43 · (GPL-1.0-or-later OR Artistic-1.0-Perl) AND Unicode-3.0 AND LicenseRef-Fedora-UltraPermissive
- fedora 44: perl-doc 5.42.3-525.fc44 · (GPL-1.0-or-later OR Artistic-1.0-Perl) AND Unicode-3.0 AND LicenseRef-Fedora-UltraPermissive
- 💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on : a language model's rewrite of the original, under its terms