PERLREGUTS(1) Perl Programmers Reference Guide PERLREGUTS(1)NAMEperlreguts - Description of the Perl regular expression engine.
DESCRIPTION
This document is an attempt to shine some light on the guts of the
regex engine and how it works. The regex engine represents a signifi‐
cant chunk of the perl codebase, but is relatively poorly understood.
This document is a meagre attempt at addressing this situation. It is
derived from the author's experience, comments in the source code,
other papers on the regex engine, feedback on the perl5-porters mail
list, and no doubt other places as well.
WARNING! It should be clearly understood that this document represents
the state of the regex engine as the author understands it at the time
of writing. It is NOT an API definition; it is purely an internals
guide for those who want to hack the regex engine, or understand how
the regex engine works. Readers of this document are expected to under‐
stand perl's regex syntax and its usage in detail. If you want to learn
about the basics of Perl's regular expressions, see perlre.
OVERVIEW
A quick note on terms
There is some debate as to whether to say "regexp" or "regex". In this
document we will use the term "regex" unless there is a special reason
not to, in which case we will explain why.
When speaking about regexes we need to distinguish between their source
code form and their internal form. In this document we will use the
term "pattern" when we speak of their textual, source code form, the
term "program" when we speak of their internal representation. These
correspond to the terms S-regex and B-regex that Mark Jason Dominus
employs in his paper on "Rx" ([1] in "REFERENCES").
What is a regular expression engine?
A regular expression engine is a program that takes a set of con‐
straints specified in a mini-language, and then applies those con‐
straints to a target string, and determines whether or not the string
satisfies the constraints. See perlre for a full definition of the lan‐
guage.
So in less grandiose terms the first part of the job is to turn a pat‐
tern into something the computer can efficiently use to find the match‐
ing point in the string, and the second part is performing the search
itself.
To do this we need to produce a program by parsing the text. We then
need to execute the program to find the point in the string that
matches. And we need to do the whole thing efficiently.
Structure of a Regexp Program
High Level
Although it is a bit confusing and some people object to the terminol‐
ogy, it is worth taking a look at a comment that has been in regexp.h
for years:
This is essentially a linear encoding of a nondeterministic finite-
state machine (aka syntax charts or "railroad normal form" in parsing
technology).
The term "railroad normal form" is a bit esoteric, with "syntax dia‐
gram/charts", or "railroad diagram/charts" being more common terms.
Nevertheless it provides a useful mental image of a regex program: each
node can be thought of as a unit of track, with a single entry and in
most cases a single exit point (there are pieces of track that fork,
but statistically not many), and the whole forms a layout with a single
entry and single exit point. The matching process can be thought of as
a car that moves along the track, with the particular route through the
system being determined by the character read at each possible connec‐
tor point. A car can fall off the track at any point but it may only
proceed as long as it matches the track.
Thus the pattern "/foo(?:\w+⎪\d+⎪\s+)bar/" can be thought of as the
following chart:
[start]
⎪
<foo>
⎪
+-----+-----+
⎪ ⎪ ⎪
<\w+> <\d+> <\s+>
⎪ ⎪ ⎪
+-----+-----+
⎪
<bar>
⎪
[end]
The truth of the matter is that perl's regular expressions these days
are much more complex than this kind of structure, but visualising it
this way can help when trying to get your bearings, and it matches the
current implementation pretty closely.
To be more precise, we will say that a regex program is an encoding of
a graph. Each node in the graph corresponds to part of the original
regex pattern, such as a literal string or a branch, and has a pointer
to the nodes representing the next component to be matched. Since
"node" and "opcode" already have other meanings in the perl source, we
will call the nodes in a regex program "regops".
The program is represented by an array of "regnode" structures, one or
more of which represent a single regop of the program. Struct "regnode"
is the smallest struct needed, and has a field structure which is
shared with all the other larger structures.
The "next" pointers of all regops except "BRANCH" implement concatena‐
tion; a "next" pointer with a "BRANCH" on both ends of it is connecting
two alternatives. [Here we have one of the subtle syntax dependencies:
an individual "BRANCH" (as opposed to a collection of them) is never
concatenated with anything because of operator precedence.]
The operand of some types of regop is a literal string; for others, it
is a regop leading into a sub-program. In particular, the operand of a
"BRANCH" node is the first regop of the branch.
NOTE: As the railroad metaphor suggests, this is not a tree structure:
the tail of the branch connects to the thing following the set of
"BRANCH"es. It is a like a single line of railway track that splits as
it goes into a station or railway yard and rejoins as it comes out the
other side.
Regops
The base structure of a regop is defined in regexp.h as follows:
struct regnode {
U8 flags; /* Various purposes, sometimes overridden */
U8 type; /* Opcode value as specified by regnodes.h */
U16 next_off; /* Offset in size regnode */
};
Other larger "regnode"-like structures are defined in regcomp.h. They
are almost like subclasses in that they have the same fields as "regn‐
ode", with possibly additional fields following in the structure, and
in some cases the specific meaning (and name) of some of base fields
are overridden. The following is a more complete description.
"regnode_1"
"regnode_2"
"regnode_1" structures have the same header, followed by a single
four-byte argument; "regnode_2" structures contain two two-byte
arguments instead:
regnode_1 U32 arg1;
regnode_2 U16 arg1; U16 arg2;
"regnode_string"
"regnode_string" structures, used for literal strings, follow the
header with a one-byte length and then the string data. Strings are
padded on the end with zero bytes so that the total length of the
node is a multiple of four bytes:
regnode_string char string[1];
U8 str_len; /* overrides flags */
"regnode_charclass"
Character classes are represented by "regnode_charclass" struc‐
tures, which have a four-byte argument and then a 32-byte (256-bit)
bitmap indicating which characters are included in the class.
regnode_charclass U32 arg1;
char bitmap[ANYOF_BITMAP_SIZE];
"regnode_charclass_class"
There is also a larger form of a char class structure used to rep‐
resent POSIX char classes called "regnode_charclass_class" which
has an additional 4-byte (32-bit) bitmap indicating which POSIX
char class have been included.
regnode_charclass_class U32 arg1;
char bitmap[ANYOF_BITMAP_SIZE];
char classflags[ANYOF_CLASSBITMAP_SIZE];
regnodes.h defines an array called "regarglen[]" which gives the size
of each opcode in units of "size regnode" (4-byte). A macro is used to
calculate the size of an "EXACT" node based on its "str_len" field.
The regops are defined in regnodes.h which is generated from reg‐
comp.sym by regcomp.pl. Currently the maximum possible number of dis‐
tinct regops is restricted to 256, with about a quarter already used.
A set of macros makes accessing the fields easier and more consistent.
These include "OP()", which is used to determine the type of a "regn‐
ode"-like structure; "NEXT_OFF()", which is the offset to the next node
(more on this later); "ARG()", "ARG1()", "ARG2()", "ARG_SET()", and
equivalents for reading and setting the arguments; and "STR_LEN()",
"STRING()" and "OPERAND()" for manipulating strings and regop bearing
types.
What regop is next?
There are three distinct concepts of "next" in the regex engine, and it
is important to keep them clear.
· There is the "next regnode" from a given regnode, a value which is
rarely useful except that sometimes it matches up in terms of value
with one of the others, and that sometimes the code assumes this to
always be so.
· There is the "next regop" from a given regop/regnode. This is the
regop physically located after the the current one, as determined
by the size of the current regop. This is often useful, such as
when dumping the structure we use this order to traverse. Sometimes
the code assumes that the "next regnode" is the same as the "next
regop", or in other words assumes that the sizeof a given regop
type is always going to be one regnode large.
· There is the "regnext" from a given regop. This is the regop which
is reached by jumping forward by the value of "NEXT_OFF()", or in a
few cases for longer jumps by the "arg1" field of the "regnode_1"
structure. The subroutine "regnext()" handles this transparently.
This is the logical successor of the node, which in some cases,
like that of the "BRANCH" regop, has special meaning.
Process Overview
Broadly speaking, performing a match of a string against a pattern
involves the following steps:
A. Compilation
1. Parsing for size
2. Parsing for construction
3. Peep-hole optimisation and analysis
B. Execution
4. Start position and no-match optimisations
5. Program execution
Where these steps occur in the actual execution of a perl program is
determined by whether the pattern involves interpolating any string
variables. If interpolation occurs, then compilation happens at run
time. If it does not, then compilation is performed at compile time.
(The "/o" modifier changes this, as does "qr//" to a certain extent.)
The engine doesn't really care that much.
Compilation
This code resides primarily in regcomp.c, along with the header files
regcomp.h, regexp.h and regnodes.h.
Compilation starts with "pregcomp()", which is mostly an initialisation
wrapper which farms work out to two other routines for the heavy lift‐
ing: the first is "reg()", which is the start point for parsing; the
second, "study_chunk()", is responsible for optimisation.
Initialisation in "pregcomp()" mostly involves the creation and data-
filling of a special structure, "RExC_state_t" (defined in regcomp.c).
Almost all internally-used routines in regcomp.h take a pointer to one
of these structures as their first argument, with the name
"pRExC_state". This structure is used to store the compilation state
and contains many fields. Likewise there are many macros which operate
on this variable: anything that looks like "RExC_xxxx" is a macro that
operates on this pointer/structure.
Parsing for size
In this pass the input pattern is parsed in order to calculate how much
space is needed for each regop we would need to emit. The size is also
used to determine whether long jumps will be required in the program.
This stage is controlled by the macro "SIZE_ONLY" being set.
The parse proceeds pretty much exactly as it does during the construc‐
tion phase, except that most routines are short-circuited to change the
size field "RExC_size" and not do anything else.
Parsing for construction
Once the size of the program has been determined, the pattern is parsed
again, but this time for real. Now "SIZE_ONLY" will be false, and the
actual construction can occur.
"reg()" is the start of the parse process. It is responsible for pars‐
ing an arbitrary chunk of pattern up to either the end of the string,
or the first closing parenthesis it encounters in the pattern. This
means it can be used to parse the top-level regex, or any section
inside of a grouping parenthesis. It also handles the "special parens"
that perl's regexes have. For instance when parsing "/x(?:foo)y/"
"reg()" will at one point be called to parse from the "?" symbol up to
and including the ")".
Additionally, "reg()" is responsible for parsing the one or more
branches from the pattern, and for "finishing them off" by correctly
setting their next pointers. In order to do the parsing, it repeatedly
calls out to "regbranch()", which is responsible for handling up to the
first "⎪" symbol it sees.
"regbranch()" in turn calls "regpiece()" which handles "things" fol‐
lowed by a quantifier. In order to parse the "things", "regatom()" is
called. This is the lowest level routine which parses out constant
strings, character classes, and the various special symbols like "$".
If "regatom()" encounters a "(" character it in turn calls "reg()".
The routine "regtail()" is called by both "reg()", "regbranch()" in
order to "set the tail pointer" correctly. When executing and we get to
the end of a branch, we need to go to the node following the grouping
parens. When parsing, however, we don't know where the end will be
until we get there, so when we do we must go back and update the off‐
sets as appropriate. "regtail" is used to make this easier.
A subtlety of the parsing process means that a regex like "/foo/" is
originally parsed into an alternation with a single branch. It is only
afterwards that the optimiser converts single branch alternations into
the simpler form.
Parse Call Graph and a Grammar
The call graph looks like this:
reg() # parse a top level regex, or inside of parens
regbranch() # parse a single branch of an alternation
regpiece() # parse a pattern followed by a quantifier
regatom() # parse a simple pattern
regclass() # used to handle a class
reg() # used to handle a parenthesised subpattern
....
...
regtail() # finish off the branch
...
regtail() # finish off the branch sequence. Tie each
# branch's tail to the tail of the sequence
# (NEW) In Debug mode this is
# regtail_study().
A grammar form might be something like this:
atom : constant ⎪ class
quant : '*' ⎪ '+' ⎪ '?' ⎪ '{min,max}'
_branch: piece
⎪ piece _branch
⎪ nothing
branch: _branch
⎪ _branch '⎪' branch
group : '(' branch ')'
_piece: atom ⎪ group
piece : _piece
⎪ _piece quant
Debug Output
In the 5.9.x development version of perl you can "use re Debug =>
'PARSE'" to see some trace information about the parse process. We will
start with some simple patterns and build up to more complex patterns.
So when we parse "/foo/" we see something like the following table. The
left shows what is being parsed, and the number indicates where the
next regop would go. The stuff on the right is the trace output of the
graph. The names are chosen to be short to make it less dense on the
screen. 'tsdy' is a special form of "regtail()" which does some extra
analysis.
>foo< 1 reg
brnc
piec
atom
>< 4 tsdy~ EXACT <foo> (EXACT) (1)
~ attach to END (3) offset to 2
The resulting program then looks like:
1: EXACT <foo>(3)
3: END(0)
As you can see, even though we parsed out a branch and a piece, it was
ultimately only an atom. The final program shows us how things work. We
have an "EXACT" regop, followed by an "END" regop. The number in parens
indicates where the "regnext" of the node goes. The "regnext" of an
"END" regop is unused, as "END" regops mean we have successfully
matched. The number on the left indicates the position of the regop in
the regnode array.
Now let's try a harder pattern. We will add a quantifier, so now we
have the pattern "/foo+/". We will see that "regbranch()" calls "reg‐
piece()" twice.
>foo+< 1 reg
brnc
piec
atom
>o+< 3 piec
atom
>< 6 tail~ EXACT <fo> (1)
7 tsdy~ EXACT <fo> (EXACT) (1)
~ PLUS (END) (3)
~ attach to END (6) offset to 3
And we end up with the program:
1: EXACT <fo>(3)
3: PLUS(6)
4: EXACT <o>(0)
6: END(0)
Now we have a special case. The "EXACT" regop has a "regnext" of 0.
This is because if it matches it should try to match itself again. The
"PLUS" regop handles the actual failure of the "EXACT" regop and acts
appropriately (going to regnode 6 if the "EXACT" matched at least once,
or failing if it didn't).
Now for something much more complex: "/x(?:foo*⎪b[a][rR])(foo⎪bar)$/"
>x(?:foo*⎪b... 1 reg
brnc
piec
atom
>(?:foo*⎪b[... 3 piec
atom
>?:foo*⎪b[a... reg
>foo*⎪b[a][... brnc
piec
atom
>o*⎪b[a][rR... 5 piec
atom
>⎪b[a][rR])... 8 tail~ EXACT <fo> (3)
>b[a][rR])(... 9 brnc
10 piec
atom
>[a][rR])(f... 12 piec
atom
>a][rR])(fo... clas
>[rR])(foo⎪... 14 tail~ EXACT <b> (10)
piec
atom
>rR])(foo⎪b... clas
>)(foo⎪bar)... 25 tail~ EXACT <a> (12)
tail~ BRANCH (3)
26 tsdy~ BRANCH (END) (9)
~ attach to TAIL (25) offset to 16
tsdy~ EXACT <fo> (EXACT) (4)
~ STAR (END) (6)
~ attach to TAIL (25) offset to 19
tsdy~ EXACT <b> (EXACT) (10)
~ EXACT <a> (EXACT) (12)
~ ANYOF[Rr] (END) (14)
~ attach to TAIL (25) offset to 11
>(foo⎪bar)$< tail~ EXACT <x> (1)
piec
atom
>foo⎪bar)$< reg
28 brnc
piec
atom
>⎪bar)$< 31 tail~ OPEN1 (26)
>bar)$< brnc
32 piec
atom
>)$< 34 tail~ BRANCH (28)
36 tsdy~ BRANCH (END) (31)
~ attach to CLOSE1 (34) offset to 3
tsdy~ EXACT <foo> (EXACT) (29)
~ attach to CLOSE1 (34) offset to 5
tsdy~ EXACT <bar> (EXACT) (32)
~ attach to CLOSE1 (34) offset to 2
>$< tail~ BRANCH (3)
~ BRANCH (9)
~ TAIL (25)
piec
atom
>< 37 tail~ OPEN1 (26)
~ BRANCH (28)
~ BRANCH (31)
~ CLOSE1 (34)
38 tsdy~ EXACT <x> (EXACT) (1)
~ BRANCH (END) (3)
~ BRANCH (END) (9)
~ TAIL (END) (25)
~ OPEN1 (END) (26)
~ BRANCH (END) (28)
~ BRANCH (END) (31)
~ CLOSE1 (END) (34)
~ EOL (END) (36)
~ attach to END (37) offset to 1
Resulting in the program
1: EXACT <x>(3)
3: BRANCH(9)
4: EXACT <fo>(6)
6: STAR(26)
7: EXACT <o>(0)
9: BRANCH(25)
10: EXACT <ba>(14)
12: OPTIMIZED (2 nodes)
14: ANYOF[Rr](26)
25: TAIL(26)
26: OPEN1(28)
28: TRIE-EXACT(34)
[StS:1 Wds:2 Cs:6 Uq:5 #Sts:7 Mn:3 Mx:3 Stcls:bf]
<foo>
<bar>
30: OPTIMIZED (4 nodes)
34: CLOSE1(36)
36: EOL(37)
37: END(0)
Here we can see a much more complex program, with various optimisations
in play. At regnode 10 we see an example where a character class with
only one character in it was turned into an "EXACT" node. We can also
see where an entire alternation was turned into a "TRIE-EXACT" node. As
a consequence, some of the regnodes have been marked as optimised away.
We can see that the "$" symbol has been converted into an "EOL" regop,
a special piece of code that looks for "\n" or the end of the string.
The next pointer for "BRANCH"es is interesting in that it points at
where execution should go if the branch fails. When executing if the
engine tries to traverse from a branch to a "regnext" that isn't a
branch then the engine will know that the entire set of branches have
failed.
Peep-hole Optimisation and Analysis
The regular expression engine can be a weighty tool to wield. On long
strings and complex patterns it can end up having to do a lot of work
to find a match, and even more to decide that no match is possible.
Consider a situation like the following pattern.
'ababababababababababab' =~ /(a⎪b)*z/
The "(a⎪b)*" part can match at every char in the string, and then fail
every time because there is no "z" in the string. So obviously we can
avoid using the regex engine unless there is a "z" in the string.
Likewise in a pattern like:
/foo(\w+)bar/
In this case we know that the string must contain a "foo" which must be
followed by "bar". We can use Fast Boyer-Moore matching as implemented
in "fbm_instr()" to find the location of these strings. If they don't
exist then we don't need to resort to the much more expensive regex
engine. Even better, if they do exist then we can use their positions
to reduce the search space that the regex engine needs to cover to
determine if the entire pattern matches.
There are various aspects of the pattern that can be used to facilitate
optimisations along these lines:
* anchored fixed strings
* floating fixed strings
* minimum and maximum length requirements
* start class
* Beginning/End of line positions
Another form of optimisation that can occur is post-parse "peep-hole"
optimisations, where inefficient constructs are replaced by more effi‐
cient constructs. An example of this are "TAIL" regops which are used
during parsing to mark the end of branches and the end of groups. These
regops are used as place-holders during construction and "always match"
so they can be "optimised away" by making the things that point to the
"TAIL" point to thing that the "TAIL" points to, thus "skipping" the
node.
Another optimisation that can occur is that of ""EXACT" merging" which
is where two consecutive "EXACT" nodes are merged into a single regop.
An even more aggressive form of this is that a branch sequence of the
form "EXACT BRANCH ... EXACT" can be converted into a "TRIE-EXACT"
regop.
All of this occurs in the routine "study_chunk()" which uses a special
structure "scan_data_t" to store the analysis that it has performed,
and does the "peep-hole" optimisations as it goes.
The code involved in "study_chunk()" is extremely cryptic. Be careful.
:-)
Execution
Execution of a regex generally involves two phases, the first being
finding the start point in the string where we should match from, and
the second being running the regop interpreter.
If we can tell that there is no valid start point then we don't bother
running interpreter at all. Likewise, if we know from the analysis
phase that we cannot detect a short-cut to the start position, we go
straight to the interpreter.
The two entry points are "re_intuit_start()" and "pregexec()". These
routines have a somewhat incestuous relationship with overlap between
their functions, and "pregexec()" may even call "re_intuit_start()" on
its own. Nevertheless other parts of the the perl source code may call
into either, or both.
Execution of the interpreter itself used to be recursive. Due to the
efforts of Dave Mitchell in the 5.9.x development track, it is now
iterative. Now an internal stack is maintained on the heap and the rou‐
tine is fully iterative. This can make it tricky as the code is quite
conservative about what state it stores, with the result that that two
consecutive lines in the code can actually be running in totally dif‐
ferent contexts due to the simulated recursion.
Start position and no-match optimisations
"re_intuit_start()" is responsible for handling start points and no-
match optimisations as determined by the results of the analysis done
by "study_chunk()" (and described in "Peep-hole Optimisation and Analy‐
sis").
The basic structure of this routine is to try to find the start- and/or
end-points of where the pattern could match, and to ensure that the
string is long enough to match the pattern. It tries to use more effi‐
cient methods over less efficient methods and may involve considerable
cross-checking of constraints to find the place in the string that
matches. For instance it may try to determine that a given fixed
string must be not only present but a certain number of chars before
the end of the string, or whatever.
It calls several other routines, such as "fbm_instr()" which does Fast
Boyer Moore matching and "find_byclass()" which is responsible for
finding the start using the first mandatory regop in the program.
When the optimisation criteria have been satisfied, "reg_try()" is
called to perform the match.
Program execution
"pregexec()" is the main entry point for running a regex. It contains
support for initialising the regex interpreter's state, running
"re_intuit_start()" if needed, and running the interpreter on the
string from various start positions as needed. When it is necessary to
use the regex interpreter "pregexec()" calls "regtry()".
"regtry()" is the entry point into the regex interpreter. It expects as
arguments a pointer to a "regmatch_info" structure and a pointer to a
string. It returns an integer 1 for success and a 0 for failure. It
is basically a set-up wrapper around "regmatch()".
"regmatch" is the main "recursive loop" of the interpreter. It is basi‐
cally a giant switch statement that implements a state machine, where
the possible states are the regops themselves, plus a number of addi‐
tional intermediate and failure states. A few of the states are imple‐
mented as subroutines but the bulk are inline code.
MISCELLANEOUS
Unicode and Localisation Support
When dealing with strings containing characters that cannot be repre‐
sented using an eight-bit character set, perl uses an internal repre‐
sentation that is a permissive version of Unicode's UTF-8 encoding[2].
This uses single bytes to represent characters from the ASCII character
set, and sequences of two or more bytes for all other characters. (See
perlunitut for more information about the relationship between UTF-8
and perl's encoding, utf8 -- the difference isn't important for this
discussion.)
No matter how you look at it, Unicode support is going to be a pain in
a regex engine. Tricks that might be fine when you have 256 possible
characters often won't scale to handle the size of the UTF-8 character
set. Things you can take for granted with ASCII may not be true with
Unicode. For instance, in ASCII, it is safe to assume that
"sizeof(char1) == sizeof(char2)", but in UTF-8 it isn't. Unicode case
folding is vastly more complex than the simple rules of ASCII, and even
when not using Unicode but only localised single byte encodings, things
can get tricky (for example, LATIN SMALL LETTER SHARP S (U+00DF, ss)
should match 'SS' in localised case-insensitive matching).
Making things worse is that UTF-8 support was a later addition to the
regex engine (as it was to perl) and this necessarily made things a
lot more complicated. Obviously it is easier to design a regex engine
with Unicode support in mind from the beginning than it is to retrofit
it to one that wasn't.
Nearly all regops that involve looking at the input string have two
cases, one for UTF-8, and one not. In fact, it's often more complex
than that, as the pattern may be UTF-8 as well.
Care must be taken when making changes to make sure that you handle
UTF-8 properly, both at compile time and at execution time, including
when the string and pattern are mismatched.
The following comment in regcomp.h gives an example of exactly how
tricky this can be:
Two problematic code points in Unicode casefolding of EXACT nodes:
U+0390 - GREEK SMALL LETTER IOTA WITH DIALYTIKA AND TONOS
U+03B0 - GREEK SMALL LETTER UPSILON WITH DIALYTIKA AND TONOS
which casefold to
Unicode UTF-8
U+03B9 U+0308 U+0301 0xCE 0xB9 0xCC 0x88 0xCC 0x81
U+03C5 U+0308 U+0301 0xCF 0x85 0xCC 0x88 0xCC 0x81
This means that in case-insensitive matching (or "loose matching",
as Unicode calls it), an EXACTF of length six (the UTF-8 encoded
byte length of the above casefolded versions) can match a target
string of length two (the byte length of UTF-8 encoded U+0390 or
U+03B0). This would rather mess up the minimum length computation.
What we'll do is to look for the tail four bytes, and then peek
at the preceding two bytes to see whether we need to decrease
the minimum length by four (six minus two).
Thanks to the design of UTF-8, there cannot be false matches:
A sequence of valid UTF-8 bytes cannot be a subsequence of
another valid sequence of UTF-8 bytes.
Base Struct
regexp.h contains the base structure definition:
typedef struct regexp {
I32 *startp;
I32 *endp;
regnode *regstclass;
struct reg_substr_data *substrs;
char *precomp; /* pre-compilation regular expression */
struct reg_data *data; /* Additional data. */
char *subbeg; /* saved or original string
so \digit works forever. */
U32 *offsets; /* offset annotations 20001228 MJD */
I32 sublen; /* Length of string pointed by subbeg */
I32 refcnt;
I32 minlen; /* minimum possible length of $& */
I32 prelen; /* length of precomp */
U32 nparens; /* number of parentheses */
U32 lastparen; /* last paren matched */
U32 lastcloseparen; /* last paren matched */
U32 reganch; /* Internal use only +
Tainted information used by regexec? */
regnode program[1]; /* Unwarranted chumminess with compiler. */
} regexp;
"program", and "data" are the primary fields of concern in terms of
program structure. "program" is the actual array of nodes, and "data"
is an array of "whatever", with each whatever being typed by letter,
and freed or cloned as needed based on this type. regops use the data
array to store reference data that isn't convenient to store in the
regop itself. It also means memory management code doesn't need to tra‐
verse the program to find pointers. So for instance, if a regop needs a
pointer, the normal procedure is use a "regnode_arg1" store the data
index in the "ARG" field and look it up from the data array.
- "startp", "endp", "nparens", "lasparen", and "lastcloseparen" are
used to manage capture buffers.
- "subbeg" and optional "saved_copy" are used during the execution
phase for managing replacements.
- "offsets" and "precomp" are used for debugging purposes.
- The rest are used for start point optimisations.
De-allocation and Cloning
Any patch that adds data items to the regexp will need to include
changes to sv.c ("Perl_re_dup()") and regcomp.c ("pregfree()"). This
involves freeing or cloning items in the regexes data array based on
the data item's type.
SEE ALSO
perlre
perlunitut
AUTHOR
by Yves Orton, 2006.
With excerpts from Perl, and contributions and suggestions from Ronald
J. Kimball, Dave Mitchell, Dominic Dunlop, Mark Jason Dominus, Stephen
McCamant, and David Landgren.
LICENCE
Same terms as Perl.
REFERENCES
[1] <http://perl.plover.com/Rx/paper/>
[2] <http://www.unicode.org>
perl v5.8.8 2008-09-19 PERLREGUTS(1)
