PERLREGUTS(1) Perl Programmers Reference Guide PERLREGUTS(1)
NAME
perlreguts - 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 significant 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.
NOTICE! It should be clearly understood that the behavior and structures
discussed in this represents the state of the engine as the author
understood 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 understand perl's regex syntax and its usage in detail. If
you want to learn about the basics of Perl's regular expressions, see
perlre. And if you want to replace the regex engine with your own, see
perlreapi.
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, and 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 constraints
specified in a mini-language, and then applies those constraints to a
target string, and determines whether or not the string satisfies the
constraints. See perlre for a full definition of the language.
In less grandiose terms, the first part of the job is to turn a pattern
into something the computer can efficiently use to find the matching
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
terminology, 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
diagram/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 connector
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. (Outside this document, the term
"regnode" is sometimes used to mean "regop", which could be confusing.)
The "next" pointers of all regops except "BRANCH" implement
concatenation; 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
"regnode", 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 tail 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"
Bracketed character classes are represented by "regnode_charclass"
structures, which have a four-byte argument and then a 32-byte
(256-bit) bitmap indicating which characters in the Latin1 range are
included in the class.
regnode_charclass U32 arg1;
char bitmap[ANYOF_BITMAP_SIZE];
Various flags whose names begin with "ANYOF_" are used for special
situations. Above Latin1 matches and things not known until run-
time are stored in "Perl's pprivate structure".
"regnode_charclass_posixl"
There is also a larger form of a char class structure used to
represent POSIX char classes under "/l" matching, called
"regnode_charclass_posixl" which has an additional 32-bit bitmap
indicating which POSIX char classes have been included.
regnode_charclass_posixl U32 arg1;
char bitmap[ANYOF_BITMAP_SIZE];
U32 classflags;
regnodes.h defines an array called "PL_regnode_arg_len[]" 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 regcomp.sym
by regcomp.pl. Currently the maximum possible number of distinct 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
"regnode"-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 regnode is next?
There are two distinct concepts of "next regnode" in the regex engine,
and it is important to keep them distinct in your thinking as they
overlap conceptually in many places, but where they don't overlap the
difference is critical. For the majority of regnode types the two
concepts are (nearly) identical in practice. The two types are
"REGNODE_AFTER" which is used heavily during compilation but only
occasionally during execution and "regnext" which is used heavily during
execution, and only occasionally during compilation.
"REGNODE_AFTER"
This is the "positionally next regnode" in the compiled regex
program. For the smaller regnode types it is "regnode_ptr+1" under
the hood, but as regnode sizes vary and can change over time we
offer macros which hide the gory details.
It is heavily used in the compiler phase but is only used by a few
select regnode types in the execution phase. It is also heavily used
in the code for dumping the regexp program for debugging.
There are a selection of macros which can be used to compute this as
efficiently as possible depending on the circumstances. The
canonical macro is REGNODE_AFTER(), which is the most powerful and
should handle any case we have, but is also potentially the slowest.
There are two additional macros for the special case that you KNOW
the current regnode size is constant, and you know its type or
opcode. In which case you can use REGNODE_AFTER_opcode() or
REGNODE_AFTER_type().
In older versions of the regex engine REGNODE_AFTER() was called
"NEXTOPER" but this was found to be confusing and it was renamed.
There is also a REGNODE_BEFORE(), but it is unsafe and should not be
used in new code.
"regnext"
This is the regnode which can be reached by jumping forward by the
value of the NEXT_OFF() member of the regnode, or in a few cases for
longer jumps by the "arg1" field of the "regnode_1" structure. The
subroutine regnext() handles this transparently. In the majority of
cases the "regnext" for a regnode is the regnode which should be
executed after the current one has successfully matched, but in some
cases this may not be true. In loop control and branch control
regnode types the regnext may signify something special, for BRANCH
nodes "regnext" is the next BRANCH that should be executed if the
current one fails execution, and some loop control regnodes set the
regnext to be the end of the loop so they can jump to their cleanup
if the current iteration fails to match.
Most regnode types do not create a branch in the execution flow, and
leaving aside optimizations the two concepts of "next" are the same.
For instance the "regnext" and "REGNODE_AFTER" of a SBOL opcode are the
same during compilation phase. The main place this is not true is
"BRANCH" regnodes where the "REGNODE_AFTER" represents the start of the
pattern in the branch and the "regnext" represents the linkage to the
next BRANCH should this one fail to match, or 0 if it is the last
branch. The looping logic for quantifiers also makes similar use of the
distinction between the two types, with "REGNODE_AFTER" being the inside
of the loop construct, and the "regnext" pointing at the end of the
loop.
During compilation the engine may not know what the regnext is for a
given node, so during compilation "regnext" is only used where it must
be used and is known to be correct. At the very end of the compilation
phase we walk the regex program and correct the regnext data as
appropriate, and also perform various optimizations which may result in
regnodes that were required during construction becoming redundant, or
we may replace a large regnode with a much smaller one and filling in
the gap with OPTIMIZED regnodes. Thus we might start with something like
this:
BRANCH
EXACT "foo"
BRANCH
EXACT "bar"
EXACT "!"
and replace it with something like:
TRIE foo|bar
OPTIMIZED
OPTIMIZED
OPTIMIZED
EXACT "!"
the "REGNODE_AFTER" for the "TRIE" node would be an "OPTIMIZED" regnode,
and in theory the "regnext" would be the same as the "REGNODE_AFTER".
But it would be inefficient to execute the OPTIMIZED regnode as a noop
three times, so the optimizer fixes the "regnext" so such nodes are
skipped during execution phase.
During execution phases we use the regnext() almost exclusively, and
only use "REGNODE_AFTER" in special cases where it has a well defined
meaning for a given regnode type. For instance /x+/ results in
PLUS
EXACT "x"
END
the "regnext" of the "PLUS" regnode is the "END" regnode, and the
"REGNODE_AFTER" of the "PLUS" regnode is the "EXACT" regnode. The
"regnext" and "REGNODE_AFTER" of the "EXACT" regnode is the "END"
regnode.
Process Overview
Broadly speaking, performing a match of a string against a pattern
involves the following steps:
A. Compilation
1. Parsing
2. Peep-hole optimisation and analysis
B. Execution
3. Start position and no-match optimisations
4. 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
lifting: 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.
reg() is the start of the parse process. It is responsible for parsing
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" followed 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().
There used to be two main passes involved in parsing, the first to
calculate the size of the compiled program, and the second to actually
compile it. But now there is only one main pass, with an initial crude
guess based on the length of the input pattern, which is increased if
necessary as parsing proceeds, and afterwards, trimmed to the actual
amount used.
However, it may happen that parsing must be restarted at the beginning
when various circumstances occur along the way. An example is if the
program turns out to be so large that there are jumps in it that won't
fit in the normal 16 bits available. There are two special regops that
can hold bigger jump destinations, BRANCHJ and LONGBRANCH. The parse is
restarted, and these are used instead of the normal shorter ones.
Whenever restarting the parse is required, the function returns failure
and sets a flag as to what needs to be done. This is passed up to the
top level routine which takes the appropriate action and restarts from
scratch. In the case of needing longer jumps, the "RExC_use_BRANCHJ"
flag is set in the "RExC_state_t" structure, which the functions know to
inspect before deciding how to do branches.
In most instances, the function that discovers the issue sets the causal
flag and returns failure immediately. "Parsing complications" contains
an explicit example of how this works. In other cases, such as a
forward reference to a numbered parenthetical grouping, we need to
finish the parse to know if that numbered grouping actually appears in
the pattern. In those cases, the parse is just redone at the end, with
the knowledge of how many groupings occur in it.
The routine regtail() is called by both reg() and 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 offsets 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
Parsing complications
The implication of the above description is that a pattern containing
nested parentheses will result in a call graph which cycles through
reg(), regbranch(), regpiece(), regatom(), reg(), regbranch() etc
multiple times, until the deepest level of nesting is reached. All the
above routines return a pointer to a "regnode", which is usually the
last regnode added to the program. However, one complication is that
reg() returns NULL for parsing "(?:)" syntax for embedded modifiers,
setting the flag "TRYAGAIN". The "TRYAGAIN" propagates upwards until it
is captured, in some cases by regatom(), but otherwise unconditionally
by regbranch(). Hence it will never be returned by regbranch() to reg().
This flag permits patterns such as "(?i)+" to be detected as errors
(Quantifier follows nothing in regex; marked by <-- HERE in m/(?i)+ <--
HERE /).
Another complication is that the representation used for the program
differs if it needs to store Unicode, but it's not always possible to
know for sure whether it does until midway through parsing. The Unicode
representation for the program is larger, and cannot be matched as
efficiently. (See "Unicode and Localisation Support" below for more
details as to why.) If the pattern contains literal Unicode, it's
obvious that the program needs to store Unicode. Otherwise, the parser
optimistically assumes that the more efficient representation can be
used, and starts sizing on this basis. However, if it then encounters
something in the pattern which must be stored as Unicode, such as an
"\x{...}" escape sequence representing a character literal, then this
means that all previously calculated sizes need to be redone, using
values appropriate for the Unicode representation. This is another
instance where the parsing needs to be restarted, and it can and is done
immediately. The function returns failure, and sets the flag
"RESTART_UTF8" (encapsulated by using the macro "REQUIRE_UTF8"). This
restart request is propagated up the call chain in a similar fashion,
until it is "caught" in Perl_re_op_compile(), which marks the pattern as
containing Unicode, and restarts the sizing pass. It is also possible
for constructions within run-time code blocks to turn out to need
Unicode representation., which is signalled by S_compile_runtime_code()
returning false to Perl_re_op_compile().
The restart was previously implemented using a "longjmp" in regatom()
back to a "setjmp" in Perl_re_op_compile(), but this proved to be
problematic as the latter is a large function containing many automatic
variables, which interact badly with the emergent control flow of
"setjmp".
Debug Output
Starting 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 regpiece()
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 has
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 the post-parse "peep-
hole" optimisation, where inefficient constructs are replaced by more
efficient constructs. The "TAIL" regops which are used during parsing to
mark the end of branches and the end of groups are examples of this.
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 the thing that "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 the 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 perl source code may call into
either, or both.
Execution of the interpreter itself used to be recursive, but thanks to
the efforts of Dave Mitchell in the 5.9.x development track, that has
changed: now an internal stack is maintained on the heap and the routine
is fully iterative. This can make it tricky as the code is quite
conservative about what state it stores, with the result that two
consecutive lines in the code can actually be running in totally
different 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 Analysis").
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
efficient 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
basically a giant switch statement that implements a state machine,
where the possible states are the regops themselves, plus a number of
additional intermediate and failure states. A few of the states are
implemented as subroutines but the bulk are inline code.
MISCELLANEOUS
Unicode and Localisation Support
When dealing with strings containing characters that cannot be
represented using an eight-bit character set, perl uses an internal
representation 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.
Base Structures
The "regexp" structure described in perlreapi is common to all regex
engines. Two of its fields are intended for the private use of the regex
engine that compiled the pattern. These are the "intflags" and pprivate
members. The "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. In the case of the stock
engine the structure pointed to by "pprivate" is called
"regexp_internal".
Its "pprivate" and "intflags" fields contain data specific to each
engine.
There are two structures used to store a compiled regular expression.
One, the "regexp" structure described in perlreapi is populated by the
engine currently being used and some of its fields read by perl to
implement things such as the stringification of "qr//".
The other structure is pointed to by the "regexp" struct's "pprivate"
and is in addition to "intflags" in the same struct considered to be the
property of the regex engine which compiled the regular expression;
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. The "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.
As mentioned earlier, in the case of the default engines, the "pprivate"
will be a pointer to a regexp_internal structure which holds the
compiled program and any additional data that is private to the regex
engine implementation.
Perl's "pprivate" structure
The following structure is used as the "pprivate" struct by perl's regex
engine. Since it is specific to perl it is only of curiosity value to
other engine implementations.
typedef struct regexp_internal {
regnode *regstclass;
struct reg_data *data;
struct reg_code_blocks *code_blocks;
U32 proglen;
U32 name_list_idx;
regnode program[1];
} regexp_internal;
Description of the attributes is as follows:
"regstclass"
Special regop that is used by re_intuit_start() to check if a
pattern can match at a certain position. For instance if the regex
engine knows that the pattern must start with a 'Z' then it can
scan the string until it finds one and then launch the regex engine
from there. The routine that handles this is called
find_by_class(). Sometimes this field points at a regop embedded in
the program, and sometimes it points at an independent synthetic
regop that has been constructed by the optimiser.
"data"
This field points at a "reg_data" structure, which is defined as
follows
struct reg_data {
U32 count;
U8 *what;
void* data[1];
};
This structure is used for handling data structures that the regex
engine needs to handle specially during a clone or free operation
on the compiled product. Each element in the data array has a
corresponding element in the what array. During compilation regops
that need special structures stored will add an element to each
array using the add_data() routine and then store the index in the
regop.
In modern perls the 0th element of this structure is reserved and
is NEVER used to store anything of use. This is to allow things
that need to index into this array to represent "no value".
"code_blocks"
This optional structure is used to manage "(?{})" constructs in the
pattern. It is made up of the following structures.
/* record the position of a (?{...}) within a pattern */
struct reg_code_block {
STRLEN start;
STRLEN end;
OP *block;
REGEXP *src_regex;
};
/* array of reg_code_block's plus header info */
struct reg_code_blocks {
int refcnt; /* we may be pointed to from a regex
and from the savestack */
int count; /* how many code blocks */
struct reg_code_block *cb; /* array of reg_code_block's */
};
"proglen"
Stores the length of the compiled program in units of regops.
"name_list_idx"
This is the index into the data array where an AV is stored that
contains the names of any named capture buffers in the pattern,
should there be any. This is only used in the debugging version of
the regex engine and when RXp_PAREN_NAMES(prog) is true. It will be
0 if there is no such data.
"program"
Compiled program. Inlined into the structure so the entire struct
can be treated as a single blob.
SEE ALSO
perlreapi
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.
Now maintained by Perl 5 Porters.
LICENCE
Same terms as Perl.
REFERENCES
[1] <https://perl.plover.com/Rx/paper/>
[2] <https://www.unicode.org/>
perl v5.40.1 2026-08-30 PERLREGUTS(1)
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