PERLREGUTS(1) Perl Programmers Reference Guide PERLREGUTS(1) #
PERLREGUTS(1) Perl Programmers Reference Guide PERLREGUTS(1)
NNAAMMEE #
perlreguts - Description of the Perl regular expression engine.
DDEESSCCRRIIPPTTIIOONN #
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.
NNOOTTIICCEE!! 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 NNOOTT 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.
OOVVEERRVVIIEEWW #
AA qquuiicckk nnoottee oonn tteerrmmss 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_-_r_e_g_e_x and _B_-_r_e_g_e_x that Mark Jason Dominus
employs in his paper on "Rx" ([1] in "REFERENCES").
WWhhaatt iiss aa rreegguullaarr eexxpprreessssiioonn eennggiinnee?? 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.
SSttrruuccttuurree ooff aa RReeggeexxpp PPrrooggrraamm _H_i_g_h _L_e_v_e_l
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 _r_e_g_e_x_p_._h for
years:
_T_h_i_s _i_s _e_s_s_e_n_t_i_a_l_l_y _a _l_i_n_e_a_r _e_n_c_o_d_i_n_g _o_f _a _n_o_n_d_e_t_e_r_m_i_n_i_s_t_i_c _f_i_n_i_t_e_-_s_t_a_t_e
_m_a_c_h_i_n_e _(_a_k_a _s_y_n_t_a_x _c_h_a_r_t_s _o_r _"_r_a_i_l_r_o_a_d _n_o_r_m_a_l _f_o_r_m_" _i_n _p_a_r_s_i_n_g
_t_e_c_h_n_o_l_o_g_y_)_.
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.
NNOOTTEE: As the railroad metaphor suggests, this is nnoott 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.
_R_e_g_o_p_s
The base structure of a regop is defined in _r_e_g_e_x_p_._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 _r_e_g_c_o_m_p_._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;
_r_e_g_n_o_d_e_s_._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 _r_e_g_n_o_d_e_s_._h which is generated from _r_e_g_c_o_m_p_._s_y_m
by _r_e_g_c_o_m_p_._p_l. 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.
_W_h_a_t _r_e_g_o_p _i_s _n_e_x_t_?
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 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.
PPrroocceessss OOvveerrvviieeww 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.
CCoommppiillaattiioonn This code resides primarily in _r_e_g_c_o_m_p_._c, along with the header files _r_e_g_c_o_m_p_._h, _r_e_g_e_x_p_._h and _r_e_g_n_o_d_e_s_._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 _r_e_g_c_o_m_p_._c).
Almost all internally-used routines in _r_e_g_c_o_m_p_._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.
_P_a_r_s_e _C_a_l_l _G_r_a_p_h _a_n_d _a _G_r_a_m_m_a_r
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
_P_a_r_s_i_n_g _c_o_m_p_l_i_c_a_t_i_o_n_s
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()"
_e_t_c 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
rreegg(()) 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 (_Q_u_a_n_t_i_f_i_e_r _f_o_l_l_o_w_s _n_o_t_h_i_n_g _i_n _r_e_g_e_x_; _m_a_r_k_e_d _b_y _<_-_- _H_E_R_E _i_n
_m_/_(_?_i_)_+ _<_-_- _H_E_R_E _/).
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".
_D_e_b_u_g _O_u_t_p_u_t
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.
_P_e_e_p_-_h_o_l_e _O_p_t_i_m_i_s_a_t_i_o_n _a_n_d _A_n_a_l_y_s_i_s
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.
:-)
EExxeeccuuttiioonn 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.
_S_t_a_r_t _p_o_s_i_t_i_o_n _a_n_d _n_o_-_m_a_t_c_h _o_p_t_i_m_i_s_a_t_i_o_n_s
"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.
_P_r_o_g_r_a_m _e_x_e_c_u_t_i_o_n
"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.
MMIISSCCEELLLLAANNEEOOUUSS #
UUnniiccooddee aanndd LLooccaalliissaattiioonn SSuuppppoorrtt 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, LLAATTIINN SSMMAALLLL LLEETTTTEERR SSHHAARRPP SS (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.
BBaassee SSttrruuccttuurreess 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.
_P_e_r_l_'_s _"_p_p_r_i_v_a_t_e_" _s_t_r_u_c_t_u_r_e
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 aadddd__ddaattaa(()) 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.
SSEEEE AALLSSOO #
perlreapi
perlre
perlunitut
AAUUTTHHOORR #
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.
LLIICCEENNCCEE #
Same terms as Perl.
RREEFFEERREENNCCEESS #
[1] <https://perl.plover.com/Rx/paper/>
[2] <https://www.unicode.org/>
perl v5.36.3 2023-02-15 PERLREGUTS(1)