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.
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 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 "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
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 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 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 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.
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 construction 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
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()".
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.
Currently, all regular expression constructions which can trigger this are
parsed by code in "regatom()".
To avoid wasted work when a restart is needed, the sizing pass is abandoned -
"regatom()" immediately returns NULL, setting the flag
"RESTART_UTF8". (This action is encapsulated 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
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 {
U32 *offsets; /* offset annotations 20001228 MJD
* data about mapping the program to
* the string*/
regnode *regstclass; /* Optional startclass as identified or
* constructed by the optimiser */
struct reg_data *data; /* Additional miscellaneous data used
* by the program. Used to make it
* easier to clone and free arbitrary
* data that the regops need. Often the
* ARG field of a regop is an index
* into this structure */
regnode program[1]; /* Unwarranted chumminess with
* compiler. */
} regexp_internal;
- "offsets"
- Offsets holds a mapping of offset in the "program" to offset in
the "precomp" string. This is only used by ActiveState's visual
regex debugger.
- "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.
- "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.
LICENCE¶
Same terms as Perl.
REFERENCES¶
[1] <
http://perl.plover.com/Rx/paper/>
[2] <
http://www.unicode.org>