perlreguts
(1)
名称
perlreguts - Description of the Perl regular expression
engine.
用法概要
Please see following description for synopsis
描述
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.
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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.
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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"
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"regnode_1" structures have the same header, followed by
a single four-byte argument; "regnode_2" structures
contain two two-byte arguments instead:
regnode_1 U32 arg1;
regnode_2 U16 arg1; U16 arg2;
"regnode_string"
"regnode_string" structures, used for literal strings,
follow the header with a one-byte length and then the
string data. Strings are padded on the end with zero
bytes so that the total length of the node is a multiple
of four bytes:
regnode_string char string[1];
U8 str_len; /* overrides flags */
"regnode_charclass"
Character classes are represented by "regnode_charclass"
structures, which have a four-byte argument and then a
32-byte (256-bit) bitmap indicating which characters are
included in the class.
regnode_charclass U32 arg1;
char bitmap[ANYOF_BITMAP_SIZE];
"regnode_charclass_class"
There is also a larger form of a char class structure
used to represent POSIX char classes called
"regnode_charclass_class" which has an additional 4-byte
(32-bit) bitmap indicating which POSIX char classes have
been included.
regnode_charclass_class U32 arg1;
char bitmap[ANYOF_BITMAP_SIZE];
char classflags[ANYOF_CLASSBITMAP_SIZE];
regnodes.h defines an array called "regarglen[]" which gives
the size of each opcode in units of "size regnode" (4-byte).
A macro is used to calculate the size of an "EXACT" node
based on its "str_len" field.
The regops are defined in regnodes.h which is generated from
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
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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.
o 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.
o 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.
o 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.
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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 ")".
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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:
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atom : constant | class
quant : '*' | '+' | '?' | '{min,max}'
_branch: piece
| piece _branch
| nothing
branch: _branch
| _branch '|' branch
group : '(' branch ')'
_piece: atom | group
piece : _piece
| _piece quant
Debug Output
In the 5.9.x development version of perl you can "use re
Debug => 'PARSE'" to see some trace information about the
parse process. We will start with some simple patterns and
build up to more complex patterns.
So when we parse "/foo/" we see something like the following
table. The left shows what is being parsed, and the number
indicates where the next regop would go. The stuff on the
right is the trace output of the graph. The names are chosen
to be short to make it less dense on the screen. 'tsdy' is a
special form of "regtail()" which does some extra analysis.
>foo< 1 reg
brnc
piec
atom
>< 4 tsdy~ EXACT <foo> (EXACT) (1)
~ attach to END (3) offset to 2
The resulting program then looks like:
1: EXACT <foo>(3)
3: END(0)
As you can see, even though we parsed out a branch and a
piece, it was ultimately only an atom. The final program
shows us how things work. We have an "EXACT" regop, followed
by an "END" regop. The number in parens indicates where the
"regnext" of the node goes. The "regnext" of an "END" regop
is unused, as "END" regops mean we have successfully
matched. The number on the left indicates the position of
the regop in the regnode array.
Now let's try a harder pattern. We will add a quantifier, so
now we have the pattern "/foo+/". We will see that
"regbranch()" calls "regpiece()" twice.
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>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)
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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
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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:
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/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:
o anchored fixed strings
o floating fixed strings
o minimum and maximum length requirements
o start class
o 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
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interpreter.
If we can tell that there is no valid start point then we
don't bother running interpreter at all. Likewise, if we
know from the analysis phase that we cannot detect a short-
cut to the start position, we go straight to the
interpreter.
The two entry points are "re_intuit_start()" and
"pregexec()". These routines have a somewhat incestuous
relationship with overlap between their functions, and
"pregexec()" may even call "re_intuit_start()" on its own.
Nevertheless other parts of the 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
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"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.
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Nearly all regops that involve looking at the input string
have two cases, one for UTF-8, and one not. In fact, it's
often more complex than that, as the pattern may be UTF-8 as
well.
Care must be taken when making changes to make sure that you
handle UTF-8 properly, both at compile time and at execution
time, including when the string and pattern are mismatched.
The following comment in regcomp.h gives an example of
exactly how tricky this can be:
Two problematic code points in Unicode casefolding of EXACT nodes:
U+0390 - GREEK SMALL LETTER IOTA WITH DIALYTIKA AND TONOS
U+03B0 - GREEK SMALL LETTER UPSILON WITH DIALYTIKA AND TONOS
which casefold to
Unicode UTF-8
U+03B9 U+0308 U+0301 0xCE 0xB9 0xCC 0x88 0xCC 0x81
U+03C5 U+0308 U+0301 0xCF 0x85 0xCC 0x88 0xCC 0x81
This means that in case-insensitive matching (or "loose matching",
as Unicode calls it), an EXACTF of length six (the UTF-8 encoded
byte length of the above casefolded versions) can match a target
string of length two (the byte length of UTF-8 encoded U+0390 or
U+03B0). This would rather mess up the minimum length computation.
What we'll do is to look for the tail four bytes, and then peek
at the preceding two bytes to see whether we need to decrease
the minimum length by four (six minus two).
Thanks to the design of UTF-8, there cannot be false matches:
A sequence of valid UTF-8 bytes cannot be a subsequence of
another valid sequence of UTF-8 bytes.
Base Structures
The "regexp" structure described in perlreapi is common to
all regex engines. Two of its 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. 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.
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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 be 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 {
regexp_paren_ofs *swap; /* Swap copy of *startp / *endp */
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. */
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} regexp_internal;
"swap"
"swap" formerly was an extra set of startp/endp stored
in a "regexp_paren_ofs" struct. This was used when the
last successful match was from the same pattern as the
current pattern, so that a partial match didn't
overwrite the previous match's results, but it caused a
problem with re-entrant code such as trying to build
the UTF-8 swashes. Currently unused and left for
backward compatibility with 5.10.0.
"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.
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ATTRIBUTES
See attributes(5) for descriptions of the following
attributes:
+---------------+------------------+
|ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+---------------+------------------+
|Availability | runtime/perl-512 |
+---------------+------------------+
|Stability | Uncommitted |
+---------------+------------------+
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>
NOTES
This software was built from source available at
https://java.net/projects/solaris-userland. The original
community source was downloaded from
http://www.cpan.org/src/5.0/perl-5.12.5.tar.bz2
Further information about this software can be found on the
open source community website at http://www.perl.org/.
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