|
Документ взят из кэша поисковой машины. Адрес
оригинального документа
: http://old.master.cmc.msu.ru/php/pcre.pattern.syntax.html
Дата изменения: Sun Feb 3 22:54:14 2002 Дата индексирования: Tue Oct 2 04:22:35 2012 Кодировка: |
The PCRE library is a set of functions that implement regular expression pattern matching using the same syntax and semantics as Perl 5, with just a few differences (see below). The current implementation corresponds to Perl 5.005.
The differences described here are with respect to Perl 5.005.
By default, a whitespace character is any character that the C library function isspace() recognizes, though it is possible to compile PCRE with alternative character type tables. Normally isspace() matches space, formfeed, newline, carriage return, horizontal tab, and vertical tab. Perl 5 no longer includes vertical tab in its set of whitespace characters. The \v escape that was in the Perl documentation for a long time was never in fact recognized. However, the character itself was treated as whitespace at least up to 5.002. In 5.004 and 5.005 it does not match \s.
PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits them, but they do not mean what you might think. For example, (?!a){3} does not assert that the next three characters are not "a". It just asserts that the next character is not "a" three times.
Capturing subpatterns that occur inside negative looka- head assertions are counted, but their entries in the offsets vector are never set. Perl sets its numerical vari- ables from any such patterns that are matched before the assertion fails to match something (thereby succeeding), but only if the negative lookahead assertion contains just one branch.
Though binary zero characters are supported in the sub- ject string, they are not allowed in a pattern string because it is passed as a normal C string, terminated by zero. The escape sequence "\0" can be used in the pattern to represent a binary zero.
The following Perl escape sequences are not supported: \l, \u, \L, \U, \E, \Q. In fact these are implemented by Perl's general string-handling and are not part of its pat- tern matching engine.
The Perl \G assertion is not supported as it is not relevant to single pattern matches.
Fairly obviously, PCRE does not support the (?{code}) construction.
There are at the time of writing some oddities in Perl 5.005_02 concerned with the settings of captured strings when part of a pattern is repeated. For example, matching "aba" against the pattern /^(a(b)?)+$/ sets $2 to the value "b", but matching "aabbaa" against /^(aa(bb)?)+$/ leaves $2 unset. However, if the pattern is changed to /^(aa(b(b))?)+$/ then $2 (and $3) get set. In Perl 5.004 $2 is set in both cases, and that is also TRUE of PCRE. If in the future Perl changes to a consistent state that is different, PCRE may change to follow.
Another as yet unresolved discrepancy is that in Perl 5.005_02 the pattern /^(a)?(?(1)a|b)+$/ matches the string "a", whereas in PCRE it does not. However, in both Perl and PCRE /^(a)?a/ matched against "a" leaves $1 unset.
PCRE provides some extensions to the Perl regular expression facilities:
Although lookbehind assertions must match fixed length strings, each alternative branch of a lookbehind assertion can match a different length of string. Perl 5.005 requires them all to have the same length.
If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $ meta- character matches only at the very end of the string.
If PCRE_EXTRA is set, a backslash followed by a letter with no special meaning is faulted.
If PCRE_UNGREEDY is set, the greediness of the repeti- tion quantifiers is inverted, that is, by default they are not greedy, but if followed by a question mark they are.
The syntax and semantics of the regular expressions sup- ported by PCRE are described below. Regular expressions are also described in the Perl documentation and in a number of other books, some of which have copious examples. Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly (ISBN 1-56592-257-3), covers them in great detail. The description here is intended as reference documentation. A regular expression is a pattern that is matched against a subject string from left to right. Most characters stand for themselves in a pattern, and match the corresponding charac- ters in the subject. As a trivial example, the pattern The quick brown fox matches a portion of a subject string that is identical to itself.
The power of regular expressions comes from the ability to include alternatives and repetitions in the pat- tern. These are encoded in the pattern by the use of meta- characters, which do not stand for themselves but instead are interpreted in some special way.
There are two different sets of meta-characters: those that are recognized anywhere in the pattern except within square brackets, and those that are recognized in square brackets. Outside square brackets, the meta-characters are as follows:
general escape character with several uses
assert start of subject (or line, in multiline mode)
assert end of subject (or line, in multiline mode)
match any character except newline (by default)
start character class definition
end character class definition
start of alternative branch
start subpattern
end subpattern
extends the meaning of (, also 0 or 1 quantifier, also quantifier minimizer
0 or more quantifier
1 or more quantifier
start min/max quantifier
end min/max quantifier
general escape character
negate the class, but only if the first character
indicates character range
terminates the character class
The backslash character has several uses. Firstly, if it is followed by a non-alphameric character, it takes away any special meaning that character may have. This use of backslash as an escape character applies both inside and outside character classes.
For example, if you want to match a "*" character, you write "\*" in the pattern. This applies whether or not the follow- ing character would otherwise be interpreted as a meta- character, so it is always safe to precede a non-alphameric with "\" to specify that it stands for itself. In particu- lar, if you want to match a backslash, you write "\\".
If a pattern is compiled with the PCRE_EXTENDED option, whi- tespace in the pattern (other than in a character class) and characters between a "#" outside a character class and the next newline character are ignored. An escaping backslash can be used to include a whitespace or "#" character as part of the pattern.
A second use of backslash provides a way of encoding non- printing characters in patterns in a visible manner. There is no restriction on the appearance of non-printing charac- ters, apart from the binary zero that terminates a pattern, but when a pattern is being prepared by text editing, it is usually easier to use one of the following escape sequences than the binary character it represents:
alarm, that is, the BEL character (hex 07)
"control-x", where x is any character
escape (hex 1B)
formfeed (hex 0C)
newline (hex 0A)
carriage return (hex 0D)
tab (hex 09)
character with hex code hh
character with octal code ddd, or backreference
The precise effect of "\cx" is as follows: if "x" is a lower case letter, it is converted to upper case. Then bit 6 of the character (hex 40) is inverted. Thus "\cz" becomes hex 1A, but "\c{" becomes hex 3B, while "\c;" becomes hex 7B.
After "\x", up to two hexadecimal digits are read (letters can be in upper or lower case).
After "\0" up to two further octal digits are read. In both cases, if there are fewer than two digits, just those that are present are used. Thus the sequence "\0\x\07" specifies two binary zeros followed by a BEL character. Make sure you supply two digits after the initial zero if the character that follows is itself an octal digit.
The handling of a backslash followed by a digit other than 0 is complicated. Outside a character class, PCRE reads it and any following digits as a decimal number. If the number is less than 10, or if there have been at least that many previous capturing left parentheses in the expression, the entire sequence is taken as a back reference. A description of how this works is given later, following the discussion of parenthesized subpatterns.
Inside a character class, or if the decimal number is greater than 9 and there have not been that many capturing subpatterns, PCRE re-reads up to three octal digits follow- ing the backslash, and generates a single byte from the least significant 8 bits of the value. Any subsequent digits stand for themselves. For example:
is another way of writing a space
is the same, provided there are fewer than 40 previous capturing subpatterns
is always a back reference
might be a back reference, or another way of writing a tab
is always a tab
is a tab followed by the character "3"
is the character with octal code 113 (since there can be no more than 99 back references)
is a byte consisting entirely of 1 bits
is either a back reference, or a binary zero followed by the two characters "8" and "1"
Note that octal values of 100 or greater must not be intro- duced by a leading zero, because no more than three octal digits are ever read.
All the sequences that define a single byte value can be used both inside and outside character classes. In addition, inside a character class, the sequence "\b" is interpreted as the backspace character (hex 08). Outside a character class it has a different meaning (see below).
The third use of backslash is for specifying generic charac- ter types:
any decimal digit
any character that is not a decimal digit
any whitespace character
any character that is not a whitespace character
any "word" character
any "non-word" character
Each pair of escape sequences partitions the complete set of characters into two disjoint sets. Any given character matches one, and only one, of each pair.
A "word" character is any letter or digit or the underscore character, that is, any character which can be part of a Perl "word". The definition of letters and digits is controlled by PCRE's character tables, and may vary if locale-specific matching is taking place (see "Locale support" above). For example, in the "fr" (French) locale, some char- acter codes greater than 128 are used for accented letters, and these are matched by \w.
These character type sequences can appear both inside and outside character classes. They each match one character of the appropriate type. If the current matching point is at the end of the subject string, all of them fail, since there is no character to match.
The fourth use of backslash is for certain simple asser- tions. An assertion specifies a condition that has to be met at a particular point in a match, without consuming any characters from the subject string. The use of subpatterns for more complicated assertions is described below. The backslashed assertions are
word boundary
not a word boundary
start of subject (independent of multiline mode)
end of subject or newline at end (independent of multiline mode)
end of subject (independent of multiline mode)
These assertions may not appear in character classes (but note that "\b" has a different meaning, namely the backspace character, inside a character class).
A word boundary is a position in the subject string where the current character and the previous character do not both match \w or \W (i.e. one matches \w and the other matches \W), or the start or end of the string if the first or last character matches \w, respectively.
The \A, \Z, and \z assertions differ from the traditional circumflex and dollar (described below) in that they only ever match at the very start and end of the subject string, whatever options are set. They are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options. The difference between \Z and \z is that \Z matches before a newline that is the last character of the string as well as at the end of the string, whereas \z matches only at the end.
Outside a character class, in the default matching mode, the
circumflex character is an assertion which is true only if
the current matching point is at the start of the subject
string. Inside a character class, circumflex has an entirely
different meaning (see below).
Circumflex need not be the first character of the pattern if
a number of alternatives are involved, but it should be the
first thing in each alternative in which it appears if the
pattern is ever to match that branch. If all possible alter-
natives start with a circumflex, that is, if the pattern is
constrained to match only at the start of the subject, it is
said to be an "anchored" pattern. (There are also other con-
structs that can cause a pattern to be anchored.)
A dollar character is an assertion which is TRUE only if the
current matching point is at the end of the subject string,
or immediately before a newline character that is the last
character in the string (by default). Dollar need not be the
last character of the pattern if a number of alternatives
are involved, but it should be the last item in any branch
in which it appears. Dollar has no special meaning in a
character class.
The meaning of dollar can be changed so that it matches only
at the very end of the string, by setting the
PCRE_DOLLAR_ENDONLY option at compile or matching time. This
does not affect the \Z assertion.
The meanings of the circumflex and dollar characters are
changed if the PCRE_MULTILINE option is set. When this is
the case, they match immediately after and immediately
before an internal "\n" character, respectively, in addition
to matching at the start and end of the subject string. For
example, the pattern /^abc$/ matches the subject string
"def\nabc" in multiline mode, but not otherwise. Conse-
quently, patterns that are anchored in single line mode
because all branches start with "^" are not anchored in mul-
tiline mode. The PCRE_DOLLAR_ENDONLY option is ignored if
PCRE_MULTILINE is set.
Note that the sequences \A, \Z, and \z can be used to match
the start and end of the subject in both modes, and if all
branches of a pattern start with \A is it always anchored,
whether PCRE_MULTILINE is set or not.
Outside a character class, a dot in the pattern matches any
one character in the subject, including a non-printing
character, but not (by default) newline. If the PCRE_DOTALL
option is set, then dots match newlines as well. The han-
dling of dot is entirely independent of the handling of cir-
cumflex and dollar, the only relationship being that they
both involve newline characters. Dot has no special meaning
in a character class.
An opening square bracket introduces a character class, ter-
minated by a closing square bracket. A closing square
bracket on its own is not special. If a closing square
bracket is required as a member of the class, it should be
the first data character in the class (after an initial cir-
cumflex, if present) or escaped with a backslash.
A character class matches a single character in the subject;
the character must be in the set of characters defined by
the class, unless the first character in the class is a&