pcre - Perl-compatible regular expressions: expresion syntax.
REGULAR EXPRESSION DETAILS [Toc] [Back] The syntax and semantics of the regular expressions supported 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), covers them in great detail.
The description here is intended as reference documentation. The basic
operation of PCRE is on strings of bytes. However, there is the beginnings
of some support for UTF-8 character strings. To use this support
you must configure PCRE to include it, and then call pcre_compile()
with the PCRE_UTF8 option. How this affects the pattern matching is
described in the final section of this document.
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 characters 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 pattern. 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
| 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
Part of a pattern that is in square brackets is called a "character
class". In a character class the only meta-characters are:
\ general escape character
^ negate the class, but only if the first character
- indicates character range
] terminates the character class
The following sections describe the use of each of the meta-characters.
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 following 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 particular, if you want to match a backslash, you write "\\".
If a pattern is compiled with the PCRE_EXTENDED option, whitespace 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 characters, 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:
\a alarm, that is, the BEL character (hex 07)
\cx "control-x", where x is any character
\e escape (hex 1B)
\f formfeed (hex 0C)
\n newline (hex 0A)
\r carriage return (hex 0D)
\t tab (hex 09)
\xhh character with hex code hh
\ddd 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 following the backslash, and generates a single
byte from the least significant 8 bits of the value. Any subsequent
digits stand for themselves. For example:
\040 is another way of writing a space
\40 is the same, provided there are fewer than 40
previous capturing subpatterns
\7 is always a back reference
\11 might be a back reference, or another way of
writing a tab
\011 is always a tab
\0113 is a tab followed by the character "3"
\113 is the character with octal code 113 (since there
can be no more than 99 back references)
\377 is a byte consisting entirely of 1 bits
\81 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 introduced 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 character types:
\d any decimal digit
\D any character that is not a decimal digit
\s any whitespace character
\S any character that is not a whitespace character
\w any "word" character
\W 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 character
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 assertions. 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
\b word boundary
\B not a word boundary
\A start of subject (independent of multiline mode)
\Z end of subject or newline at end (independent of multiline
mode)
\z 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. If the
startoffset argument of pcre_exec() is non-zero, \A can never match.
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.
CIRCUMFLEX AND DOLLAR [Toc] [Back] 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. If the startoffset argument
of pcre_exec() is non-zero, circumflex can never match. 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 alternatives 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
constructs 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. Consequently, patterns
that are anchored in single line mode because all branches start with
"^" are not anchored in multiline mode, and a match for circumflex is
possible when the startoffset argument of pcre_exec() is non-zero. 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.
FULL STOP (PERIOD, DOT)
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, dots match newlines
as well. The handling of dot is entirely independent of the handling
of circumflex 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, terminated 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
circumflex, 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 circumflex, in which case the subject
character must not be in the set defined by the class. If a circumflex
is actually required as a member of the class, ensure it is not the
first character, or escape it with a backslash.
For example, the character class [aeiou] matches any lower case vowel,
while [^aeiou] matches any character that is not a lower case vowel.
Note that a circumflex is just a convenient notation for specifying the
characters which are in the class by enumerating those that are not. It
is not an assertion: it still consumes a character from the subject
string, and fails if the current pointer is at the end of the string.
When caseless matching is set, any letters in a class represent both
their upper case and lower case versions, so for example, a caseless
[aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not
match "A", whereas a caseful version would.
The newline character is never treated in any special way in character
classes, whatever the setting of the PCRE_DOTALL or PCRE_MULTILINE
options is. A class such as [^a] will always match a newline.
The minus (hyphen) character can be used to specify a range of characters
in a character class. For example, [d-m] matches any letter
between d and m, inclusive. If a minus character is required in a
class, it must be escaped with a backslash or appear in a position
where it cannot be interpreted as indicating a range, typically as the
first or last character in the class.
It is not possible to have the literal character "]" as the end character
of a range. A pattern such as [W-]46] is interpreted as a class of
two characters ("W" and "-") followed by a literal string "46]", so it
would match "W46]" or "-46]". However, if the "]" is escaped with a
backslash it is interpreted as the end of range, so [W-\]46] is interpreted
as a single class containing a range followed by two separate
characters. The octal or hexadecimal representation of "]" can also be
used to end a range.
Ranges operate in ASCII collating sequence. They can also be used for
characters specified numerically, for example [\000-\037]. If a range
that includes letters is used when caseless matching is set, it matches
the letters in either case. For example, [W-c] is equivalent to
[][\^_`wxyzabc], matched caselessly, and if character tables for the
"fr" locale are in use, [\xc8-\xcb] matches accented E characters in
both cases.
The character types \d, \D, \s, \S, \w, and \W may also appear in a
character class, and add the characters that they match to the class.
For example, [\dABCDEF] matches any hexadecimal digit. A circumflex can
conveniently be used with the upper case character types to specify a
more restricted set of characters than the matching lower case type.
For example, the class [^\W_] matches any letter or digit, but not
underscore.
All non-alphameric characters other than \, -, ^ (at the start) and the
terminating ] are non-special in character classes, but it does no harm
if they are escaped.
POSIX CHARACTER CLASSES [Toc] [Back] Perl 5.6 (not yet released at the time of writing) is going to support
the POSIX notation for character classes, which uses names enclosed by
[: and :] within the enclosing square brackets. PCRE supports this
notation. For example,
[01[:alpha:]%]
matches "0", "1", any alphabetic character, or "%". The supported class
names are
alnum letters and digits
alpha letters
ascii character codes 0 - 127
cntrl control characters
digit decimal digits (same as \d)
graph printing characters, excluding space
lower lower case letters
print printing characters, including space
punct printing characters, excluding letters and digits
space white space (same as \s)
upper upper case letters
word "word" characters (same as \w)
xdigit hexadecimal digits
The names "ascii" and "word" are Perl extensions. Another Perl extension
is negation, which is indicated by a ^ character after the colon.
For example,
[12[:^digit:]]
matches "1", "2", or any non-digit. PCRE (and Perl) also recogize the
POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but
these are not supported, and an error is given if they are encountered.
Vertical bar characters are used to separate alternative patterns. For
example, the pattern
gilbert|sullivan
matches either "gilbert" or "sullivan". Any number of alternatives may
appear, and an empty alternative is permitted (matching the empty
string). The matching process tries each alternative in turn, from
left to right, and the first one that succeeds is used. If the alternatives
are within a subpattern (defined below), "succeeds" means matching
the rest of the main pattern as well as the alternative in the subpattern.
INTERNAL OPTION SETTING [Toc] [Back] The settings of PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
PCRE_EXTENDED can be changed from within the pattern by a sequence of
Perl option letters enclosed between "(?" and ")". The option letters
are
i for PCRE_CASELESS
m for PCRE_MULTILINE
s for PCRE_DOTALL
x for PCRE_EXTENDED
For example, (?im) sets caseless, multiline matching. It is also possible
to unset these options by preceding the letter with a hyphen, and a
combined setting and unsetting such as (?im-sx), which sets PCRE_CASELESS
and PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED,
is also permitted. If a letter appears both before and after the
hyphen, the option is unset.
The scope of these option changes depends on where in the pattern the
setting occurs. For settings that are outside any subpattern (defined
below), the effect is the same as if the options were set or unset at
the start of matching. The following patterns all behave in exactly the
same way:
(?i)abc
a(?i)bc
ab(?i)c
abc(?i)
which in turn is the same as compiling the pattern abc with PCRE_CASELESS
set. In other words, such "top level" settings apply to the whole
pattern (unless there are other changes inside subpatterns). If there
is more than one setting of the same option at top level, the rightmost
setting is used.
If an option change occurs inside a subpattern, the effect is different.
This is a change of behaviour in Perl 5.005. An option change
inside a subpattern affects only that part of the subpattern that follows
it, so
(a(?i)b)c
matches abc and aBc and no other strings (assuming PCRE_CASELESS is not
used). By this means, options can be made to have different settings
in different parts of the pattern. Any changes made in one alternative
do carry on into subsequent branches within the same subpattern. For
example,
(a(?i)b|c)
matches "ab", "aB", "c", and "C", even though when matching "C" the
first branch is abandoned before the option setting. This is because
the effects of option settings happen at compile time. There would be
some very weird behaviour otherwise.
The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can be changed
in the same way as the Perl-compatible options by using the characters
U and X respectively. The (?X) flag setting is special in that it must
always occur earlier in the pattern than any of the additional features
it turns on, even when it is at top level. It is best put at the start.
Subpatterns are delimited by parentheses (round brackets), which can be
nested. Marking part of a pattern as a subpattern does two things:
1. It localizes a set of alternatives. For example, the pattern
cat(aract|erpillar|)
matches one of the words "cat", "cataract", or "caterpillar". Without
the parentheses, it would match "cataract", "erpillar" or the empty
string.
2. It sets up the subpattern as a capturing subpattern (as defined
above). When the whole pattern matches, that portion of the subject
string that matched the subpattern is passed back to the caller via the
ovector argument of pcre_exec(). Opening parentheses are counted from
left to right (starting from 1) to obtain the numbers of the capturing
subpatterns.
For example, if the string "the red king" is matched against the pattern
the ((red|white) (king|queen))
the captured substrings are "red king", "red", and "king", and are numbered
1, 2, and 3.
The fact that plain parentheses fulfil two functions is not always
helpful. There are often times when a grouping subpattern is required
without a capturing requirement. If an opening parenthesis is followed
by "?:", the subpattern does not do any capturing, and is not counted
when computing the number of any subsequent capturing subpatterns. For
example, if the string "the white queen" is matched against the pattern
the ((?:red|white) (king|queen))
the captured substrings are "white queen" and "queen", and are numbered
1 and 2. The maximum number of captured substrings is 99, and the maximum
number of all subpatterns, both capturing and non-capturing, is
200.
As a convenient shorthand, if any option settings are required at the
start of a non-capturing subpattern, the option letters may appear
between the "?" and the ":". Thus the two patterns
(?i:saturday|sunday)
(?:(?i)saturday|sunday)
match exactly the same set of strings. Because alternative branches are
tried from left to right, and options are not reset until the end of
the subpattern is reached, an option setting in one branch does affect
subsequent branches, so the above patterns match "SUNDAY" as well as
"Saturday".
Repetition is specified by quantifiers, which can follow any of the
following items:
a single character, possibly escaped
the . metacharacter
a character class
a back reference (see next section)
a parenthesized subpattern (unless it is an assertion - see below)
The general repetition quantifier specifies a minimum and maximum number
of permitted matches, by giving the two numbers in curly brackets
(braces), separated by a comma. The numbers must be less than 65536,
and the first must be less than or equal to the second. For example:
z{2,4}
matches "zz", "zzz", or "zzzz". A closing brace on its own is not a
special character. If the second number is omitted, but the comma is
present, there is no upper limit; if the second number and the comma
are both omitted, the quantifier specifies an exact number of required
matches. Thus
[aeiou]{3,}
matches at least 3 successive vowels, but may match many more, while
\d{8}
matches exactly 8 digits. An opening curly bracket that appears in a
position where a quantifier is not allowed, or one that does not match
the syntax of a quantifier, is taken as a literal character. For example,
{,6} is not a quantifier, but a literal string of four characters.
The quantifier {0} is permitted, causing the expression to behave as if
the previous item and the quantifier were not present.
For convenience (and historical compatibility) the three most common
quantifiers have single-character abbreviations:
* is equivalent to {0,}
+ is equivalent to {1,}
? is equivalent to {0,1}
It is possible to construct infinite loops by following a subpattern
that can match no characters with a quantifier that has no upper limit,
for example:
(a?)*
Earlier versions of Perl and PCRE used to give an error at compile time
for such patterns. However, because there are cases where this can be
useful, such patterns are now accepted, but if any repetition of the
subpattern does in fact match no characters, the loop is forcibly broken.
By default, the quantifiers are "greedy", that is, they match as much
as possible (up to the maximum number of permitted times), without
causing the rest of the pattern to fail. The classic example of where
this gives problems is in trying to match comments in C programs. These
appear between the sequences /* and */ and within the sequence, individual
* and / characters may appear. An attempt to match C comments by
applying the pattern
/\*.*\*/
to the string
/* first command */ not comment /* second comment */
fails, because it matches the entire string due to the greediness of
the .* item.
However, if a quantifier is followed by a question mark, it ceases to
be greedy, and instead matches the minimum number of times possible, so
the pattern
/\*.*?\*/
does the right thing with the C comments. The meaning of the various
quantifiers is not otherwise changed, just the preferred number of
matches. Do not confuse this use of question mark with its use as a
quantifier in its own right. Because it has two uses, it can sometimes
appear doubled, as in
\d??\d
which matches one digit by preference, but can match two if that is the
only way the rest of the pattern matches.
If the PCRE_UNGREEDY option is set (an option which is not available in
Perl), the quantifiers are not greedy by default, but individual ones
can be made greedy by following them with a question mark. In other
words, it inverts the default behaviour.
When a parenthesized subpattern is quantified with a minimum repeat
count that is greater than 1 or with a limited maximum, more store is
required for the compiled pattern, in proportion to the size of the
minimum or maximum.
If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
to Perl's /s) is set, thus allowing the . to match newlines, the
pattern is implicitly anchored, because whatever follows will be tried
against every character position in the subject string, so there is no
point in retrying the overall match at any position after the first.
PCRE treats such a pattern as though it were preceded by \A. In cases
where it is known that the subject string contains no newlines, it is
worth setting PCRE_DOTALL when the pattern begins with .* in order to
obtain this optimization, or alternatively using ^ to indicate anchoring
explicitly.
When a capturing subpattern is repeated, the value captured is the substring
that matched the final iteration. For example, after
(tweedle[dume]{3}\s*)+
has matched "tweedledum tweedledee" the value of the captured substring
is "tweedledee". However, if there are nested capturing subpatterns,
the corresponding captured values may have been set in previous iterations.
For example, after
/(a|(b))+/
matches "aba" the value of the second captured substring is "b".
Outside a character class, a backslash followed by a digit greater than
0 (and possibly further digits) is a back reference to a capturing subpattern
earlier (i.e. to its left) in the pattern, provided there have
been that many previous capturing left parentheses.
However, if the decimal number following the backslash is less than 10,
it is always taken as a back reference, and causes an error only if
there are not that many capturing left parentheses in the entire pattern.
In other words, the parentheses that are referenced need not be
to the left of the reference for numbers less than 10. See the section
entitled "Backslash" above for further details of the handling of digits
following a backslash.
A back reference matches whatever actually matched the capturing subpattern
in the current subject string, rather than anything matching
the subpattern itself. So the pattern
(sens|respons)e and \1ibility
matches "sense and sensibility" and "response and responsibility", but
not "sense and responsibility". If caseful matching is in force at the
time of the back reference, the case of letters is relevant. For example,
((?i)rah)\s+\1
matches "rah rah" and "RAH RAH", but not "RAH rah", even though the
original capturing subpattern is matched caselessly.
There may be more than one back reference to the same subpattern. If a
subpattern has not actually been used in a particular match, any back
references to it always fail. For example, the pattern
(a|(bc))\2
always fails if it starts to match "a" rather than "bc". Because there
may be up to 99 back references, all digits following the backslash are
taken as part of a potential back reference number. If the pattern continues
with a digit character, some delimiter must be used to terminate
the back reference. If the PCRE_EXTENDED option is set, this can be
whitespace. Otherwise an empty comment can be used.
A back reference that occurs inside the parentheses to which it refers
fails when the subpattern is first used, so, for example, (a\1) never
matches. However, such references can be useful inside repeated subpatterns.
For example, the pattern
(a|b\1)+
matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration
of the subpattern, the back reference matches the character
string corresponding to the previous iteration. In order for this to
work, the pattern must be such that the first iteration does not need
to match the back reference. This can be done using alternation, as in
the example above, or by a quantifier with a minimum of zero.
An assertion is a test on the characters following or preceding the
current matching point that does not actually consume any characters.
The simple assertions coded as \b, \B, \A, \Z, \z, ^ and $ are
described above. More complicated assertions are coded as subpatterns.
There are two kinds: those that look ahead of the current position in
the subject string, and those that look behind it.
An assertion subpattern is matched in the normal way, except that it
does not cause the current matching position to be changed. Lookahead
assertions start with (?= for positive assertions and (?! for negative
assertions. For example,
\w+(?=;)
matches a word followed by a semicolon, but does not include the semicolon
in the match, and
foo(?!bar)
matches any occurrence of "foo" that is not followed by "bar". Note
that the apparently similar pattern
(?!foo)bar
does not find an occurrence of "bar" that is preceded by something
other than "foo"; it finds any occurrence of "bar" whatsoever, because
the assertion (?!foo) is always true when the next three characters are
"bar". A lookbehind assertion is needed to achieve this effect.
Lookbehind assertions start with (?<= for positive assertions and (?<!
for negative assertions. For example,
(?<!foo)bar
does find an occurrence of "bar" that is not preceded by "foo". The
contents of a lookbehind assertion are restricted such that all the
strings it matches must have a fixed length. However, if there are several
alternatives, they do not all have to have the same fixed length.
Thus
(?<=bullock|donkey)
is permitted, but
(?<!dogs?|cats?)
causes an error at compile time. Branches that match different length
strings are permitted only at the top level of a lookbehind assertion.
This is an extension compared with Perl 5.005, which requires all
branches to match the same length of string. An assertion such as
(?<=ab(c|de))
is not permitted, because its single top-level branch can match two
different lengths, but it is acceptable if rewritten to use two toplevel
branches:
(?<=abc|abde)
The implementation of lookbehind assertions is, for each alternative,
to temporarily move the current position back by the fixed width and
then try to match. If there are insufficient characters before the current
position, the match is deemed to fail. Lookbehinds in conjunction
with once-only subpatterns can be particularly useful for matching at
the ends of strings; an example is given at the end of the section on
once-only subpatterns.
Several assertions (of any sort) may occur in succession. For example,
(?<=\d{3})(?<!999)foo
matches "foo" preceded by three digits that are not "999". Notice that
each of the assertions is applied independently at the same point in
the subject string. First there is a check that the previous three
characters are all digits, and then there is a check that the same
three characters are not "999". This pattern does not match "foo" preceded
by six characters, the first of which are digits and the last
three of which are not "999". For example, it doesn't match "123abcfoo".
A pattern to do that is
(?<=\d{3}...)(?<!999)foo
This time the first assertion looks at the preceding six characters,
checking that the first three are digits, and then the second assertion
checks that the preceding three characters are not "999".
Assertions can be nested in any combination. For example,
(?<=(?<!foo)bar)baz
matches an occurrence of "baz" that is preceded by "bar" which in turn
is not preceded by "foo", while
(?<=\d{3}(?!999)...)foo
is another pattern which matches "foo" preceded by three digits and any
three characters that are not "999".
Assertion subpatterns are not capturing subpatterns, and may not be
repeated, because it makes no sense to assert the same thing several
times. If any kind of assertion contains capturing subpatterns within
it, these are counted for the purposes of numbering the capturing subpatterns
in the whole pattern. However, substring capturing is carried
out only for positive assertions, because it does not make sense for
negative assertions.
Assertions count towards the maximum of 200 parenthesized subpatterns.
ONCE-ONLY SUBPATTERNS [Toc] [Back] With both maximizing and minimizing repetition, failure of what follows
normally causes the repeated item to be re-evaluated to see if a different
number of repeats allows the rest of the pattern to match. Sometimes
it is useful to prevent this, either to change the nature of the
match, or to cause it fail earlier than it otherwise might, when the
author of the pattern knows there is no point in carrying on.
Consider, for example, the pattern \d+foo when applied to the subject
line
123456bar
After matching all 6 digits and then failing to match "foo", the normal
action of the matcher is to try again with only 5 digits matching the
\d+ item, and then with 4, and so on, before ultimately failing. Onceonly
subpatterns provide the means for specifying that once a portion
of the pattern has matched, it is not to be re-evaluated in this way,
so the matcher would give up immediately on failing to match "foo" the
first time. The notation is another kind of special parenthesis, starting
with (?> as in this example:
(?>\d+)bar
This kind of parenthesis "locks up" the part of the pattern it contains
once it has matched, and a failure further into the pattern is
prevented from backtracking into it. Backtracking past it to previous
items, however, works as normal.
An alternative description is that a subpattern of this type matches
the string of characters that an identical standalone pattern would
match, if anchored at the current point in the subject string.
Once-only subpatterns are not capturing subpatterns. Simple cases such
as the above example can be thought of as a maximizing repeat that must
swallow everything it can. So, while both \d+ and \d+? are prepared to
adjust the number of digits they match in order to make the rest of the
pattern match, (?>\d+) can only match an entire sequence of digits.
This construction can of course contain arbitrarily complicated subpatterns,
and it can be nested.
Once-only subpatterns can be used in conjunction with lookbehind assertions
to specify efficient matching at the end of the subject string.
Consider a simple pattern such as
abcd$
when applied to a long string which does not match. Because matching
proceeds from left to right, PCRE will look for each "a" in the subject
and then see if what follows matches the rest of the pattern. If the
pattern is specified as
^.*abcd$
the initial .* matches the entire string at first, but when this fails
(because there is no following "a"), it backtracks to match all but the
last character, then all but the last two characters, and so on. Once
again the search for "a" covers the entire string, from right to left,
so we are no better off. However, if the pattern is written as
^(?>.*)(?<=abcd)
there can be no backtracking for the .* item; it can match only the
entire string. The subsequent lookbehind assertion does a single test
on the last four characters. If it fails, the match fails immediately.
For long strings, this approach makes a significant difference to the
processing time.
When a pattern contains an unlimited repeat inside a subpattern that
can itself be repeated an unlimited number of times, the use of a onceonly
subpattern is the only way to avoid some failing matches taking a
very long time indeed. The pattern
(\D+|<\d+>)*[!?]
matches an unlimited number of substrings that either consist of nondigits,
or digits enclosed in <>, followed by either ! or ?. When it
matches, it runs quickly. However, if it is applied to
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
it takes a long time before reporting failure. This is because the
string can be divided between the two repeats in a large number of
ways, and all have to be tried. (The example used [!?] rather than a
single character at the end, because both PCRE and Perl have an optimization
that allows for fast failure when a single character is used.
They remember the last single character that is required for a match,
and fail early if it is not present in the string.) If the pattern is
changed to
((?>\D+)|<\d+>)*[!?]
sequences of non-digits cannot be broken, and failure happens quickly.
CONDITIONAL SUBPATTERNS [Toc] [Back] It is possible to cause the matching process to obey a subpattern conditionally
or to choose between two alternative subpatterns, depending
on the result of an assertion, or whether a previous capturing subpattern
matched or not. The two possible forms of conditional subpattern
are
(?(condition)yes-pattern)
(?(condition)yes-pattern|no-pattern)
If the condition is satisfied, the yes-pattern is used; otherwise the
no-pattern (if present) is used. If there are more than two alternatives
in the subpattern, a compile-time error occurs.
There are two kinds of condition. If the text between the parentheses
consists of a sequence of digits, the condition is satisfied if the
capturing subpattern of that number has previously matched. Consider
the following pattern, which contains non-significant white space to
make it more readable (assume the PCRE_EXTENDED option) and to divide
it into three parts for ease of discussion:
( \( )? [^()]+ (?(1) \) )
The first part matches an optional opening parenthesis, and if that
character is present, sets it as the first captured substring. The second
part matches one or more characters that are not parentheses. The
third part is a conditional subpattern that tests whether the first set
of parentheses matched or not. If they did, that is, if subject started
with an opening parenthesis, the condition is true, and so the yes-pattern
is executed and a closing parenthesis is required. Otherwise,
since no-pattern is not present, the subpattern matches nothing. In
other words, this pattern matches a sequence of non-parentheses,
optionally enclosed in parentheses.
If the condition is not a sequence of digits, it must be an assertion.
This may be a positive or negative lookahead or lookbehind assertion.
Consider this pattern, again containing non-significant white space,
and with the two alternatives on the second line:
(?(?=[^a-z]*[a-z])
\d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
The condition is a positive lookahead assertion that matches an
optional sequence of non-letters followed by a letter. In other words,
it tests for the presence of at least one letter in the subject. If a
letter is found, the subject is matched against the first alternative;
otherwise it is matched against the second. This pattern matches
strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are
letters and dd are digits.
The sequence (?# marks the start of a comment which continues up to the
next closing parenthesis. Nested parentheses are not permitted. The
characters that make up a comment play no part in the pattern matching
at all.
If the PCRE_EXTENDED option is set, an unescaped # character outside a
character class introduces a comment that continues up to the next newline
character in the pattern.
Consider the problem of matching a string in parentheses, allowing for
unlimited nested parentheses. Without the use of recursion, the best
that can be done is to use a pattern that matches up to some fixed
depth of nesting. It is not possible to handle an arbitrary nesting
depth. Perl 5.6 has provided an experimental facility that allows regular
expressions to recurse (amongst other things). It does this by
interpolating Perl code in the expression at run time, and the code can
refer to the expression itself. A Perl pattern to solve the parentheses
problem can be created like this:
$re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;
The (?p{...}) item interpolates Perl code at run time, and in this case
refers recursively to the pattern in which it appears. Obviously, PCRE
cannot support the interpolation of Perl code. Instead, the special
item (?R) is provided for the specific case of recursion. This PCRE
pattern solves the parentheses problem (assume the PCRE_EXTENDED option
is set so that white space is ignored):
\( ( (?>[^()]+) | (?R) )* \)
First it matches an opening parenthesis. Then it matches any number of
substrings which can either be a sequence of non-parentheses, or a
recursive match of the pattern itself (i.e. a correctly parenthesized
substring). Finally there is a closing parenthesis.
This particular example pattern contains nested unlimited repeats, and
so the use of a once-only subpattern for matching strings of non-parentheses
is important when applying the pattern to strings that do not
match. For example, when it is applied to
(aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
it yields "no match" quickly. However, if a once-only subpattern is not
used, the match runs for a very long time indeed because there are so
many different ways the + and * repeats can carve up the subject, and
all have to be tested before failure can be reported.
The values set for any capturing subpatterns are those from the outermost
level of the recursion at which the subpattern value is set. If
the pattern above is matched against
(ab(cd)ef)
the value for the capturing parentheses is "ef", which is the last
value taken on at the top level. If additional parentheses are added,
giving
\( ( ( (?>[^()]+) | (?R) )* ) \)
^ ^
^ ^ the string they capture is "ab(cd)ef",
the contents of the top level parentheses. If there are more than 15
capturing parentheses in a pattern, PCRE has to obtain extra memory to
store data during a recursion, which it does by using pcre_malloc,
freeing it via pcre_free afterwards. If no memory can be obtained, it
saves data for the first 15 capturing parentheses only, as there is no
way to give an out-of-memory error from within a recursion.
Certain items that may appear in patterns are more efficient than others.
It is more efficient to use a character class like [aeiou] than a
set of alternatives such as (a|e|i|o|u). In general, the simplest construction
that provides the required behaviour is usually the most
efficient. Jeffrey Friedl's book contains a lot of discussion about
optimizing regular expressions for efficient performance.
When a pattern begins with .* and the PCRE_DOTALL option is set, the
pattern is implicitly anchored by PCRE, since it can match only at the
start of a subject string. However, if PCRE_DOTALL is not set, PCRE
cannot make this optimization, because the . metacharacter does not
then match a newline, and if the subject string contains newlines, the
pattern may match from the character immediately following one of them
instead of from the very start. For example, the pattern
(.*) second
matches the subject "first\nand second" (where \n stands for a newline
character) with the first captured substring being "and". In order to
do this, PCRE has to retry the match starting after every newline in
the subject.
If you are using such a pattern with subject strings that do not contain
newlines, the best performance is obtained by setting PCRE_DOTALL,
or starting the pattern with ^.* to indicate explicit anchoring. That
saves PCRE from having to scan along the subject looking for a newline
to restart at.
Beware of patterns that contain nested indefinite repeats. These can
take a long time to run when applied to a string that does not match.
Consider the pattern fragment
(a+)*
This can match "aaaa" in 33 different ways, and this number increases
very rapidly as the string gets longer. (The * repeat can match 0, 1,
2, 3, or 4 times, and for each of those cases other than 0, the +
repeats can match different numbers of times.) When the remainder of
the pattern is such that the entire match is going to fail, PCRE has in
principle to try every possible variation, and this can take an
extremely long time.
An optimization catches some of the more simple cases such as
(a+)*b
where a literal character follows. Before embarking on the standard
matching procedure, PCRE checks that there is a "b" later in the subject
string, and if there is not, it fails the match immediately. However,
when there is no following literal this optimization cannot be
used. You can see the difference by comparing the behaviour of
(a+)*\d
with the pattern above. The former gives a failure almost instantly
when applied to a whole line of "a" characters, whereas the latter
takes an appreciable time with strings longer than about 20 characters.
Starting at release 3.3, PCRE has some support for character strings
encoded in the UTF-8 format. This is incomplete, and is regarded as
experimental. In order to use it, you must configure PCRE to include
UTF-8 support in the code, and, in addition, you must call pcre_com-
pile() with the PCRE_UTF8 option flag. When you do this, both the pattern
and any subject strings that are matched against it are treated as
UTF-8 strings instead of just strings of bytes, but only in the cases
that are mentioned below.
If you compile PCRE with UTF-8 support, but do not use it at run time,
the library will be a bit bigger, but the additional run time overhead
is limited to testing the PCRE_UTF8 flag in several places, so should
not be very large.
PCRE assumes that the strings it is given contain valid UTF-8 codes. It
does not diagnose invalid UTF-8 strings. If you pass invalid UTF-8
strings to PCRE, the results are undefined.
Running with PCRE_UTF8 set causes these changes in the way PCRE works:
1. In a pattern, the escape sequence \x{...}, where the contents of the
braces is a string of hexadecimal digits, is interpreted as a UTF-8
character whose code number is the given hexadecimal number, for example:
\x{1234}. This inserts from one to six literal bytes into the pattern,
using the UTF-8 encoding. If a non-hexadecimal digit appears
between the braces, the item is not recognized.
2. The original hexadecimal escape sequence, \xhh, generates a two-byte
UTF-8 character if its value is greater than 127.
3. Repeat quantifiers are NOT correctly handled if they follow a multibyte
character. For example, \x{100}* and \xc3+ do not work. If you
want to repeat such characters, you must enclose them in non-capturing
parentheses, for example (?:\x{100}), at present.
4. The dot metacharacter matches one UTF-8 character instead of a single
byte.
5. Unlike literal UTF-8 characters, the dot metacharacter followed by a
repeat quantifier does operate correctly on UTF-8 characters instead of
single bytes.
4. Although the \x{...} escape is permitted in a character class, characters
whose values are greater than 255 cannot be included in a class.
5. A class is matched against a UTF-8 character instead of just a single
byte, but it can match only characters whose values are less than
256. Characters with greater values always fail to match a class.
6. Repeated classes work correctly on multiple characters.
7. Classes containing just a single character whose value is greater
than 127 (but less than 256), for example, [\x80] or [^\x{93}], do not
work because these are optimized into single byte matches. In the first
case, of course, the class brackets are just redundant.
8. Lookbehind assertions move backwards in the subject by a fixed number
of characters instead of a fixed number of bytes. Simple cases have
been tested to work correctly, but there may be hidden gotchas herein.
9. The character types such as \d and \w do not work correctly with
UTF-8 characters. They continue to test a single byte.
10. Anything not explicitly mentioned here continues to work in bytes
rather than in characters.
DIFFERENCES FROM PERL [Toc] [Back] The differences described here are with respect to Perl 5.005.
1. 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.
2. 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.
3. Capturing subpatterns that occur inside negative lookahead assertions
are counted, but their entries in the offsets vector are never
set. Perl sets its numerical variables 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.
4. Though binary zero characters are supported in the subject 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.
5. 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 pattern matching engine.
6. The Perl \G assertion is not supported as it is not relevant to single
pattern matches.
7. Fairly obviously, PCRE does not support the (?{code}) and (?p{code})
constructions. However, there is some experimental support for recursive
patterns using the non-Perl item (?R).
8. 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) are set.
In Perl 5.004 $2 is set in both cases, and that is also true of PCRE.
If in the future Perl changes to a consistent state that is different,
PCRE may change to follow.
9. Another as yet unresolved discrepancy is that in Perl 5.005_02 the
pattern /^(a)?(?(1)a|b)+$/ matches the string "a", whereas in PCRE it
does not. However, in both Perl and PCRE /^(a)?a/ matched against "a"
leaves $1 unset.
10. The following UTF-8 features of Perl 5.6 are not implemented:
a. The escape sequence \C to match a single byte.
b. The use of Unicode tables and properties and escapes \p, \P, and \X.
11. PCRE provides some extensions to the Perl regular expression facilities:
(a) 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.
(b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
meta- character matches only at the very end of the string.
(c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
meaning is faulted.
(d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers
is inverted, that is, by default they are not greedy, but if followed
by a question mark they are.
(e) PCRE_ANCHORED can be used to force a pattern to be tried only at
the start of the subject.
(f) The PCRE_NOTBOL, PCRE_NOTEOL, and PCRE_NOTEMPTY options for
pcre_exec() have no Perl equivalents.
(g) The (?R) construct allows for recursive pattern matching (Perl 5.6
can do this using the (?p{code}) construct, which PCRE cannot of course
support.)
Philip Hazel <[email protected]>
University Computing Service,
New Museums Site,
Cambridge CB2 3QG, England.
Phone: +44 1223 334714
|
