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PCRE(3)
NAME
PCRE - Perl-compatible regular expressions
PCRE REGULAR EXPRESSION DETAILS
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, 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 also
support for UTF-8 character strings. To use this support you must build
PCRE to include UTF-8 support, and then call pcre_compile() with the
PCRE_UTF8 option. How this affects the pattern matching is mentioned in
several places below. There is also a summary of UTF-8 features in the
section on UTF-8 support in the main pcre page.
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 string (or line, in multiline mode)
$ assert end of string (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
also "possessive 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
[ POSIX character class (only if followed by POSIX
syntax)
] terminates the character class
The following sections describe the use of each of the meta-characters.
BACKSLASH
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 escaping action 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 backslash 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.
If you want to remove the special meaning from a sequence of characters,
you can do so by putting them between \Q and \E. This is different from
Perl in that $ and @ are handled as literals in \Q...\E sequences in PCRE,
whereas in Perl, $ and @ cause variable interpolation. Note the following
examples:
Pattern PCRE matches Perl matches
\Qabc$xyz\E abc$xyz abc followed by the
contents of $xyz
\Qabc\$xyz\E abc\$xyz abc\$xyz
\Qabc\E\$\Qxyz\E abc$xyz abc$xyz
The \Q...\E sequence is recognized both inside and outside character
classes.
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)
\ddd character with octal code ddd, or backreference
\xhh character with hex code hh
\x{hhh..} character with hex code hhh... (UTF-8 mode only)
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, from zero to two hexadecimal digits are read (letters can be in
upper or lower case). In UTF-8 mode, any number of hexadecimal digits may
appear between \x{ and }, but the value of the character code must be less
than 2**31 (that is, the maximum hexadecimal value is 7FFFFFFF). If
characters other than hexadecimal digits appear between \x{ and }, or if
there is no terminating }, this form of escape is not recognized. Instead,
the initial \x will be interpreted as a basic hexadecimal escape, with no
following digits, giving a byte whose value is zero.
Characters whose value is less than 256 can be defined by either of the two
syntaxes for \x when PCRE is in UTF-8 mode. There is no difference in the
way they are handled. For example, \xdc is exactly the same as \x{dc}.
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
(code value 7). 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 might be a back reference, otherwise the
character with octal code 113
\377 might be a back reference, otherwise
the 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 or a single UTF-8
character (in UTF-8 mode) 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.
In UTF-8 mode, characters with values greater than 255 never match \d, \s,
or \w, and always match \D, \S, and \W.
For compatibility with Perl, \s does not match the VT character (code 11).
This makes it different from the the POSIX "space" class. The \s characters
are HT (9), LF (10), FF (12), CR (13), and space (32).
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" in the
pcreapi page). 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 matches at a word boundary
\B matches when not at a word boundary
\A matches at start of subject
\Z matches at end of subject or before newline at end
\z matches at end of subject
\G matches at first matching position in subject
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. Thus, they are
independent of multiline mode.
They are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options. If the
startoffset argument of pcre_exec() is non-zero, indicating that matching
is to start at a point other than the beginning of the subject, \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.
The \G assertion is true only when the current matching position is at the
start point of the match, as specified by the startoffset argument of
pcre_exec(). It differs from \A when the value of startoffset is non-zero.
By calling pcre_exec() multiple times with appropriate arguments, you can
mimic Perl's /g option, and it is in this kind of implementation where \G
can be useful.
Note, however, that PCRE's interpretation of \G, as the start of the
current match, is subtly different from Perl's, which defines it as the end
of the previous match. In Perl, these can be different when the previously
matched string was empty. Because PCRE does just one match at a time, it
cannot reproduce this behaviour.
If all the alternatives of a pattern begin with \G, the expression is
anchored to the starting match position, and the "anchored" flag is set in
the compiled regular expression.
CIRCUMFLEX AND DOLLAR
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 if the PCRE_MULTILINE
option is unset. 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
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 newline 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 it is 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. In UTF-8 mode, a dot matches any UTF-8 character, which might be
more than one byte long, except (by default) for 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.
MATCHING A SINGLE BYTE
Outside a character class, the escape sequence \C matches any one byte,
both in and out of UTF-8 mode. Unlike a dot, it always matches a newline.
The feature is provided in Perl in order to match individual bytes in UTF-8
mode. Because it breaks up UTF-8 characters into individual bytes, what
remains in the string may be a malformed UTF-8 string. For this reason it
is best avoided.
PCRE does not allow \C to appear in lookbehind assertions (see below),
because in UTF-8 mode it makes it impossible to calculate the length of the
lookbehind.
SQUARE BRACKETS
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. In UTF-8 mode,
the character may occupy more than one byte. A matched character must be in
the set of characters defined by the class, unless the first character in
the class definition 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.
In UTF-8 mode, characters with values greater than 255 can be included in a
class as a literal string of bytes, or by using the \x{ escaping mechanism.
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. PCRE does not support the concept of case
for characters with values greater than 255.
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 the collating sequence of character values. They can also
be used for characters specified numerically, for example [\000-\037]. In
UTF-8 mode, ranges can include characters whose values are greater than
255, for example [\x{100}-\x{2ff}].
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
Perl supports the POSIX notation for character classes, which uses names
enclosed by [: and :] within the enclosing square brackets. PCRE also
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
blank space or tab only
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 (not quite the same as \s)
upper upper case letters
word "word" characters (same as \w)
xdigit hexadecimal digits
The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
space (32). Notice that this list includes the VT character (code 11). This
makes "space" different to \s, which does not include VT (for Perl
compatibility).
The name "word" is a Perl extension, and "blank" is a GNU extension from
Perl 5.8. 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 recognize 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.
In UTF-8 mode, characters with values greater than 255 do not match any of
the POSIX character classes.
VERTICAL BAR
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
The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
PCRE_EXTENDED options 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.
When an option change occurs at top level (that is, not inside subpattern
parentheses), the change applies to the remainder of the pattern that
follows. If the change is placed right at the start of a pattern, PCRE
extracts it into the global options (and it will therefore show up in data
extracted by the pcre_fullinfo() function).
An option change within a subpattern affects only that part of the current
pattern 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
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, respectively.
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 a question
mark and a colon, 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 capturing subpatterns is 65535, and the
maximum depth of nesting 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".
NAMED SUBPATTERNS
Identifying capturing parentheses by number is simple, but it can be very
hard to keep track of the numbers in complicated regular expressions.
Furthermore, if an expression is modified, the numbers may change. To help
with the difficulty, PCRE supports the naming of subpatterns, something
that Perl does not provide. The Python syntax (?P<name>...) is used. Names
consist of alphanumeric characters and underscores, and must be unique
within a pattern.
Named capturing parentheses are still allocated numbers as well as names.
The PCRE API provides function calls for extracting the name-to-number
translation table from a compiled pattern. For further details see the
pcreapi documentation.
REPETITION
Repetition is specified by quantifiers, which can follow any of the
following items:
a literal data character
the . metacharacter
the \C escape sequence
escapes such as \d that match single characters
a character class
a back reference (see next section)
a parenthesized subpattern (unless it is an assertion)
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.
In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to
individual bytes. Thus, for example, \x{100}{2} matches two UTF-8
characters, each of which is represented by a two-byte sequence.
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 owing 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 normally 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 in order to obtain this optimization, or
alternatively using ^ to indicate anchoring explicitly.
However, there is one situation where the optimization cannot be used. When
.* is inside capturing parentheses that are the subject of a backreference
elsewhere in the pattern, a match at the start may fail, and a later one
succeed. Consider, for example:
(.*)abc\1
If the subject is "xyz123abc123" the match point is the fourth character.
For this reason, such a pattern is not implicitly anchored.
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".
ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS
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. "Atomic
grouping" (a term taken from Jeffrey Friedl's book) provides the means for
specifying that once a subpattern has matched, it is not to be re-evaluated
in this way.
If we use atomic grouping for the previous example, the matcher would give
up immediately on failing to match "foo" the first time. The notation is a
kind of special parenthesis, starting with (?> as in this example:
(?>\d+)foo
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.
Atomic grouping 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.
Atomic groups in general can of course contain arbitrarily complicated
subpatterns, and can be nested. However, when the subpattern for an atomic
group is just a single repeated item, as in the example above, a simpler
notation, called a "possessive quantifier" can be used. This consists of an
additional + character following a quantifier. Using this notation, the
previous example can be rewritten as
\d++bar
Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
option is ignored. They are a convenient notation for the simpler forms of
atomic group. However, there is no difference in the meaning or processing
of a possessive quantifier and the equivalent atomic group.
The possessive quantifier syntax is an extension to the Perl syntax. It
originates in Sun's Java package.
When a pattern contains an unlimited repeat inside a subpattern that can
itself be repeated an unlimited number of times, the use of an atomic group
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 non-
digits, 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.
BACK REFERENCES
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 (that is, 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 (see "Subpatterns as subroutines" below for a way of doing that). 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.
Back references to named subpatterns use the Python syntax (?P=name). We
could rewrite the above example as follows:
(?<p1>(?i)rah)\s+(?P=p1)
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 many capturing parentheses in a pattern, 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.
ASSERTIONS
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, \G, \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.
If you want to force a matching failure at some point in a pattern, the
most convenient way to do it is with (?!) because an empty string always
matches, so an assertion that requires there not to be an empty string must
always fail.
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 (at least for 5.8), 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 top-level
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.
PCRE does not allow the \C escape (which matches a single byte in UTF-8
mode) to appear in lookbehind assertions, because it makes it impossible to
calculate the length of the lookbehind.
Atomic groups 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 that 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)
or, equivalently,
^.*+(?<=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.
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.
CONDITIONAL SUBPATTERNS
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 three 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. The number must
be greater than zero. 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 the string (R), it is satisfied if a recursive call to
the pattern or subpattern has been made. At "top level", the condition is
false. This is a PCRE extension. Recursive patterns are described in the
next section.
If the condition is not a sequence of digits or (R), 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.
COMMENTS
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.
RECURSIVE PATTERNS
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 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, it supports some
special syntax for recursion of the entire pattern, and also for individual
subpattern recursion.
The special item that consists of (? followed by a number greater than zero
and a closing parenthesis is a recursive call of the subpattern of the
given number, provided that it occurs inside that subpattern. (If not, it
is a "subroutine" call, which is described in the next section.) The
special item (?R) is a recursive call of the entire regular expression.
For example, this PCRE pattern solves the nested 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 (that is a correctly parenthesized
substring). Finally there is a closing parenthesis.
If this were part of a larger pattern, you would not want to recurse the
entire pattern, so instead you could use this:
( \( ( (?>[^()]+) | (?1) )* \) )
We have put the pattern into parentheses, and caused the recursion to refer
to them instead of the whole pattern. In a larger pattern, keeping track of
parenthesis numbers can be tricky. It may be more convenient to use named
parentheses instead. For this, PCRE uses (?P>name), which is an extension
to the Python syntax that PCRE uses for named parentheses (Perl does not
provide named parentheses). We could rewrite the above example as follows:
(?P<pn> \( ( (?>[^()]+) | (?P>pn) )* \) )
This particular example pattern contains nested unlimited repeats, and so
the use of atomic grouping for matching strings of non-parentheses is
important when applying the pattern to strings that do not match. For
example, when this pattern is applied to
(aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
it yields "no match" quickly. However, if atomic grouping 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.
At the end of a match, the values set for any capturing subpatterns are
those from the outermost level of the recursion at which the subpattern
value is set. If you want to obtain intermediate values, a callout
function can be used (see below and the pcrecallout documentation). 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, the match fails with the PCRE_ERROR_NOMEMORY error.
Do not confuse the (?R) item with the condition (R), which tests for
recursion. Consider this pattern, which matches text in angle brackets,
allowing for arbitrary nesting. Only digits are allowed in nested brackets
(that is, when recursing), whereas any characters are permitted at the
outer level.
< (?: (?(R) \d++ | [^<>]*+) | (?R)) * >
In this pattern, (?(R) is the start of a conditional subpattern, with two
different alternatives for the recursive and non-recursive cases. The (?R)
item is the actual recursive call.
SUBPATTERNS AS SUBROUTINES
If the syntax for a recursive subpattern reference (either by number or by
name) is used outside the parentheses to which it refers, it operates like
a subroutine in a programming language. An earlier example pointed out that
the pattern
(sens|respons)e and \1ibility
matches "sense and sensibility" and "response and responsibility", but not
"sense and responsibility". If instead the pattern
(sens|respons)e and (?1)ibility
is used, it does match "sense and responsibility" as well as the other two
strings. Such references must, however, follow the subpattern to which they
refer.
CALLOUTS
Perl has a feature whereby using the sequence (?{...}) causes arbitrary
Perl code to be obeyed in the middle of matching a regular expression. This
makes it possible, amongst other things, to extract different substrings
that match the same pair of parentheses when there is a repetition.
PCRE provides a similar feature, but of course it cannot obey arbitrary
Perl code. The feature is called "callout". The caller of PCRE provides an
external function by putting its entry point in the global variable
pcre_callout. By default, this variable contains NULL, which disables all
calling out.
Within a regular expression, (?C) indicates the points at which the
external function is to be called. If you want to identify different
callout points, you can put a number less than 256 after the letter C. The
default value is zero. For example, this pattern has two callout points:
(?C1)abc(?C2)def
During matching, when PCRE reaches a callout point (and pcre_callout is
set), the external function is called. It is provided with the number of
the callout, and, optionally, one item of data originally supplied by the
caller of pcre_exec(). The callout function may cause matching to
backtrack, or to fail altogether. A complete description of the interface
to the callout function is given in the pcrecallout documentation.
Last updated: 03 February 2003
Copyright (c) 1997-2003 University of Cambridge.
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