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PCRE(3) 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 books, some of which have copious examples. Jeffrey Friedl’s "Mastering
Regular Expressions", published by O’Reilly, covers regular expressions in great
detail. This description of PCRE’s regular expressions is intended as reference
material.
The original operation of PCRE was on strings of one-byte characters. However,
there is now also support for UTF-8 character strings. To use this, you must build
PCRE to include UTF-8 support, and then call pcre_compile() with the PCRE_UTF8
option. How this affects 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 metacharacters,
which do not stand for themselves but instead are interpreted in some special way.
There are two different sets of metacharacters: those that are recognized anywhere
in the pattern except within square brackets, and those that are recognized in
square brackets. Outside square brackets, the metacharacters 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 metacharacters 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 metacharacters.
BACKSLASH
The backslash character has several uses. Firstly, if it is followed by a non-
alphanumeric character, it takes away any special meaning that character may have.
This use of backslash as an escape character applies both inside and outside char-
acter 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 metacharacter, so it is always safe to precede a non-alphanumeric
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.
Non-printing characters
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 con-
verted 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 character whose value is zero.
Characters whose value is less than 256 can be defined by either of the two syn-
taxes 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 pattern character that follows
is itself an octal digit.
The handling of a backslash followed by a digit other than 0 is complicated. Out-
side 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 dis-
cussion 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), and the sequence \X is interpreted as the character "X". Outside a char-
acter class, these sequences have different meanings (see below).
Generic character types
The third use of backslash is for specifying generic character types. The following
are always recognized:
\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.
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.
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 an underscore or any character less than 256 that is a letter
or digit. The definition of letters and digits is controlled by PCRE’s low-valued
character tables, and may vary if locale-specific matching is taking place (see
"Locale support" in the pcreapi page). For example, in the "fr_FR" (French) locale,
some character codes greater than 128 are used for accented letters, and these are
matched by \w.
In UTF-8 mode, characters with values greater than 128 never match \d, \s, or \w,
and always match \D, \S, and \W. This is true even when Unicode character property
support is available.
Unicode character properties
When PCRE is built with Unicode character property support, three additional escape
sequences to match generic character types are available when UTF-8 mode is
selected. They are:
\p{xx} a character with the xx property
\P{xx} a character without the xx property
\X an extended Unicode sequence
The property names represented by xx above are limited to the Unicode general cate-
gory properties. Each character has exactly one such property, specified by a two-
letter abbreviation. For compatibility with Perl, negation can be specified by
including a circumflex between the opening brace and the property name. For exam-
ple, \p{^Lu} is the same as \P{Lu}.
If only one letter is specified with \p or \P, it includes all the properties that
start with that letter. In this case, in the absence of negation, the curly brack-
ets in the escape sequence are optional; these two examples have the same effect:
\p{L}
\pL
The following property codes are supported:
C Other
Cc Control
Cf Format
Cn Unassigned
Co Private use
Cs Surrogate
L Letter
Ll Lower case letter
Lm Modifier letter
Lo Other letter
Lt Title case letter
Lu Upper case letter
M Mark
Mc Spacing mark
Me Enclosing mark
Mn Non-spacing mark
N Number
Nd Decimal number
Nl Letter number
No Other number
P Punctuation
Pc Connector punctuation
Pd Dash punctuation
Pe Close punctuation
Pf Final punctuation
Pi Initial punctuation
Po Other punctuation
Ps Open punctuation
S Symbol
Sc Currency symbol
Sk Modifier symbol
Sm Mathematical symbol
So Other symbol
Z Separator
Zl Line separator
Zp Paragraph separator
Zs Space separator
Extended properties such as "Greek" or "InMusicalSymbols" are not supported by
PCRE.
Specifying caseless matching does not affect these escape sequences. For example,
\p{Lu} always matches only upper case letters.
The \X escape matches any number of Unicode characters that form an extended Uni-
code sequence. \X is equivalent to
(?>\PM\pM*)
That is, it matches a character without the "mark" property, followed by zero or
more characters with the "mark" property, and treats the sequence as an atomic
group (see below). Characters with the "mark" property are typically accents that
affect the preceding character.
Matching characters by Unicode property is not fast, because PCRE has to search a
structure that contains data for over fifteen thousand characters. That is why the
traditional escape sequences such as \d and \w do not use Unicode properties in
PCRE.
Simple assertions
The fourth use of backslash is for certain simple assertions. An assertion speci-
fies a condition that has to be met at a particular point in a match, without con-
suming any characters from the subject string. The use of subpatterns for more com-
plicated 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 dif-
ferent 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 in the next section) 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. These three assertions are not affected by the PCRE_NOTBOL or
PCRE_NOTEOL options, which affect only the behaviour of the circumflex and dollar
metacharacters. However, 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 that 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, circum-
flex 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 alterna-
tives 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 alterna-
tives 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 that 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 char-
acter 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_MULTI-
LINE option is set. When this is the case, they match immediately after and immedi-
ately 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" (where \n represents a newline character) 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_MULTI-
LINE 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) 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 can match 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, the \C escape sequence is best avoided.
PCRE does not allow \C to appear in lookbehind assertions (described below),
because in UTF-8 mode this would make it impossible to calculate the length of the
lookbehind.
SQUARE BRACKETS AND CHARACTER CLASSES
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 circum-
flex is just a convenient notation for specifying the characters that are in the
class by enumerating those that are not. A class that starts with a circumflex is
not an assertion: it still consumes a character from the subject string, and there-
fore it 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. When running in UTF-8 mode, PCRE supports the concept of case for characters
with values greater than 128 only when it is compiled with Unicode property sup-
port.
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, typi-
cally 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 class containing a range followed by two
other 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 in non-UTF-8 mode, if character tables for the "fr_FR"
locale are in use, [\xc8-\xcb] matches accented E characters in both cases. In
UTF-8 mode, PCRE supports the concept of case for characters with values greater
than 128 only when it is compiled with Unicode property support.
The character types \d, \D, \p, \P, \s, \S, \w, and \W may also appear in a charac-
ter 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.
The only metacharacters that are recognized in character classes are backslash,
hyphen (only where it can be interpreted as specifying a range), circumflex (only
at the start), opening square bracket (only when it can be interpreted as introduc-
ing a POSIX class name - see the next section), and the terminating closing square
bracket. However, escaping other non-alphanumeric characters does no harm.
POSIX CHARACTER CLASSES
Perl supports the POSIX notation for character classes. This 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 128 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 pro-
cess 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 parenthe-
ses), 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 pat-
tern than any of the additional features it turns on, even when it is at top level.
It is best to put it at the start.
SUBPATTERNS
Subpatterns are delimited by parentheses (round brackets), which can be nested.
Turning part of a pattern into 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 parenthe-
ses, it would match "cataract", "erpillar" or the empty string.
2. It sets up the subpattern as a capturing subpattern. This means that, when the
whole pattern matches, that portion of the subject string that matched the subpat-
tern 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 numbers for
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 require-
ment. 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 this 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. There is also a convenience function for extracting a cap-
tured substring by name. 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
the \X escape sequence (in UTF-8 mode with Unicode properties)
an escape such as \d that matches a single character
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 permit-
ted 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 charac-
ter. 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 speci-
fies 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 lit-
eral 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. Similarly, when Unicode property support is
available, \X{3} matches three Unicode extended sequences, each of which may be
several bytes long (and they may be of different lengths).
The quantifier {0} is permitted, causing the expression to behave as if the previ-
ous 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 pat-
tern to fail. The classic example of where this gives problems is in trying to
match comments in C programs. These appear between /* and */ and within the com-
ment, individual * and / characters may appear. An attempt to match C comments by
applying the pattern
/\*.*\*/
to the string
/* first comment */ 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 memory 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 "twee-
dledee". However, if there are nested capturing subpatterns, the corresponding cap-
tured 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 oth-
erwise 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 Jef-
frey 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 imme-
diately 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 sin-
gle repeated item, as in the example above, a simpler notation, called a "posses-
sive quantifier" can be used. This consists of an additional + character following
a quantifier. Using this notation, the previous example can be rewritten as
\d++foo
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 quan-
tifier 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 internal \D+ repeat and the external * repeat in a large number
of ways, and all have to be tried. (The example uses [!?] 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 so that it uses an atomic group, like this:
((?>\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 pos-
sibly 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 cap-
turing 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 paren-
theses that are referenced need not be to the left of the reference for numbers
less than 10. See the subsection entitled "Non-printing characters" above for fur-
ther 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 refer-
ence, 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 cap-
turing 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
(see "Comments" below) 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 match-
ing 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.
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 pur-
poses of numbering the capturing subpatterns in the whole pattern. However, sub-
string capturing is carried out only for positive assertions, because it does not
make sense for negative assertions.
Lookahead assertions
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 appar-
ently 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 the other effect.
If you want to force a matching failure at some point in a pattern, the most conve-
nient 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
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 temporar-
ily 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. The \X escape, which can match different numbers of
bytes, is also not permitted.
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 fol-
lows 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, using the possessive quantifier syntax,
^.*+(?<=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.
Using multiple assertions
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 pre-
ceded by "foo", while
(?<=\d{3}(?!999)...)foo
is another pattern that matches "foo" preceded by three digits and any three char-
acters that are not "999".
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 asser-
tion, 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 subpat-
tern 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 paren-
theses.
If the condition is the string (R), it is satisfied if a recursive call to the pat-
tern 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 pat-
tern, 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 that 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 provides a 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, pro-
vided 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 pat-
tern 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 apply-
ing 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 the next
section 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 possi-
ble, 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 func-
tion 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
If the PCRE_AUTO_CALLOUT flag is passed to pcre_compile(), callouts are automati-
cally installed before each item in the pattern. They are all numbered 255.
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, the
position in the pattern, and, optionally, one item of data originally supplied by
the caller of pcre_exec(). The callout function may cause matching to proceed, to
backtrack, or to fail altogether. A complete description of the interface to the
callout function is given in the pcrecallout documentation.
Last updated: 09 September 2004
Copyright (c) 1997-2004 University of Cambridge.
PCRE(3)
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