-This file contains the PCRE man page that describes the regular expressions
-supported by PCRE version 5.0. Note that not all of the features are relevant
+This file contains the PCRE man page that describes the regular expressions
+supported by PCRE version 6.7. Note that not all of the features are relevant
in the context of Exim. In particular, the version of PCRE that is compiled
with Exim does not include UTF-8 support, there is no mechanism for changing
the options with which the PCRE functions are called, and features such as
callout are not accessible.
-----------------------------------------------------------------------------
-PCRE(3) PCRE(3)
-
+PCREPATTERN(3) PCREPATTERN(3)
NAME
PCRE - Perl-compatible regular expressions
+
PCRE REGULAR EXPRESSION DETAILS
The syntax and semantics of the regular expressions supported by PCRE
of UTF-8 features in the section on UTF-8 support in the main pcre
page.
+ The remainder of this document discusses the patterns that are sup-
+ ported by PCRE when its main matching function, pcre_exec(), is used.
+ From release 6.0, PCRE offers a second matching function,
+ pcre_dfa_exec(), which matches using a different algorithm that is not
+ Perl-compatible. The advantages and disadvantages of the alternative
+ function, and how it differs from the normal function, are discussed in
+ the pcrematching 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
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 alterna-
- tives 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 recog-
- nized anywhere in the pattern except within square brackets, and those
- that are recognized in square brackets. Outside square brackets, the
+ matches a portion of a subject string that is identical to itself. When
+ caseless matching is specified (the PCRE_CASELESS option), letters are
+ matched independently of case. In UTF-8 mode, PCRE always understands
+ the concept of case for characters whose values are less than 128, so
+ caseless matching is always possible. For characters with higher val-
+ ues, the concept of case is supported if PCRE is compiled with Unicode
+ property support, but not otherwise. If you want to use caseless
+ matching for characters 128 and above, you must ensure that PCRE is
+ compiled with Unicode property support as well as with UTF-8 support.
+
+ 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 recog-
+ nized 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
also "possessive quantifier"
{ start min/max quantifier
- Part of a pattern that is in square brackets is called a "character
+ 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
syntax)
] terminates the character class
- The following sections describe the use of each of the metacharacters.
+ 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
+ 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 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 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 back-
+ 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 back-
slash, 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 # charac-
- ter as part of the pattern.
+ 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 are ignored. An escap-
+ ing 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 charac-
- ters, you can do so by putting them between \Q and \E. This is differ-
- ent from Perl in that $ and @ are handled as literals in \Q...\E
- sequences in PCRE, whereas in Perl, $ and @ cause variable interpola-
+ If you want to remove the special meaning from a sequence of charac-
+ ters, you can do so by putting them between \Q and \E. This is differ-
+ ent from Perl in that $ and @ are handled as literals in \Q...\E
+ sequences in PCRE, whereas in Perl, $ and @ cause variable interpola-
tion. Note the following examples:
Pattern PCRE matches Perl matches
\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
+ 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 char-
- acters 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
+ acters 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)
\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)
+ \x{hhh..} character with hex code hhh..
- 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;
+ 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 dig-
- its 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 hex-
- adecimal escape, with no following digits, giving a character whose
- value is zero.
+ After \x, from zero to two hexadecimal digits are read (letters can be
+ in upper or lower case). Any number of hexadecimal digits may appear
+ between \x{ and }, but the value of the character code must be less
+ than 256 in non-UTF-8 mode, and less than 2**31 in UTF-8 mode (that is,
+ the maximum hexadecimal value is 7FFFFFFF). If characters other than
+ hexadecimal digits appear between \x{ and }, or if there is no termi-
+ nating }, 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 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}.
+ two syntaxes for \x. There is no difference in the way they are han-
+ dled. 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.
+ After \0 up to two further octal digits are read. 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 compli-
cated. Outside a character class, PCRE reads it and any following dig-
- its as a decimal number. If the number is less than 10, or if there
+ its 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
+ 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 sin-
- gle byte from the least significant 8 bits of the value. Any subsequent
- digits stand for themselves. For example:
+ 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, ane uses them to gen-
+ erate a data character. Any subsequent digits stand for themselves. In
+ non-UTF-8 mode, the value of a character specified in octal must be
+ less than \400. In UTF-8 mode, values up to \777 are permitted. For
+ example:
\040 is another way of writing a space
\40 is the same, provided there are fewer than 40
\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
+ 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 character class, these
- sequences have different meanings (see below).
+ All the sequences that define a single character value can be used both
+ inside and outside character classes. In addition, inside a character
+ class, the sequence \b is interpreted as the backspace character (hex
+ 08), and the sequence \X is interpreted as the character "X". Outside a
+ character class, these sequences have different meanings (see below).
Generic character types
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).
+ characters are HT (9), LF (10), FF (12), CR (13), and space (32). (If
+ "use locale;" is included in a Perl script, \s may match the VT charac-
+ ter. In PCRE, it never does.)
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 con-
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 Uni-
- code character property support is available.
+ code character property support is available. The use of locales with
+ Unicode is discouraged.
Unicode character properties
When PCRE is built with Unicode character property support, three addi-
- tional escape sequences to match generic character types are available
+ tional escape sequences to match character properties 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 category properties. Each character has exactly one such prop-
- erty, 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 example, \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 brackets in the escape sequence are optional; these
- two examples have the same effect:
+ \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
+ script names, the general category properties, and "Any", which matches
+ any character (including newline). Other properties such as "InMusical-
+ Symbols" are not currently supported by PCRE. Note that \P{Any} does
+ not match any characters, so always causes a match failure.
+
+ Sets of Unicode characters are defined as belonging to certain scripts.
+ A character from one of these sets can be matched using a script name.
+ For example:
+
+ \p{Greek}
+ \P{Han}
+
+ Those that are not part of an identified script are lumped together as
+ "Common". The current list of scripts is:
+
+ Arabic, Armenian, Bengali, Bopomofo, Braille, Buginese, Buhid, Cana-
+ dian_Aboriginal, Cherokee, Common, Coptic, Cypriot, Cyrillic, Deseret,
+ Devanagari, Ethiopic, Georgian, Glagolitic, Gothic, Greek, Gujarati,
+ Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hiragana, Inherited, Kannada,
+ Katakana, Kharoshthi, Khmer, Lao, Latin, Limbu, Linear_B, Malayalam,
+ Mongolian, Myanmar, New_Tai_Lue, Ogham, Old_Italic, Old_Persian, Oriya,
+ Osmanya, Runic, Shavian, Sinhala, Syloti_Nagri, Syriac, Tagalog, Tag-
+ banwa, Tai_Le, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh,
+ Ugaritic, Yi.
+
+ Each character has exactly one general category 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 example, \p{^Lu} is the same as \P{Lu}.
+
+ If only one letter is specified with \p or \P, it includes all the gen-
+ eral category properties that start with that letter. In this case, in
+ the absence of negation, the curly brackets in the escape sequence are
+ optional; these two examples have the same effect:
\p{L}
\pL
- The following property codes are supported:
+ The following general category property codes are supported:
C Other
Cc Control
Zp Paragraph separator
Zs Space separator
- Extended properties such as "Greek" or "InMusicalSymbols" are not sup-
- ported by PCRE.
+ The special property L& is also supported: it matches a character that
+ has the Lu, Ll, or Lt property, in other words, a letter that is not
+ classified as a modifier or "other".
+
+ The long synonyms for these properties that Perl supports (such as
+ \p{Letter}) are not supported by PCRE, nor is it permitted to prefix
+ any of these properties with "Is".
+
+ No character that is in the Unicode table has the Cn (unassigned) prop-
+ erty. Instead, this property is assumed for any code point that is not
+ in the Unicode table.
- Specifying caseless matching does not affect these escape sequences.
+ 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
+ The \X escape matches any number of Unicode characters that form an
extended Unicode 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"
+ 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
+ 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 asser-
- tion 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.
+ tion 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
\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
+ These assertions may not appear in character classes (but note that \b
has a different meaning, namely the backspace character, inside a char-
acter 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
+ 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
+ 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 asser-
+ at the very start and end of the subject string, whatever options are
+ set. Thus, they are independent of multiline mode. These three asser-
tions 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, indi-
+ affect only the behaviour of the circumflex and dollar metacharacters.
+ However, if the startoffset argument of pcre_exec() is non-zero, indi-
cating 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 subject, \A can never match. The difference between \Z and \z is
+ that \Z matches before a newline at the end of the string as well as at
+ the very end, 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
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 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
+ before a newline at the end of 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 immedi-
- ately after and immediately before an internal newline character,
- respectively, in addition to matching at the start and end of the sub-
- ject string. For example, the pattern /^abc$/ matches the subject
- string "def\nabc" (where \n represents a newline character) in multi-
- line mode, but not otherwise. Consequently, patterns that are anchored
- in single line mode because all branches start with ^ are not anchored
- in multiline mode, and a match for circumflex is possible when the
- startoffset argument of pcre_exec() is non-zero. The PCRE_DOL-
- LAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
+ PCRE_MULTILINE option is set. When this is the case, a circumflex
+ matches immediately after internal newlines as well as at the start of
+ the subject string. It does not match after a newline that ends the
+ string. A dollar matches before any newlines in the string, as well as
+ at the very end, when PCRE_MULTILINE is set. When newline is specified
+ as the two-character sequence CRLF, isolated CR and LF characters do
+ not indicate newlines.
+
+ For example, the pattern /^abc$/ matches the subject string "def\nabc"
+ (where \n represents a newline) 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.
+ start with \A it is always anchored, whether or not PCRE_MULTILINE is
+ set.
FULL STOP (PERIOD, DOT)
Outside a character class, a dot in the pattern matches any one charac-
- ter 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 han-
- dling 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.
+ ter in the subject string except (by default) a character that signi-
+ fies the end of a line. In UTF-8 mode, the matched character may be
+ more than one byte long. When a line ending is defined as a single
+ character (CR or LF), dot never matches that character; when the two-
+ character sequence CRLF is used, dot does not match CR if it is immedi-
+ ately followed by LF, but otherwise it matches all characters (includ-
+ ing isolated CRs and LFs).
+
+ The behaviour of dot with regard to newlines can be changed. If the
+ PCRE_DOTALL option is set, a dot matches any one character, without
+ exception. If newline is defined as the two-character sequence CRLF, it
+ takes two dots to match it.
+
+ The handling of dot is entirely independent of the handling of circum-
+ flex and dollar, the only relationship being that they both involve
+ newlines. 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
+ both in and out of UTF-8 mode. Unlike a dot, it always matches CR and
+ LF. 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 calcu-
+ PCRE does not allow \C to appear in lookbehind assertions (described
+ below), because in UTF-8 mode this would make it impossible to calcu-
late the length of the lookbehind.
An opening square bracket introduces a character class, terminated by a
closing square bracket. A closing square bracket on its own is not spe-
cial. 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
+ 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
+ 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
+ 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.
+ 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 that are in the class by enumerating those that are not. A
- class that starts with a circumflex is not an assertion: it still con-
- sumes a character from the subject string, and therefore it fails if
+ 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 con-
+ sumes a character from the subject string, and therefore 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
+ 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 support.
-
- 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.
+ 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. In UTF-8 mode, PCRE always
+ understands the concept of case for characters whose values are less
+ than 128, so caseless matching is always possible. For characters with
+ higher values, the concept of case is supported if PCRE is compiled
+ with Unicode property support, but not otherwise. If you want to use
+ caseless matching for characters 128 and above, you must ensure that
+ PCRE is compiled with Unicode property support as well as with UTF-8
+ support.
+
+ Characters that might indicate line breaks (CR and LF) are never
+ treated in any special way when matching character classes, whatever
+ line-ending sequence is in use, and whatever setting of the PCRE_DOTALL
+ and PCRE_MULTILINE options is used. A class such as [^a] always matches
+ one of these characters.
The minus (hyphen) character can be used to specify a range of charac-
ters in a character class. For example, [d-m] matches any letter
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 alterna-
- tives are within a subpattern (defined below), "succeeds" means match-
- ing the rest of the main pattern as well as the alternative in the sub-
- pattern.
+ 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 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 to put it at the
- start.
+ The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA
+ can be changed in the same way as the Perl-compatible options by using
+ the characters J, U and X respectively.
SUBPATTERNS
cat(aract|erpillar|)
- matches one of the words "cat", "cataract", or "caterpillar". Without
- the parentheses, it would match "cataract", "erpillar" or the empty
+ 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. This means
- that, when the whole pattern matches, that portion of the subject
+ 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 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 numbers for the capturing
+ 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 pat-
+ For example, if the string "the red king" is matched against the pat-
tern
the ((red|white) (king|queen))
the captured substrings are "red king", "red", and "king", and are num-
bered 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 captur-
- ing, and is not counted when computing the number of any subsequent
- capturing subpatterns. For example, if the string "the white queen" is
+ 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 captur-
+ ing, 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-
+ 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
+ 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
+ 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 expres-
- sions. Furthermore, if an expression is modified, the numbers may
- change. To help with this difficulty, PCRE supports the naming of sub-
- patterns, 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
+ Identifying capturing parentheses by number is simple, but it can be
+ very hard to keep track of the numbers in complicated regular expres-
+ sions. Furthermore, if an expression is modified, the numbers may
+ change. To help with this difficulty, PCRE supports the naming of sub-
+ patterns, something that Perl does not provide. The Python syntax
+ (?P<name>...) is used. References to capturing parentheses from other
+ parts of the pattern, such as backreferences, recursion, and condi-
+ tions, can be made by name as well as by number.
+
+ Names consist of up to 32 alphanumeric characters and underscores.
+ 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 con-
- venience function for extracting a captured substring by name. For fur-
- ther details see the pcreapi documentation.
+ number translation table from a compiled pattern. There is also a con-
+ venience function for extracting a captured substring by name.
+
+ By default, a name must be unique within a pattern, but it is possible
+ to relax this constraint by setting the PCRE_DUPNAMES option at compile
+ time. This can be useful for patterns where only one instance of the
+ named parentheses can match. Suppose you want to match the name of a
+ weekday, either as a 3-letter abbreviation or as the full name, and in
+ both cases you want to extract the abbreviation. This pattern (ignoring
+ the line breaks) does the job:
+
+ (?P<DN>Mon|Fri|Sun)(?:day)?|
+ (?P<DN>Tue)(?:sday)?|
+ (?P<DN>Wed)(?:nesday)?|
+ (?P<DN>Thu)(?:rsday)?|
+ (?P<DN>Sat)(?:urday)?
+
+ There are five capturing substrings, but only one is ever set after a
+ match. The convenience function for extracting the data by name
+ returns the substring for the first, and in this example, the only,
+ subpattern of that name that matched. This saves searching to find
+ which numbered subpattern it was. If you make a reference to a non-
+ unique named subpattern from elsewhere in the pattern, the one that
+ corresponds to the lowest number is used. For further details of the
+ interfaces for handling named subpatterns, see the pcreapi documenta-
+ tion.
REPETITION
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.
+ The possessive quantifier syntax is an extension to the Perl syntax.
+ Jeffrey Friedl originated the idea (and the name) in the first edition
+ of his book. Mike McCloskey liked it, so implemented it when he built
+ Sun's Java package, and PCRE copied it from there.
When a pattern contains an unlimited repeat inside a subpattern that
can itself be repeated an unlimited number of times, the use of an
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 pat-
tern. In other words, the parentheses that are referenced need not be
- to the left of the reference for numbers less than 10. See the subsec-
- tion entitled "Non-printing characters" above for further details of
- the handling of digits following a backslash.
+ to the left of the reference for numbers less than 10. A "forward back
+ reference" of this type can make sense when a repetition is involved
+ and the subpattern to the right has participated in an earlier itera-
+ tion.
+
+ It is not possible to have a numerical "forward back reference" to sub-
+ pattern whose number is 10 or more. However, a back reference to any
+ subpattern is possible using named parentheses (see below). See also
+ the subsection entitled "Non-printing characters" above for further
+ details of the handling of digits following a backslash.
- A back reference matches whatever actually matched the capturing sub-
- pattern in the current subject string, rather than anything matching
+ A back reference matches whatever actually matched the capturing sub-
+ pattern 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 exam-
+ 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 exam-
ple,
((?i)rah)\s+\1
- matches "rah rah" and "RAH RAH", but not "RAH rah", even though the
+ 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).
+ 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)
+ (?P<p1>(?i)rah)\s+(?P=p1)
+
+ A subpattern that is referenced by name may appear in the pattern
+ before or after the reference.
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
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 sev-
- eral alternatives, they do not all have to have the same fixed length.
- Thus
+ eral top-level alternatives, they do not all have to have the same
+ fixed length. Thus
(?<=bullock|donkey)
tives 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 con-
- tains 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:
+ consists of a sequence of digits, or a sequence of alphanumeric charac-
+ ters and underscores, the condition is satisfied if the capturing sub-
+ pattern of that number or name has previously matched. There is a pos-
+ sible ambiguity here, because subpattern names may consist entirely of
+ digits. PCRE looks first for a named subpattern; if it cannot find one
+ and the text consists entirely of digits, it looks for a subpattern of
+ that number, which must be greater than zero. Using subpattern names
+ that consist entirely of digits is not recommended.
+
+ 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:
( \( )? [^()]+ (?(1) \) )
tern 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.
+ optionally enclosed in parentheses. Rewriting it to use a named subpat-
+ tern gives this:
+
+ (?P<OPEN> \( )? [^()]+ (?(OPEN) \) )
- 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 condi-
- tion is false. This is a PCRE extension. Recursive patterns are
- described in the next section.
+ If the condition is the string (R), and there is no subpattern with the
+ name R, the condition 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
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 new-
- line character in the pattern.
+ character class introduces a comment that continues to immediately
+ after the next newline in the pattern.
RECURSIVE PATTERNS
tion.) 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):
+ A recursive subpattern call is always treated as an atomic group. That
+ is, once it has matched some of the subject string, it is never re-
+ entered, even if it contains untried alternatives and there is a subse-
+ quent matching failure.
+
+ 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 parenthe-
+ recursive match of the pattern itself (that is, a correctly parenthe-
sized substring). Finally there is a closing parenthesis.
If this were part of a larger pattern, you would not want to recurse
(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.
+ two strings. Such references, if given numerically, must follow the
+ subpattern to which they refer. However, named references can refer to
+ later subpatterns.
+
+ Like recursive subpatterns, a "subroutine" call is always treated as an
+ atomic group. That is, once it has matched some of the subject string,
+ it is never re-entered, even if it contains untried alternatives and
+ there is a subsequent matching failure.
CALLOUTS
gether. 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.
+Last updated: 06 June 2006
+Copyright (c) 1997-2006 University of Cambridge.
git://git.exim.org / exim.git / blobdiff