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|
[;1m re[0m
This module contains regular expression matching functions for
strings and binaries.
The regular expression syntax and semantics resemble that of
Perl.
The matching algorithms of the library are based on the PCRE
library, but not all of the PCRE library is interfaced and some
parts of the library go beyond what PCRE offers. Currently PCRE
version 8.40 (release date 2017-01-11) is used. The sections of
the PCRE documentation that are relevant to this module are
included here.
Note:
The Erlang literal syntax for strings uses the "\" (backslash)
character as an escape code. You need to escape backslashes in
literal strings, both in your code and in the shell, with an
extra backslash, that is, "\\".
[;1mPerl-Like Regular Expression Syntax[0m
The following sections contain reference material for the regular
expressions used by this module. The information is based on the
PCRE documentation, with changes where this module behaves
differently to the PCRE library.
[;1mPCRE Regular Expression Details[0m
The syntax and semantics of the regular expressions supported by
PCRE are described in detail in the following sections. Perl's
regular expressions are described in its own documentation, and
regular expressions in general are covered in many books, some
with copious examples. Jeffrey Friedl's "Mastering Regular
Expressions", published by O'Reilly, covers regular expressions in
great detail. This description of the PCRE regular expressions is
intended as reference material.
The reference material is divided into the following sections:
• Special Start-of-Pattern Items
• Characters and Metacharacters
• Backslash
• Circumflex and Dollar
• Full Stop (Period, Dot) and \N
• Matching a Single Data Unit
• Square Brackets and Character Classes
• Posix Character Classes
• Vertical Bar
• Internal Option Setting
• Subpatterns
• Duplicate Subpattern Numbers
• Named Subpatterns
• Repetition
• Atomic Grouping and Possessive Quantifiers
• Back References
• Assertions
• Conditional Subpatterns
• Comments
• Recursive Patterns
• Subpatterns as Subroutines
• Oniguruma Subroutine Syntax
• Backtracking Control
[;1mSpecial Start-of-Pattern Items[0m
Some options that can be passed to [;;4mcompile/2[0m can also be set by
special items at the start of a pattern. These are not
Perl-compatible, but are provided to make these options accessible
to pattern writers who are not able to change the program that
processes the pattern. Any number of these items can appear, but
they must all be together right at the start of the pattern
string, and the letters must be in upper case.
UTF Support
Unicode support is basically UTF-8 based. To use Unicode
characters, you either call [;;4mcompile/2[0m or [;;4mrun/3[0m with option [;;4m[0m
[;;4municode[0m, or the pattern must start with one of these special
sequences:
(*UTF8)
(*UTF)
Both options give the same effect, the input string is interpreted
as UTF-8. Notice that with these instructions, the automatic
conversion of lists to UTF-8 is not performed by the [;;4mre[0m
functions. Therefore, using these sequences is not recommended.
Add option [;;4municode[0m when running [;;4mcompile/2[0m instead.
Some applications that allow their users to supply patterns can
wish to restrict them to non-UTF data for security reasons. If
option [;;4mnever_utf[0m is set at compile time, (*UTF), and so on, are
not allowed, and their appearance causes an error.
Unicode Property Support
The following is another special sequence that can appear at the
start of a pattern:
(*UCP)
This has the same effect as setting option [;;4mucp[0m: it causes
sequences such as \d and \w to use Unicode properties to determine
character types, instead of recognizing only characters with codes <
256 through a lookup table.
Disabling Startup Optimizations
If a pattern starts with [;;4m(*NO_START_OPT)[0m, it has the same effect
as setting option [;;4mno_start_optimize[0m at compile time.
Newline Conventions
PCRE supports five conventions for indicating line breaks in
strings: a single CR (carriage return) character, a single LF
(line feed) character, the two-character sequence CRLF, any of the
three preceding, and any Unicode newline sequence.
A newline convention can also be specified by starting a pattern
string with one of the following five sequences:
[;;4m(*CR)[0m:
Carriage return
[;;4m(*LF)[0m:
Line feed
[;;4m(*CRLF)[0m:
>Carriage return followed by line feed
[;;4m(*ANYCRLF)[0m:
Any of the three above
[;;4m(*ANY)[0m:
All Unicode newline sequences
These override the default and the options specified to [;;4mcompile/2[0m.
For example, the following pattern changes the convention to CR:
(*CR)a.b
This pattern matches [;;4ma\nb[0m, as LF is no longer a newline. If more
than one of them is present, the last one is used.
The newline convention affects where the circumflex and dollar
assertions are true. It also affects the interpretation of the dot
metacharacter when [;;4mdotall[0m is not set, and the behavior of \N.
However, it does not affect what the \R escape sequence matches.
By default, this is any Unicode newline sequence, for Perl
compatibility. However, this can be changed; see the description
of \R in section Newline Sequences. A change of the \R setting
can be combined with a change of the newline convention.
Setting Match and Recursion Limits
The caller of [;;4mrun/3[0m can set a limit on the number of times the
internal match() function is called and on the maximum depth of
recursive calls. These facilities are provided to catch runaway
matches that are provoked by patterns with huge matching trees (a
typical example is a pattern with nested unlimited repeats) and to
avoid running out of system stack by too much recursion. When one
of these limits is reached, [;;4mpcre_exec()[0m gives an error return.
The limits can also be set by items at the start of the pattern of
the following forms:
(*LIMIT_MATCH=d)
(*LIMIT_RECURSION=d)
Here d is any number of decimal digits. However, the value of the
setting must be less than the value set by the caller of [;;4mrun/3[0m
for it to have any effect. That is, the pattern writer can lower
the limit set by the programmer, but not raise it. If there is
more than one setting of one of these limits, the lower value is
used.
The default value for both the limits is 10,000,000 in the Erlang
VM. Notice that the recursion limit does not affect the stack
depth of the VM, as PCRE for Erlang is compiled in such a way that
the match function never does recursion on the C stack.
Note that [;;4mLIMIT_MATCH[0m and [;;4mLIMIT_RECURSION[0m can only reduce the
value of the limits set by the caller, not increase them.
[;1mCharacters and Metacharacters[0m
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 following pattern matches a
portion of a subject string that is identical to itself:
The quick brown fox
When caseless matching is specified (option [;;4mcaseless[0m), letters
are matched independently of case.
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.
Two sets of metacharacters exist: those that are recognized
anywhere in the pattern except within square brackets, and those
that are recognized within square brackets. Outside square
brackets, the metacharacters are as follows:
[;;4m\[0m:
General escape character with many uses
[;;4m^[0m:
Assert start of string (or line, in multiline mode)
[;;4m$[0m:
Assert end of string (or line, in multiline mode)
[;;4m.[0m:
Match any character except newline (by default)
[;;4m[[0m:
Start character class definition
[;;4m|[0m:
Start of alternative branch
[;;4m([0m:
Start subpattern
[;;4m)[0m:
End subpattern
[;;4m?[0m:
Extends the meaning of (, also 0 or 1 quantifier, also
quantifier minimizer
[;;4m*[0m:
0 or more quantifiers
[;;4m+[0m:
1 or more quantifier, also "possessive quantifier"
[;;4m{[0m:
Start min/max quantifier
Part of a pattern within square brackets is called a "character
class". The following are the only metacharacters in a character
class:
[;;4m\[0m:
General escape character
[;;4m^[0m:
Negate the class, but only if the first character
[;;4m-[0m:
Indicates character range
[;;4m[[0m:
Posix character class (only if followed by Posix syntax)
[;;4m][0m:
Terminates the character class
The following sections describe the use of each metacharacter.
[;1mBackslash[0m
The backslash character has many uses. First, if it is followed by
a character that is not a number or a letter, it takes away any
special meaning that a character can 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 if 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, write \\.
In [;;4municode[0m mode, only ASCII numbers and letters have any special
meaning after a backslash. All other characters (in particular,
those whose code points are > 127) are treated as literals.
If a pattern is compiled with option [;;4mextended[0m, 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
escaping backslash can be used to include a whitespace or #
character as part of the pattern.
To remove the special meaning from a sequence of characters, put
them between \Q and \E. This is different from Perl in that $ and @
are handled as literals in \Q...\E sequences in PCRE, while $ and @
cause variable interpolation in Perl. Notice 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. An isolated \E that is not preceded by \Q is
ignored. If \Q is not followed by \E later in the pattern, the
literal interpretation continues to the end of the pattern (that
is, \E is assumed at the end). If the isolated \Q is inside a
character class, this causes an error, as the character class is
not terminated.
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. When a pattern is
prepared by text editing, it is often easier to use one of the
following escape sequences than the binary character it
represents:
[;;4m\a[0m:
Alarm, that is, the BEL character (hex 07)
[;;4m\cx[0m:
"Control-x", where x is any ASCII character
[;;4m\e[0m:
Escape (hex 1B)
[;;4m\f[0m:
Form feed (hex 0C)
[;;4m\n[0m:
Line feed (hex 0A)
[;;4m\r[0m:
Carriage return (hex 0D)
[;;4m\t[0m:
Tab (hex 09)
[;;4m\0dd[0m:
Character with octal code 0dd
[;;4m\ddd[0m:
Character with octal code ddd, or back reference
[;;4m\o{ddd..}[0m:
character with octal code ddd..
[;;4m\xhh[0m:
Character with hex code hh
[;;4m\x{hhh..}[0m:
Character with hex code hhh..
Note:
Note that \0dd is always an octal code, and that \8 and \9 are
the literal characters "8" and "9".
The precise effect of \cx on ASCII characters is as follows: if x
is a lowercase letter, it is converted to upper case. Then bit 6
of the character (hex 40) is inverted. Thus \cA to \cZ become hex
01 to hex 1A (A is 41, Z is 5A), but \c{ becomes hex 3B ({ is 7B),
and \c; becomes hex 7B (; is 3B). If the data item (byte or 16-bit
value) following \c has a value > 127, a compile-time error
occurs. This locks out non-ASCII characters in all modes.
The \c facility was designed for use with ASCII characters, but
with the extension to Unicode it is even less useful than it once
was.
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\015 specifies two binary zeros followed by a CR
character (code value 13). Make sure you supply two digits after
the initial zero if the pattern character that follows is itself
an octal digit.
The escape \o must be followed by a sequence of octal digits,
enclosed in braces. An error occurs if this is not the case. This
escape is a recent addition to Perl; it provides way of specifying
character code points as octal numbers greater than 0777, and it
also allows octal numbers and back references to be unambiguously
specified.
For greater clarity and unambiguity, it is best to avoid following \
by a digit greater than zero. Instead, use \o{} or \x{} to specify
character numbers, and \g{} to specify back references. The
following paragraphs describe the old, ambiguous syntax.
The handling of a backslash followed by a digit other than 0 is
complicated, and Perl has changed in recent releases, causing PCRE
also to change. Outside a character class, PCRE reads the digit
and any following digits as a decimal number. If the number is <
8, 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 provided
later, following the discussion of parenthesized subpatterns.
Inside a character class, or if the decimal number following \ is >
7 and there have not been that many capturing subpatterns, PCRE
handles \8 and \9 as the literal characters "8" and "9", and
otherwise re-reads up to three octal digits following the
backslash, and using them to generate a data character. Any
subsequent digits stand for themselves. For example:
[;;4m\040[0m:
Another way of writing an ASCII space
[;;4m\40[0m:
The same, provided there are < 40 previous capturing
subpatterns
[;;4m\7[0m:
Always a back reference
[;;4m\11[0m:
Can be a back reference, or another way of writing a tab
[;;4m\011[0m:
Always a tab
[;;4m\0113[0m:
A tab followed by character "3"
[;;4m\113[0m:
Can be a back reference, otherwise the character with octal
code 113
[;;4m\377[0m:
Can be a back reference, otherwise value 255 (decimal)
[;;4m\81[0m:
Either a back reference, or the two characters "8" and "1"
Notice that octal values >= 100 that are specified using this
syntax must not be introduced by a leading zero, as no more than
three octal digits are ever read.
By default, after \x that is not followed by {, 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
}. If a character other than a hexadecimal digit appears between
\x{ and }, or if there is no terminating }, an error occurs.
Characters whose value is less than 256 can be defined by either
of the two syntaxes for \x. There is no difference in the way they
are handled. For example, \xdc is exactly the same as \x{dc}.
Constraints on character values
Characters that are specified using octal or hexadecimal numbers
are limited to certain values, as follows:
[;;4m8-bit non-UTF mode[0m:
< 0x100
[;;4m8-bit UTF-8 mode[0m:
< 0x10ffff and a valid codepoint
Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the
so-called "surrogate" codepoints), and 0xffef.
Escape sequences in character classes
All the sequences that define a single character value can be used
both inside and outside character classes. Also, inside a
character class, \b is interpreted as the backspace character (hex
08).
\N is not allowed in a character class. \B, \R, and \X are not
special inside a character class. Like other unrecognized escape
sequences, they are treated as the literal characters "B", "R",
and "X". Outside a character class, these sequences have different
meanings.
Unsupported Escape Sequences
In Perl, the sequences \l, \L, \u, and \U are recognized by its
string handler and used to modify the case of following
characters. PCRE does not support these escape sequences.
Absolute and Relative Back References
The sequence \g followed by an unsigned or a negative number,
optionally enclosed in braces, is an absolute or relative back
reference. A named back reference can be coded as \g{name}. Back
references are discussed later, following the discussion of
parenthesized subpatterns.
Absolute and Relative Subroutine Calls
For compatibility with Oniguruma, the non-Perl syntax \g followed
by a name or a number enclosed either in angle brackets or single
quotes, is alternative syntax for referencing a subpattern as a
"subroutine". Details are discussed later. Notice that \g{...}
(Perl syntax) and \g<...> (Oniguruma syntax) are not synonymous.
The former is a back reference and the latter is a subroutine
call.
Generic Character Types
Another use of backslash is for specifying generic character
types:
[;;4m\d[0m:
Any decimal digit
[;;4m\D[0m:
Any character that is not a decimal digit
[;;4m\h[0m:
Any horizontal whitespace character
[;;4m\H[0m:
Any character that is not a horizontal whitespace character
[;;4m\s[0m:
Any whitespace character
[;;4m\S[0m:
Any character that is not a whitespace character
[;;4m\v[0m:
Any vertical whitespace character
[;;4m\V[0m:
Any character that is not a vertical whitespace character
[;;4m\w[0m:
Any "word" character
[;;4m\W[0m:
Any "non-word" character
There is also the single sequence \N, which matches a non-newline
character. This is the same as the "." metacharacter when [;;4mdotall[0m
is not set. Perl also uses \N to match characters by name, but
PCRE does not support this.
Each pair of lowercase and uppercase escape sequences partitions
the complete set of characters into two disjoint sets. Any given
character matches one, and only one, of each pair. The 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 fail, as
there is no character to match.
For compatibility with Perl, \s did not used to match the VT
character (code 11), which made it different from the the POSIX
"space" class. However, Perl added VT at release 5.18, and PCRE
followed suit at release 8.34. The default \s characters are now
HT (9), LF (10), VT (11), FF (12), CR (13), and space (32), which
are defined as white space in the "C" locale. This list may vary
if locale-specific matching is taking place. For example, in some
locales the "non-breaking space" character (\xA0) is recognized as
white space, and in others the VT character is not.
A "word" character is an underscore or any character that is a
letter or a digit. By default, the definition of letters and
digits is controlled by the PCRE low-valued character tables, in
Erlang's case (and without option [;;4municode[0m), the ISO Latin-1
character set.
By default, in [;;4municode[0m mode, characters with values > 255, that
is, all characters outside the ISO Latin-1 character set, never
match \d, \s, or \w, and always match \D, \S, and \W. These
sequences retain their original meanings from before UTF support
was available, mainly for efficiency reasons. However, if option [;;4m[0m
[;;4mucp[0m is set, the behavior is changed so that Unicode properties
are used to determine character types, as follows:
[;;4m\d[0m:
Any character that \p{Nd} matches (decimal digit)
[;;4m\s[0m:
Any character that \p{Z} or \h or \v
[;;4m\w[0m:
Any character that matches \p{L} or \p{N} matches, plus
underscore
The uppercase escapes match the inverse sets of characters. Notice
that \d matches only decimal digits, while \w matches any Unicode
digit, any Unicode letter, and underscore. Notice also that [;;4mucp[0m
affects \b and \B, as they are defined in terms of \w and \W.
Matching these sequences is noticeably slower when [;;4mucp[0m is set.
The sequences \h, \H, \v, and \V are features that were added to
Perl in release 5.10. In contrast to the other sequences, which
match only ASCII characters by default, these always match certain
high-valued code points, regardless if [;;4mucp[0m is set.
The following are the horizontal space characters:
[;;4mU+0009[0m:
Horizontal tab (HT)
[;;4mU+0020[0m:
Space
[;;4mU+00A0[0m:
Non-break space
[;;4mU+1680[0m:
Ogham space mark
[;;4mU+180E[0m:
Mongolian vowel separator
[;;4mU+2000[0m:
En quad
[;;4mU+2001[0m:
Em quad
[;;4mU+2002[0m:
En space
[;;4mU+2003[0m:
Em space
[;;4mU+2004[0m:
Three-per-em space
[;;4mU+2005[0m:
Four-per-em space
[;;4mU+2006[0m:
Six-per-em space
[;;4mU+2007[0m:
Figure space
[;;4mU+2008[0m:
Punctuation space
[;;4mU+2009[0m:
Thin space
[;;4mU+200A[0m:
Hair space
[;;4mU+202F[0m:
Narrow no-break space
[;;4mU+205F[0m:
Medium mathematical space
[;;4mU+3000[0m:
Ideographic space
The following are the vertical space characters:
[;;4mU+000A[0m:
Line feed (LF)
[;;4mU+000B[0m:
Vertical tab (VT)
[;;4mU+000C[0m:
Form feed (FF)
[;;4mU+000D[0m:
Carriage return (CR)
[;;4mU+0085[0m:
Next line (NEL)
[;;4mU+2028[0m:
Line separator
[;;4mU+2029[0m:
Paragraph separator
In 8-bit, non-UTF-8 mode, only the characters with code points <
256 are relevant.
Newline Sequences
Outside a character class, by default, the escape sequence \R
matches any Unicode newline sequence. In non-UTF-8 mode, \R is
equivalent to the following:
(?>\r\n|\n|\x0b|\f|\r|\x85)
This is an example of an "atomic group", details are provided
below.
This particular group matches either the two-character sequence CR
followed by LF, or one of the single characters LF (line feed,
U+000A), VT (vertical tab, U+000B), FF (form feed, U+000C), CR
(carriage return, U+000D), or NEL (next line, U+0085). The
two-character sequence is treated as a single unit that cannot be
split.
In Unicode mode, two more characters whose code points are > 255
are added: LS (line separator, U+2028) and PS (paragraph
separator, U+2029). Unicode character property support is not
needed for these characters to be recognized.
\R can be restricted to match only CR, LF, or CRLF (instead of the
complete set of Unicode line endings) by setting option [;;4m[0m
[;;4mbsr_anycrlf[0m either at compile time or when the pattern is
matched. (BSR is an acronym for "backslash R".) This can be made
the default when PCRE is built; if so, the other behavior can be
requested through option [;;4mbsr_unicode[0m. These settings can also be
specified by starting a pattern string with one of the following
sequences:
[;;4m(*BSR_ANYCRLF)[0m:
CR, LF, or CRLF only
[;;4m(*BSR_UNICODE)[0m:
Any Unicode newline sequence
These override the default and the options specified to the
compiling function, but they can themselves be overridden by
options specified to a matching function. Notice that these
special settings, which are not Perl-compatible, are recognized
only at the very start of a pattern, and that they must be in
upper case. If more than one of them is present, the last one is
used. They can be combined with a change of newline convention;
for example, a pattern can start with:
(*ANY)(*BSR_ANYCRLF)
They can also be combined with the (*UTF8), (*UTF), or (*UCP)
special sequences. Inside a character class, \R is treated as an
unrecognized escape sequence, and so matches the letter "R" by
default.
Unicode Character Properties
Three more escape sequences that match characters with specific
properties are available. When in 8-bit non-UTF-8 mode, these
sequences are limited to testing characters whose code points are <
256, but they do work in this mode. The following are the extra
escape sequences:
[;;4m\p{xx}[0m:
A character with property xx
[;;4m\P{xx}[0m:
A character without property xx
[;;4m\X[0m:
A Unicode extended grapheme cluster
The property names represented by xx above are limited to the
Unicode script names, the general category properties, "Any",
which matches any character (including newline), and some special
PCRE properties (described in the next section). Other Perl
properties, such as "InMusicalSymbols", are currently not
supported by PCRE. Notice that \P{Any} does not match any
characters and 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 following is the current list of
scripts:
• Arabic
• Armenian
• Avestan
• Balinese
• Bamum
• Bassa_Vah
• Batak
• Bengali
• Bopomofo
• Braille
• Buginese
• Buhid
• Canadian_Aboriginal
• Carian
• Caucasian_Albanian
• Chakma
• Cham
• Cherokee
• Common
• Coptic
• Cuneiform
• Cypriot
• Cyrillic
• Deseret
• Devanagari
• Duployan
• Egyptian_Hieroglyphs
• Elbasan
• Ethiopic
• Georgian
• Glagolitic
• Gothic
• Grantha
• Greek
• Gujarati
• Gurmukhi
• Han
• Hangul
• Hanunoo
• Hebrew
• Hiragana
• Imperial_Aramaic
• Inherited
• Inscriptional_Pahlavi
• Inscriptional_Parthian
• Javanese
• Kaithi
• Kannada
• Katakana
• Kayah_Li
• Kharoshthi
• Khmer
• Khojki
• Khudawadi
• Lao
• Latin
• Lepcha
• Limbu
• Linear_A
• Linear_B
• Lisu
• Lycian
• Lydian
• Mahajani
• Malayalam
• Mandaic
• Manichaean
• Meetei_Mayek
• Mende_Kikakui
• Meroitic_Cursive
• Meroitic_Hieroglyphs
• Miao
• Modi
• Mongolian
• Mro
• Myanmar
• Nabataean
• New_Tai_Lue
• Nko
• Ogham
• Ol_Chiki
• Old_Italic
• Old_North_Arabian
• Old_Permic
• Old_Persian
• Oriya
• Old_South_Arabian
• Old_Turkic
• Osmanya
• Pahawh_Hmong
• Palmyrene
• Pau_Cin_Hau
• Phags_Pa
• Phoenician
• Psalter_Pahlavi
• Rejang
• Runic
• Samaritan
• Saurashtra
• Sharada
• Shavian
• Siddham
• Sinhala
• Sora_Sompeng
• Sundanese
• Syloti_Nagri
• Syriac
• Tagalog
• Tagbanwa
• Tai_Le
• Tai_Tham
• Tai_Viet
• Takri
• Tamil
• Telugu
• Thaana
• Thai
• Tibetan
• Tifinagh
• Tirhuta
• Ugaritic
• Vai
• Warang_Citi
• Yi
Each character has exactly one Unicode general category property,
specified by a two-letter acronym. 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
general category properties that start with that letter. In this
case, in the absence of negation, the curly brackets in the escape
sequence are optional. The following two examples have the same
effect:
\p{L}
\pL
The following general category property codes are supported:
[;;4mC[0m:
Other
[;;4mCc[0m:
Control
[;;4mCf[0m:
Format
[;;4mCn[0m:
Unassigned
[;;4mCo[0m:
Private use
[;;4mCs[0m:
Surrogate
[;;4mL[0m:
Letter
[;;4mLl[0m:
Lowercase letter
[;;4mLm[0m:
Modifier letter
[;;4mLo[0m:
Other letter
[;;4mLt[0m:
Title case letter
[;;4mLu[0m:
Uppercase letter
[;;4mM[0m:
Mark
[;;4mMc[0m:
Spacing mark
[;;4mMe[0m:
Enclosing mark
[;;4mMn[0m:
Non-spacing mark
[;;4mN[0m:
Number
[;;4mNd[0m:
Decimal number
[;;4mNl[0m:
Letter number
[;;4mNo[0m:
Other number
[;;4mP[0m:
Punctuation
[;;4mPc[0m:
Connector punctuation
[;;4mPd[0m:
Dash punctuation
[;;4mPe[0m:
Close punctuation
[;;4mPf[0m:
Final punctuation
[;;4mPi[0m:
Initial punctuation
[;;4mPo[0m:
Other punctuation
[;;4mPs[0m:
Open punctuation
[;;4mS[0m:
Symbol
[;;4mSc[0m:
Currency symbol
[;;4mSk[0m:
Modifier symbol
[;;4mSm[0m:
Mathematical symbol
[;;4mSo[0m:
Other symbol
[;;4mZ[0m:
Separator
[;;4mZl[0m:
Line separator
[;;4mZp[0m:
Paragraph separator
[;;4mZs[0m:
Space separator
The special property L& is also supported. It matches a character
that has the Lu, Ll, or Lt property, that is, a letter that is not
classified as a modifier or "other".
The Cs (Surrogate) property applies only to characters in the
range U+D800 to U+DFFF. Such characters are invalid in Unicode
strings and so cannot be tested by PCRE. Perl does not support the
Cs property.
The long synonyms for property names supported by Perl (such as
\p{Letter}) are not supported by PCRE. It is not permitted to
prefix any of these properties with "Is".
No character in the Unicode table has the Cn (unassigned)
property. This property is instead assumed for any code point that
is not in the Unicode table.
Specifying caseless matching does not affect these escape
sequences. For example, \p{Lu} always matches only uppercase
letters. This is different from the behavior of current versions
of Perl.
Matching characters by Unicode property is not fast, as PCRE must
do a multistage table lookup to find a character property. That is
why the traditional escape sequences such as \d and \w do not use
Unicode properties in PCRE by default. However, you can make them
do so by setting option [;;4mucp[0m or by starting the pattern with
(*UCP).
Extended Grapheme Clusters
The \X escape matches any number of Unicode characters that form
an "extended grapheme cluster", and treats the sequence as an
atomic group (see below). Up to and including release 8.31, PCRE
matched an earlier, simpler definition that was equivalent to [;;4m[0m
[;;4m(?>\PM\pM*)[0m. That is, it matched a character without the "mark"
property, followed by zero or more characters with the "mark"
property. Characters with the "mark" property are typically
non-spacing accents that affect the preceding character.
This simple definition was extended in Unicode to include more
complicated kinds of composite character by giving each character
a grapheme breaking property, and creating rules that use these
properties to define the boundaries of extended grapheme clusters.
In PCRE releases later than 8.31, \X matches one of these
clusters.
\X always matches at least one character. Then it decides whether
to add more characters according to the following rules for ending
a cluster:
• End at the end of the subject string.
• Do not end between CR and LF; otherwise end after any
control character.
• Do not break Hangul (a Korean script) syllable sequences.
Hangul characters are of five types: L, V, T, LV, and LVT.
An L character can be followed by an L, V, LV, or LVT
character. An LV or V character can be followed by a V or T
character. An LVT or T character can be followed only by a T
character.
• Do not end before extending characters or spacing marks.
Characters with the "mark" property always have the "extend"
grapheme breaking property.
• Do not end after prepend characters.
• Otherwise, end the cluster.
PCRE Additional Properties
In addition to the standard Unicode properties described earlier,
PCRE supports four more that make it possible to convert
traditional escape sequences, such as \w and \s to use Unicode
properties. PCRE uses these non-standard, non-Perl properties
internally when the [;;4mucp[0m option is passed. However, they can also
be used explicitly. The properties are as follows:
[;;4mXan[0m:
Any alphanumeric character. Matches characters that have
either the L (letter) or the N (number) property.
[;;4mXps[0m:
Any Posix space character. Matches the characters tab, line
feed, vertical tab, form feed, carriage return, and any other
character that has the Z (separator) property.
[;;4mXsp[0m:
Any Perl space character. Matches the same as Xps, except that
vertical tab is excluded.
[;;4mXwd[0m:
Any Perl "word" character. Matches the same characters as Xan,
plus underscore.
Perl and POSIX space are now the same. Perl added VT to its space
character set at release 5.18 and PCRE changed at release 8.34.
Xan matches characters that have either the L (letter) or the N
(number) property. Xps matches the characters tab, linefeed,
vertical tab, form feed, or carriage return, and any other
character that has the Z (separator) property. Xsp is the same as
Xps; it used to exclude vertical tab, for Perl compatibility, but
Perl changed, and so PCRE followed at release 8.34. Xwd matches
the same characters as Xan, plus underscore.
There is another non-standard property, Xuc, which matches any
character that can be represented by a Universal Character Name in
C++ and other programming languages. These are the characters $,
@, ` (grave accent), and all characters with Unicode code points
>= U+00A0, except for the surrogates U+D800 to U+DFFF. Notice that
most base (ASCII) characters are excluded. (Universal Character
Names are of the form \uHHHH or \UHHHHHHHH, where H is a
hexadecimal digit. Notice that the Xuc property does not match
these sequences but the characters that they represent.)
Resetting the Match Start
The escape sequence \K causes any previously matched characters
not to be included in the final matched sequence. For example, the
following pattern matches "foobar", but reports that it has
matched "bar":
foo\Kbar
This feature is similar to a lookbehind assertion (described
below). However, in this case, the part of the subject before the
real match does not have to be of fixed length, as lookbehind
assertions do. The use of \K does not interfere with the setting
of captured substrings. For example, when the following pattern
matches "foobar", the first substring is still set to "foo":
(foo)\Kbar
Perl documents that the use of \K within assertions is "not well
defined". In PCRE, \K is acted upon when it occurs inside positive
assertions, but is ignored in negative assertions. Note that when
a pattern such as (?=ab\K) matches, the reported start of the
match can be greater than the end of the match.
Simple Assertions
The final use of backslash is for certain simple assertions. An
assertion specifies a condition that must 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 following are the backslashed
assertions:
[;;4m\b[0m:
Matches at a word boundary.
[;;4m\B[0m:
Matches when not at a word boundary.
[;;4m\A[0m:
Matches at the start of the subject.
[;;4m\Z[0m:
Matches at the end of the subject, and before a newline at the
end of the subject.
[;;4m\z[0m:
Matches only at the end of the subject.
[;;4m\G[0m:
Matches at the first matching position in the subject.
Inside a character class, \b has a different meaning; it matches
the backspace character. If any other of these assertions appears
in a character class, by default it matches the corresponding
literal character (for example, \B matches the letter B).
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 (that is, 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. In UTF mode, the meanings of \w and \W can be
changed by setting option [;;4mucp[0m. When this is done, it also
affects \b and \B. PCRE and Perl do not have a separate "start of
word" or "end of word" metasequence. However, whatever follows \b
normally determines which it is. For example, the fragment \ba
matches "a" at the start of a word.
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 options [;;4mnotbol[0m
or [;;4mnoteol[0m, which affect only the behavior of the circumflex and
dollar metacharacters. However, if argument [;;4mstartoffset[0m of [;;4m[0m
[;;4mrun/3[0m 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 at the end of the string and at the very end, while \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 argument [;;4m[0m
[;;4mstartoffset[0m of [;;4mrun/3[0m. It differs from \A when the value of [;;4m[0m
[;;4mstartoffset[0m is non-zero. By calling [;;4mrun/3[0m multiple times with
appropriate arguments, you can mimic the Perl option [;;4m/g[0m, and it
is in this kind of implementation where \G can be useful.
Notice, however, that the PCRE interpretation of \G, as the start
of the current match, is subtly different from Perl, which defines
it as the end of the previous match. In Perl, these can be
different when the previously matched string was empty. As PCRE
does only one match at a time, it cannot reproduce this behavior.
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.
[;1mCircumflex and Dollar[0m
The circumflex and dollar metacharacters are zero-width
assertions. That is, they test for a particular condition to be
true without consuming any characters from the subject string.
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
argument [;;4mstartoffset[0m of [;;4mrun/3[0m is non-zero, circumflex can
never match if option [;;4mmultiline[0m is unset. Inside a character
class, circumflex has an entirely different meaning (see below).
Circumflex needs not to be the first character of the pattern if
some alternatives are involved, but it is to be the first thing in
each alternative in which it appears if the pattern is ever to
match that branch. If all possible alternatives start with a
circumflex, that is, if the pattern is constrained to match only
at the start of the subject, it is said to be an "anchored"
pattern. (There are also other constructs that can cause a pattern
to be anchored.)
The 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 at the end of the string (by
default). Notice however that it does not match the newline.
Dollar needs not to be the last character of the pattern if some
alternatives are involved, but it is to 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 option [;;4mdollar_endonly[0m at
compile time. This does not affect the \Z assertion.
The meanings of the circumflex and dollar characters are changed
if option [;;4mmultiline[0m is set. When this is the case, a circumflex
matches immediately after internal newlines and 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,
and at the very end, when [;;4mmultiline[0m 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. So, 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 argument
startoffset of [;;4mrun/3[0m is non-zero. Option [;;4mdollar_endonly[0m is
ignored if [;;4mmultiline[0m is set.
Notice that the sequences \A, \Z, and \z can be used to match the
start and end of the subject in both modes. If all branches of a
pattern start with \A, it is always anchored, regardless if [;;4m[0m
[;;4mmultiline[0m is set.
[;1mFull Stop (Period, Dot) and \N[0m
Outside a character class, a dot in the pattern matches any
character in the subject string except (by default) a character
that signifies the end of a line.
When a line ending is defined as a single character, dot never
matches that character. When the two-character sequence CRLF is
used, dot does not match CR if it is immediately followed by LF,
otherwise it matches all characters (including isolated CRs and
LFs). When any Unicode line endings are recognized, dot does not
match CR, LF, or any of the other line-ending characters.
The behavior of dot regarding newlines can be changed. If option [;;4m[0m
[;;4mdotall[0m is set, a dot matches any character, without exception. If
the two-character sequence CRLF is present in the subject string,
it takes two dots to match it.
The handling of dot is entirely independent of the handling of
circumflex and dollar, the only relationship is that both involve
newlines. Dot has no special meaning in a character class.
The escape sequence \N behaves like a dot, except that it is not
affected by option [;;4mPCRE_DOTALL[0m. That is, it matches any
character except one that signifies the end of a line. Perl also
uses \N to match characters by name but PCRE does not support
this.
[;1mMatching a Single Data Unit[0m
Outside a character class, the escape sequence \C matches any data
unit, regardless if a UTF mode is set. One data unit is one byte.
Unlike a dot, \C always matches line-ending characters. The
feature is provided in Perl to match individual bytes in UTF-8
mode, but it is unclear how it can usefully be used. As \C breaks
up characters into individual data units, matching one unit with
\C in a UTF mode means that the remaining string can start with a
malformed UTF character. This has undefined results, as PCRE
assumes that it deals with valid UTF strings.
PCRE does not allow \C to appear in lookbehind assertions
(described below) in a UTF mode, as this would make it impossible
to calculate the length of the lookbehind.
The \C escape sequence is best avoided. However, one way of using
it that avoids the problem of malformed UTF characters is to use a
lookahead to check the length of the next character, as in the
following pattern, which can be used with a UTF-8 string (ignore
whitespace and line breaks):
(?| (?=[\x00-\x7f])(\C) |
(?=[\x80-\x{7ff}])(\C)(\C) |
(?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
(?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))
A group that starts with (?| resets the capturing parentheses
numbers in each alternative (see section Duplicate Subpattern
Numbers). The assertions at the start of each branch check the
next UTF-8 character for values whose encoding uses 1, 2, 3, or 4
bytes, respectively. The individual bytes of the character are
then captured by the appropriate number of groups.
[;1mSquare Brackets and Character Classes[0m
An opening square bracket introduces a character class, terminated
by a closing square bracket. A closing square bracket on its own
is not special by default. However, if option [;;4m[0m
[;;4mPCRE_JAVASCRIPT_COMPAT[0m is set, a lone closing square bracket
causes a compile-time error. If a closing square bracket is
required as a member of the class, it is to 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 a
UTF mode, the character can be more than one data unit long. 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 required as a member
of the class, ensure that it is not the first character, or escape
it with a backslash.
For example, the character class [;;4m[aeiou][0m matches any lowercase
vowel, while [;;4m[^aeiou][0m matches any character that is not a
lowercase vowel. Notice 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 consumes 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 > 255 (0xffff) can be
included in a class as a literal string of data units, or by using
the \x{ escaping mechanism.
When caseless matching is set, any letters in a class represent
both their uppercase and lowercase versions. For example, a
caseless [;;4m[aeiou][0m matches "A" and "a", and a caseless [;;4m[^aeiou][0m
does not match "A", but a caseful version would. In a UTF mode,
PCRE always understands the concept of case for characters whose
values are < 256, so caseless matching is always possible. For
characters with higher values, the concept of case is supported
only if PCRE is compiled with Unicode property support. If you
want to use caseless matching in a UTF mode for characters >=,
ensure that PCRE is compiled with Unicode property support and
with UTF support.
Characters that can indicate line breaks are never treated in any
special way when matching character classes, whatever line-ending
sequence is in use, and whatever setting of options [;;4mPCRE_DOTALL[0m
and [;;4mPCRE_MULTILINE[0m 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
characters in a character class. For example, [d-m] matches any
letter between d and m, inclusive. If a minus character is
required in a class, it must be escaped with a backslash or appear
in a position where it cannot be interpreted as indicating a
range, typically as the first or last character in the class, or
immediately after a range. For example, [b-d-z] matches letters in
the range b to d, a hyphen character, or z.
The literal character "]" cannot be 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 "]" 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.
An error is generated if a POSIX character class (see below) or an
escape sequence other than one that defines a single character
appears at a point where a range ending character is expected. For
example, [z-\xff] is valid, but [A-\d] and [A-[:digit:]] are not.
Ranges operate in the collating sequence of character values. They
can also be used for characters specified numerically, for
example, [\000-\037]. Ranges can include any characters that are
valid for the current mode.
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. In a non-UTF
mode, if character tables for a French locale are in use,
[\xc8-\xcb] matches accented E characters in both cases. In UTF
modes, PCRE supports the concept of case for characters with
values > 255 only when it is compiled with Unicode property
support.
The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v,
\V, \w, and \W can appear in a character class, and add the
characters that they match to the class. For example, [\dABCDEF]
matches any hexadecimal digit. In UTF modes, option [;;4mucp[0m affects
the meanings of \d, \s, \w and their uppercase partners, just as
it does when they appear outside a character class, as described
in section Generic Character Types earlier. The escape sequence
\b has a different meaning inside a character class; it matches
the backspace character. The sequences \B, \N, \R, and \X are not
special inside a character class. Like any other unrecognized
escape sequences, they are treated as the literal characters "B",
"N", "R", and "X".
A circumflex can conveniently be used with the uppercase character
types to specify a more restricted set of characters than the
matching lowercase type. For example, class [^\W_] matches any
letter or digit, but not underscore, while [\w] includes
underscore. A positive character class is to be read as "something
OR something OR ..." and a negative class as "NOT something AND
NOT something AND NOT ...".
Only the following metacharacters are recognized in character
classes:
• 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
introducing a Posix class name, or for a special
compatibility feature; see the next two sections)
• Terminating closing square bracket
However, escaping other non-alphanumeric characters does no harm.
[;1mPosix Character Classes[0m
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, the following
matches "0", "1", any alphabetic character, or "%":
[01[:alpha:]%]
The following are the supported class names:
[;;4malnum[0m:
Letters and digits
[;;4malpha[0m:
Letters
[;;4mascii[0m:
Character codes 0-127
[;;4mblank[0m:
Space or tab only
[;;4mcntrl[0m:
Control characters
[;;4mdigit[0m:
Decimal digits (same as \d)
[;;4mgraph[0m:
Printing characters, excluding space
[;;4mlower[0m:
Lowercase letters
[;;4mprint[0m:
Printing characters, including space
[;;4mpunct[0m:
Printing characters, excluding letters, digits, and space
[;;4mspace[0m:
Whitespace (the same as \s from PCRE 8.34)
[;;4mupper[0m:
Uppercase letters
[;;4mword[0m:
"Word" characters (same as \w)
[;;4mxdigit[0m:
Hexadecimal digits
The default "space" characters are HT (9), LF (10), VT (11), FF
(12), CR (13), and space (32). If locale-specific matching is
taking place, the list of space characters may be different; there
may be fewer or more of them. "Space" used to be different to \s,
which did not include VT, for Perl compatibility. However, Perl
changed at release 5.18, and PCRE followed at release 8.34.
"Space" and \s now match the same set of characters.
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, the
following matches "1", "2", or any non-digit:
[12[:^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.
By default, characters with values > 255 do not match any of the
Posix character classes. However, if option [;;4mPCRE_UCP[0m is passed
to [;;4mpcre_compile()[0m, some of the classes are changed so that
Unicode character properties are used. This is achieved by
replacing certain Posix classes by other sequences, as follows:
[;;4m[:alnum:][0m:
Becomes \p{Xan}
[;;4m[:alpha:][0m:
Becomes \p{L}
[;;4m[:blank:][0m:
Becomes \h
[;;4m[:digit:][0m:
Becomes \p{Nd}
[;;4m[:lower:][0m:
Becomes \p{Ll}
[;;4m[:space:][0m:
Becomes \p{Xps}
[;;4m[:upper:][0m:
Becomes \p{Lu}
[;;4m[:word:][0m:
Becomes \p{Xwd}
Negated versions, such as [:^alpha:], use \P instead of \p. Three
other POSIX classes are handled specially in UCP mode:
[;;4m[:graph:][0m:
This matches characters that have glyphs that mark the page
when printed. In Unicode property terms, it matches all
characters with the L, M, N, P, S, or Cf properties, except
for:
[;;4mU+061C[0m:
Arabic Letter Mark
[;;4mU+180E[0m:
Mongolian Vowel Separator
[;;4mU+2066 - U+2069[0m:
Various "isolate"s
[;;4m[:print:][0m:
This matches the same characters as [:graph:] plus space
characters that are not controls, that is, characters with the
Zs property.
[;;4m[:punct:][0m:
This matches all characters that have the Unicode P
(punctuation) property, plus those characters whose code
points are less than 128 that have the S (Symbol) property.
The other POSIX classes are unchanged, and match only characters
with code points less than 128.
Compatibility Feature for Word Boundaries
In the POSIX.2 compliant library that was included in 4.4BSD Unix,
the ugly syntax [[:<:]] and [[:>:]] is used for matching "start of
word" and "end of word". PCRE treats these items as follows:
[;;4m[[:<:]][0m:
is converted to \b(?=\w)
[;;4m[[:>:]][0m:
is converted to \b(?<=\w)
Only these exact character sequences are recognized. A sequence
such as [a[:<:]b] provokes error for an unrecognized POSIX class
name. This support is not compatible with Perl. It is provided to
help migrations from other environments, and is best not used in
any new patterns. Note that \b matches at the start and the end of
a word (see "Simple assertions" above), and in a Perl-style
pattern the preceding or following character normally shows which
is wanted, without the need for the assertions that are used above
in order to give exactly the POSIX behaviour.
[;1mVertical Bar[0m
Vertical bar characters are used to separate alternative patterns.
For example, the following pattern matches either "gilbert" or
"sullivan":
gilbert|sullivan
Any number of alternatives can 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 that
succeeds is used. If the alternatives are within a subpattern
(defined in section Subpatterns), "succeeds" means matching the
remaining main pattern and the alternative in the subpattern.
[;1mInternal Option Setting[0m
The settings of the Perl-compatible options [;;4mcaseless[0m, [;;4mmultiline[0m, [;;4m[0m
[;;4mdotall[0m, and [;;4mextended[0m can be changed from within the pattern by
a sequence of Perl option letters enclosed between "(?" and ")".
The option letters are as follows:
[;;4mi[0m:
For [;;4mcaseless[0m
[;;4mm[0m:
For [;;4mmultiline[0m
[;;4ms[0m:
For [;;4mdotall[0m
[;;4mx[0m:
For [;;4mextended[0m
For example, [;;4m(?im)[0m sets caseless, multiline matching. These
options can also be unset by preceding the letter with a hyphen. A
combined setting and unsetting such as [;;4m(?im-sx)[0m, which sets [;;4m[0m
[;;4mcaseless[0m and [;;4mmultiline[0m, while unsetting [;;4mdotall[0m and [;;4mextended[0m,
is also permitted. If a letter appears both before and after the
hyphen, the option is unset.
The PCRE-specific options [;;4mdupnames[0m, [;;4mungreedy[0m, and [;;4mextra[0m can
be changed in the same way as the Perl-compatible options by using
the characters J, U, and X respectively.
When one of these option changes occurs at top-level (that is, not
inside subpattern parentheses), the change applies to the
remainder of the pattern that follows.
An option change within a subpattern (see section Subpatterns)
affects only that part of the subpattern that follows it. So, the
following matches abc and aBc and no other strings (assuming [;;4m[0m
[;;4mcaseless[0m is not used):
(a(?i)b)c
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", although when matching "C" the
first branch is abandoned before the option setting. This is
because the effects of option settings occur at compile time.
There would be some weird behavior otherwise.
Note:
Other PCRE-specific options can be set by the application when
the compiling or matching functions are called. Sometimes the
pattern can contain special leading sequences, such as
(*CRLF), to override what the application has set or what has
been defaulted. Details are provided in section Newline
Sequences earlier.
The (*UTF8) and (*UCP) leading sequences can be used to set
UTF and Unicode property modes. They are equivalent to setting
options [;;4municode[0m and [;;4mucp[0m, respectively. The (*UTF) sequence
is a generic version that can be used with any of the
libraries. However, the application can set option [;;4mnever_utf[0m,
which locks out the use of the (*UTF) sequences.
[;1mSubpatterns[0m
Subpatterns are delimited by parentheses (round brackets), which
can be nested. Turning part of a pattern into a subpattern does
two things:
[;;4m1.[0m:
It localizes a set of alternatives. For example, the following
pattern matches "cataract", "caterpillar", or "cat":
cat(aract|erpillar|)
Without the parentheses, it would match "cataract",
"erpillar", or an empty string.
[;;4m2.[0m:
It sets up the subpattern as a capturing subpattern. That is,
when the complete pattern matches, that portion of the subject
string that matched the subpattern is passed back to the
caller through the return value of [;;4mrun/3[0m.
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 following
pattern, the captured substrings are "red king", "red", and
"king", and are numbered 1, 2, and 3, respectively:
the ((red|white) (king|queen))
It is not always helpful that plain parentheses fulfill two
functions. Often a grouping subpattern is required without a
capturing requirement. If an opening parenthesis is followed by a
question mark and a colon, the subpattern does not do any
capturing, and is not counted when computing the number of any
subsequent capturing subpatterns. For example, if the string "the
white queen" is matched against the following pattern, the
captured substrings are "white queen" and "queen", and are
numbered 1 and 2:
the ((?:red|white) (king|queen))
The maximum number of capturing subpatterns is 65535.
As a convenient shorthand, if any option settings are required at
the start of a non-capturing subpattern, the option letters can
appear between "?" and ":". Thus, the following two patterns match
the same set of strings:
(?i:saturday|sunday)
(?:(?i)saturday|sunday)
As 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 both "SUNDAY" and "Saturday".
[;1mDuplicate Subpattern Numbers[0m
Perl 5.10 introduced a feature where each alternative in a
subpattern uses the same numbers for its capturing parentheses.
Such a subpattern starts with [;;4m(?|[0m and is itself a non-capturing
subpattern. For example, consider the following pattern:
(?|(Sat)ur|(Sun))day
As the two alternatives are inside a [;;4m(?|[0m group, both sets of
capturing parentheses are numbered one. Thus, when the pattern
matches, you can look at captured substring number one, whichever
alternative matched. This construct is useful when you want to
capture a part, but not all, of one of many alternatives. Inside a [;;4m[0m
[;;4m(?|[0m group, parentheses are numbered as usual, but the number is
reset at the start of each branch. The numbers of any capturing
parentheses that follow the subpattern start after the highest
number used in any branch. The following example is from the Perl
documentation; the numbers underneath show in which buffer the
captured content is stored:
# before ---------------branch-reset----------- after
/ ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
# 1 2 2 3 2 3 4
A back reference to a numbered subpattern uses the most recent
value that is set for that number by any subpattern. The following
pattern matches "abcabc" or "defdef":
/(?|(abc)|(def))\1/
In contrast, a subroutine call to a numbered subpattern always
refers to the first one in the pattern with the given number. The
following pattern matches "abcabc" or "defabc":
/(?|(abc)|(def))(?1)/
If a condition test for a subpattern having matched refers to a
non-unique number, the test is true if any of the subpatterns of
that number have matched.
An alternative approach using this "branch reset" feature is to
use duplicate named subpatterns, as described in the next section.
[;1mNamed Subpatterns[0m
Identifying capturing parentheses by number is simple, but it can
be hard to keep track of the numbers in complicated regular
expressions. Also, if an expression is modified, the numbers can
change. To help with this difficulty, PCRE supports the naming of
subpatterns. This feature was not added to Perl until release
5.10. Python had the feature earlier, and PCRE introduced it at
release 4.0, using the Python syntax. PCRE now supports both the
Perl and the Python syntax. Perl allows identically numbered
subpatterns to have different names, but PCRE does not.
In PCRE, a subpattern can be named in one of three ways: [;;4m[0m
[;;4m(?<name>...)[0m or [;;4m(?'name'...)[0m as in Perl, or [;;4m(?P<name>...)[0m as
in Python. References to capturing parentheses from other parts of
the pattern, such as back references, recursion, and conditions,
can be made by name and by number.
Names consist of up to 32 alphanumeric characters and underscores,
but must start with a non-digit. Named capturing parentheses are
still allocated numbers as well as names, exactly as if the names
were not present. The [;;4mcapture[0m specification to [;;4mrun/3[0m can use
named values if they are present in the regular expression.
By default, a name must be unique within a pattern, but this
constraint can be relaxed by setting option [;;4mdupnames[0m at compile
time. (Duplicate names are also always permitted for subpatterns
with the same number, set up as described in the previous
section.) Duplicate names can be useful for patterns where only
one instance of the named parentheses can match. Suppose that 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. The following pattern (ignoring the line
breaks) does the job:
(?<DN>Mon|Fri|Sun)(?:day)?|
(?<DN>Tue)(?:sday)?|
(?<DN>Wed)(?:nesday)?|
(?<DN>Thu)(?:rsday)?|
(?<DN>Sat)(?:urday)?
There are five capturing substrings, but only one is ever set
after a match. (An alternative way of solving this problem is to
use a "branch reset" subpattern, as described in the previous
section.)
For capturing named subpatterns which names are not unique, the
first matching occurrence (counted from left to right in the
subject) is returned from [;;4mrun/3[0m, if the name is specified in the [;;4m[0m
[;;4mvalues[0m part of the [;;4mcapture[0m statement. The [;;4mall_names[0m capturing
value matches all the names in the same way.
Note:
You cannot use different names to distinguish between two
subpatterns with the same number, as PCRE uses only the
numbers when matching. For this reason, an error is given at
compile time if different names are specified to subpatterns
with the same number. However, you can specify the same name
to subpatterns with the same number, even when [;;4mdupnames[0m is
not set.
[;1mRepetition[0m
Repetition is specified by quantifiers, which can follow any of
the following items:
• A literal data character
• The dot metacharacter
• The \C escape sequence
• The \X escape sequence
• The \R escape sequence
• An escape such as \d or \pL that matches a single character
• A character class
• A back reference (see the next section)
• A parenthesized subpattern (including assertions)
• A subroutine call to a subpattern (recursive or otherwise)
The general repetition quantifier specifies a minimum and maximum
number of permitted matches, by giving the two numbers in curly
brackets (braces), separated by a comma. The numbers must be <
65536, and the first must be less than or equal to the second. For
example, the following matches "zz", "zzz", or "zzzz":
z{2,4}
A closing brace on its own is not a special character. If the
second number is omitted, but the comma is present, there is no
upper limit. If the second number and the comma are both omitted,
the quantifier specifies an exact number of required matches.
Thus, the following matches at least three successive vowels, but
can match many more:
[aeiou]{3,}
The following matches exactly eight digits:
\d{8}
An opening curly bracket that appears in a position where a
quantifier is not allowed, or one that does not match the syntax
of a quantifier, is taken as a literal character. For example,
{,6} is not a quantifier, but a literal string of four characters.
In Unicode mode, quantifiers apply to characters rather than to
individual data units. Thus, for example, \x{100}{2} matches two
characters, each of which is represented by a 2-byte sequence in a
UTF-8 string. Similarly, \X{3} matches three Unicode extended
grapheme clusters, each of which can be many data units long (and
they can be of different lengths).
The quantifier {0} is permitted, causing the expression to behave
as if the previous item and the quantifier were not present. This
can be useful for subpatterns that are referenced as subroutines
from elsewhere in the pattern (but see also section Defining
Subpatterns for Use by Reference Only). Items other than
subpatterns that have a {0} quantifier are omitted from the
compiled pattern.
For convenience, the three most common quantifiers have
single-character abbreviations:
[;;4m*[0m:
Equivalent to {0,}
[;;4m+[0m:
Equivalent to {1,}
[;;4m?[0m:
Equivalent to {0,1}
Infinite loops can be constructed 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, as there are cases where this can
be useful, such patterns are now accepted. However, if any
repetition of the subpattern matches 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 remaining pattern to fail. The classic example
of where this gives problems is in trying to match comments in C
programs. These appear between /* and */. Within the comment,
individual * and / characters can appear. An attempt to match C
comments by applying the pattern
/\*.*\*/
to the string
/* first comment */ not comment /* second comment */
fails, as 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 following pattern does the right thing with the C
comments:
/\*.*?\*/
The meaning of the various quantifiers is not otherwise changed,
only the preferred number of matches. Do not confuse this use of
question mark with its use as a quantifier in its own right. As 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 remaining pattern matches.
If option [;;4mungreedy[0m is set (an option that 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.
That is, it inverts the default behavior.
When a parenthesized subpattern is quantified with a minimum
repeat count that is > 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 option [;;4mdotall[0m
(equivalent to Perl option [;;4m/s[0m) is set, thus allowing the dot to
match newlines, the pattern is implicitly anchored, because
whatever follows is 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 if it was preceded by \A.
In cases where it is known that the subject string contains no
newlines, it is worth setting [;;4mdotall[0m to obtain this
optimization, or alternatively using ^ to indicate anchoring
explicitly.
However, there are some cases where the optimization cannot be
used. When .* is inside capturing parentheses that are the subject
of a back reference elsewhere in the pattern, a match at the start
can fail where a later one succeeds. Consider, for example:
(.*)abc\1
If the subject is "xyz123abc123", the match point is the fourth
character. Therefore, such a pattern is not implicitly anchored.
Another case where implicit anchoring is not applied is when the
leading .* is inside an atomic group. Once again, a match at the
start can fail where a later one succeeds. Consider the following
pattern:
(?>.*?a)b
It matches "ab" in the subject "aab". The use of the backtracking
control verbs (*PRUNE) and (*SKIP) also disable this optimization.
When a capturing subpattern is repeated, the value captured is the
substring that matched the final iteration. For example, after
(tweedle[dume]{3}\s*)+
has matched "tweedledum tweedledee", the value of the captured
substring is "tweedledee". However, if there are nested capturing
subpatterns, the corresponding captured values can have been set
in previous iterations. For example, after
/(a|(b))+/
matches "aba", the value of the second captured substring is "b".
[;1mAtomic Grouping and Possessive Quantifiers[0m
With both maximizing ("greedy") and minimizing ("ungreedy" or
"lazy") repetition, failure of what follows normally causes the
repeated item to be re-evaluated to see if a different number of
repeats allows the remaining pattern to match. Sometimes it is
useful to prevent this, either to change the nature of the match,
or to cause it to fail earlier than it otherwise might, when the
author of the pattern knows that there is no point in carrying on.
Consider, for example, the pattern \d+foo when applied to the
following subject line:
123456bar
After matching all six digits and then failing to match "foo", the
normal action of the matcher is to try again with only five digits
matching item \d+, and then with four, and so on, before
ultimately failing. "Atomic grouping" (a term taken from Jeffrey
Friedl's book) provides the means for specifying that once a
subpattern has matched, it is not to be re-evaluated in this way.
If atomic grouping is used for the previous example, the matcher
gives up immediately on failing to match "foo" the first time. The
notation is a kind of special parenthesis, starting with [;;4m(?>[0m as
in the following 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 to
make the remaining pattern match, [;;4m(?>\d+)[0m can only match an
entire sequence of digits.
Atomic groups in general can contain any complicated subpatterns,
and can be nested. However, when the subpattern for an atomic
group is just a single repeated item, as in the example above, a
simpler notation, called a "possessive quantifier" can be used.
This consists of an extra + character following a quantifier.
Using this notation, the previous example can be rewritten as
\d++foo
Notice that a possessive quantifier can be used with an entire
group, for example:
(abc|xyz){2,3}+
Possessive quantifiers are always greedy; the setting of option [;;4m[0m
[;;4mungreedy[0m is ignored. They are a convenient notation for the
simpler forms of an atomic group. However, there is no difference
in the meaning of a possessive quantifier and the equivalent
atomic group, but there can be a performance difference;
possessive quantifiers are probably slightly faster.
The possessive quantifier syntax is an extension to the Perl 5.8
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 the Sun Java package, and PCRE copied it from
there. It ultimately found its way into Perl at release 5.10.
PCRE has an optimization that automatically "possessifies" certain
simple pattern constructs. For example, the sequence A+B is
treated as A++B, as there is no point in backtracking into a
sequence of A:s when B must follow.
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 long time. The pattern
(\D+|<\d+>)*[!?]
matches an unlimited number of substrings that either consist of
non-digits, or digits enclosed in <>, followed by ! 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 many ways, and all must be tried. (The
example uses [!?] rather than a single character at the end, as
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 the following, sequences of
non-digits cannot be broken, and failure happens quickly:
((?>\D+)|<\d+>)*[!?]
[;1mBack References[0m
Outside a character class, a backslash followed by a digit > 0
(and possibly further digits) is a back reference to a capturing
subpattern earlier (that is, to its left) in the pattern, provided
there have been that many previous capturing left parentheses.
However, if the decimal number following the backslash is < 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. That is, the parentheses that are referenced do need not
be to the left of the reference for numbers < 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 iteration.
It is not possible to have a numerical "forward back reference" to
a subpattern whose number is 10 or more using this syntax, as a
sequence such as \50 is interpreted as a character defined in
octal. For more details of the handling of digits following a
backslash, see section Non-Printing Characters earlier. There is
no such problem when named parentheses are used. A back reference
to any subpattern is possible using named parentheses (see below).
Another way to avoid the ambiguity inherent in the use of digits
following a backslash is to use the \g escape sequence. This
escape must be followed by an unsigned number or a negative
number, optionally enclosed in braces. The following examples are
identical:
(ring), \1
(ring), \g1
(ring), \g{1}
An unsigned number specifies an absolute reference without the
ambiguity that is present in the older syntax. It is also useful
when literal digits follow the reference. A negative number is a
relative reference. Consider the following example:
(abc(def)ghi)\g{-1}
The sequence \g{-1} is a reference to the most recently started
capturing subpattern before \g, that is, it is equivalent to \2 in
this example. Similarly, \g{-2} would be equivalent to \1. The use
of relative references can be helpful in long patterns, and also
in patterns that are created by joining fragments containing
references within themselves.
A back reference matches whatever matched the capturing subpattern
in the current subject string, rather than anything matching the
subpattern itself (section Subpattern as Subroutines describes a
way of doing that). So, the following pattern matches "sense and
sensibility" and "response and responsibility", but not "sense and
responsibility":
(sens|respons)e and \1ibility
If caseful matching is in force at the time of the back reference,
the case of letters is relevant. For example, the following
matches "rah rah" and "RAH RAH", but not "RAH rah", although the
original capturing subpattern is matched caselessly:
((?i)rah)\s+\1
There are many different ways of writing back references to named
subpatterns. The .NET syntax [;;4m\k{name}[0m and the Perl syntax [;;4m[0m
[;;4m\k<name>[0m or [;;4m\k'name'[0m are supported, as is the Python syntax [;;4m[0m
[;;4m(?P=name)[0m. The unified back reference syntax in Perl 5.10, in
which \g can be used for both numeric and named references, is
also supported. The previous example can be rewritten in the
following ways:
(?<p1>(?i)rah)\s+\k<p1>
(?'p1'(?i)rah)\s+\k{p1}
(?P<p1>(?i)rah)\s+(?P=p1)
(?<p1>(?i)rah)\s+\g{p1}
A subpattern that is referenced by name can appear in the pattern
before or after the reference.
There can be more than one back reference to the same subpattern.
If a subpattern has not been used in a particular match, any back
references to it always fails. For example, the following pattern
always fails if it starts to match "a" rather than "bc":
(a|(bc))\2
As there can 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 option [;;4mextended[0m is set, this can be whitespace.
Otherwise an empty comment (see section Comments) can be used.
Recursive Back References
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 following pattern matches
any number of "a"s and also "aba", "ababbaa", and so on:
(a|b\1)+
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.
Back references of this type cause the group that they reference
to be treated as an atomic group. Once the whole group has been
matched, a subsequent matching failure cannot cause backtracking
into the middle of the group.
[;1mAssertions[0m
An assertion is a test on the characters following or preceding
the current matching point that does not consume any characters.
The simple assertions coded as \b, \B, \A, \G, \Z, \z, ^, and $
are described in the previous sections.
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. If such an
assertion contains capturing subpatterns within it, these are
counted for the purposes of numbering the capturing subpatterns in
the whole pattern. However, substring capturing is done only for
positive assertions. (Perl sometimes, but not always, performs
capturing in negative assertions.)
Warning:
If a positive assertion containing one or more capturing
subpatterns succeeds, but failure to match later in the
pattern causes backtracking over this assertion, the captures
within the assertion are reset only if no higher numbered
captures are already set. This is, unfortunately, a
fundamental limitation of the current implementation, and as
PCRE1 is now in maintenance-only status, it is unlikely ever
to change.
For compatibility with Perl, assertion subpatterns can be
repeated. However, it makes no sense to assert the same thing many
times, the side effect of capturing parentheses can occasionally
be useful. In practice, there are only three cases:
• If the quantifier is {0}, the assertion is never obeyed
during matching. However, it can contain internal capturing
parenthesized groups that are called from elsewhere through
the subroutine mechanism.
• If quantifier is {0,n}, where n > 0, it is treated as if it
was {0,1}. At runtime, the remaining pattern match is tried
with and without the assertion, the order depends on the
greediness of the quantifier.
• If the minimum repetition is > 0, the quantifier is ignored.
The assertion is obeyed only once when encountered during
matching.
Lookahead Assertions
Lookahead assertions start with (?= for positive assertions and
(?! for negative assertions. For example, the following matches a
word followed by a semicolon, but does not include the semicolon
in the match:
\w+(?=;)
The following matches any occurrence of "foo" that is not followed
by "bar":
foo(?!bar)
Notice that the apparently similar pattern
(?!foo)bar
does not find an occurrence of "bar" that is preceded by something
other than "foo". It finds any occurrence of "bar" whatsoever, as
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 convenient way to do it is with (?!), as an
empty string always matches. So, an assertion that requires there
is not to be an empty string must always fail. The backtracking
control verb (*FAIL) or (*F) is a synonym for (?!).
Lookbehind Assertions
Lookbehind assertions start with (?<= for positive assertions and
(?<! for negative assertions. For example, the following finds an
occurrence of "bar" that is not preceded by "foo":
(?<!foo)bar
The contents of a lookbehind assertion are restricted such that
all the strings it matches must have a fixed length. However, if
there are many top-level alternatives, they do not all have to
have the same fixed length. Thus, the following is permitted:
(?<=bullock|donkey)
The following causes an error at compile time:
(?<!dogs?|cats?)
Branches that match different length strings are permitted only at
the top-level of a lookbehind assertion. This is an extension
compared with Perl, which requires all branches to match the same
length of string. An assertion such as the following is not
permitted, as its single top-level branch can match two different
lengths:
(?<=ab(c|de))
However, it is acceptable to PCRE if rewritten to use two
top-level branches:
(?<=abc|abde)
Sometimes the escape sequence \K (see above) can be used instead
of a lookbehind assertion to get round the fixed-length
restriction.
The implementation of lookbehind assertions is, for each
alternative, to move the current position back temporarily by the
fixed length and then try to match. If there are insufficient
characters before the current position, the assertion fails.
In a UTF mode, PCRE does not allow the \C escape (which matches a
single data unit even in a UTF mode) to appear in lookbehind
assertions, as it makes it impossible to calculate the length of
the lookbehind. The \X and \R escapes, which can match different
numbers of data units, are not permitted either.
"Subroutine" calls (see below), such as (?2) or (?&X), are
permitted in lookbehinds, as long as the subpattern matches a
fixed-length string. Recursion, however, is not supported.
Possessive quantifiers can be used with lookbehind assertions to
specify efficient matching of fixed-length strings at the end of
subject strings. Consider the following simple pattern when
applied to a long string that does not match:
abcd$
As matching proceeds from left to right, PCRE looks for each "a"
in the subject and then sees if what follows matches the remaining
pattern. If the pattern is specified as
^.*abcd$
the initial .* matches the entire string at first. However, when
this fails (as there is no following "a"), it backtracks to match
all but the last character, then all but the last two characters,
and so on. Once again the search for "a" covers the entire string,
from right to left, so we are no better off. However, if the
pattern is written as
^.*+(?<=abcd)
there can be no backtracking for the .*+ item; it can match only
the entire string. The subsequent lookbehind assertion does a
single test on the last four characters. If it fails, the match
fails immediately. For long strings, this approach makes a
significant difference to the processing time.
Using Multiple Assertions
Many assertions (of any sort) can occur in succession. For
example, the following matches "foo" preceded by three digits that
are not "999":
(?<=\d{3})(?<!999)foo
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 does not match "123abcfoo". A pattern to do that is
the following:
(?<=\d{3}...)(?<!999)foo
This time the first assertion looks at the preceding six
characters, checks 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, the
following matches an occurrence of "baz" that is preceded by
"bar", which in turn is not preceded by "foo":
(?<=(?<!foo)bar)baz
The following pattern matches "foo" preceded by three digits and
any three characters that are not "999":
(?<=\d{3}(?!999)...)foo
[;1mConditional Subpatterns[0m
It is possible to cause the matching process to obey a subpattern
conditionally or to choose between two alternative subpatterns,
depending on the result of an assertion, or whether a specific
capturing subpattern has already been matched. The following are
the two possible forms of conditional subpattern:
(?(condition)yes-pattern)
(?(condition)yes-pattern|no-pattern)
If the condition is satisfied, the yes-pattern is used, otherwise
the no-pattern (if present). If more than two alternatives exist
in the subpattern, a compile-time error occurs. Each of the two
alternatives can itself contain nested subpatterns of any form,
including conditional subpatterns; the restriction to two
alternatives applies only at the level of the condition. The
following pattern fragment is an example where the alternatives
are complex:
(?(1) (A|B|C) | (D | (?(2)E|F) | E) )
There are four kinds of condition: references to subpatterns,
references to recursion, a pseudo-condition called DEFINE, and
assertions.
Checking for a Used Subpattern By Number
If the text between the parentheses consists of a sequence of
digits, the condition is true if a capturing subpattern of that
number has previously matched. If more than one capturing
subpattern with the same number exists (see section Duplicate
Subpattern Numbers earlier), the condition is true if any of them
have matched. An alternative notation is to precede the digits
with a plus or minus sign. In this case, the subpattern number is
relative rather than absolute. The most recently opened
parentheses can be referenced by (?(-1), the next most recent by
(?(-2), and so on. Inside loops, it can also make sense to refer
to subsequent groups. The next parentheses to be opened can be
referenced as (?(+1), and so on. (The value zero in any of these
forms is not used; it provokes a compile-time error.)
Consider the following pattern, which contains non-significant
whitespace to make it more readable (assume option [;;4mextended[0m) and
to divide it into three parts for ease of discussion:
( \( )? [^()]+ (?(1) \) )
The first part matches an optional opening parenthesis, and if
that character is present, sets it as the first captured
substring. The second part matches one or more characters that are
not parentheses. The third part is a conditional subpattern that
tests whether the first set of parentheses matched or not. If they
did, that is, if subject started with an opening parenthesis, the
condition is true, and so the yes-pattern is executed and a
closing parenthesis is required. Otherwise, as no-pattern is not
present, the subpattern matches nothing. That is, this pattern
matches a sequence of non-parentheses, optionally enclosed in
parentheses.
If this pattern is embedded in a larger one, a relative reference
can be used:
...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
This makes the fragment independent of the parentheses in the
larger pattern.
Checking for a Used Subpattern By Name
Perl uses the syntax (?(<name>)...) or (?('name')...) to test for
a used subpattern by name. For compatibility with earlier versions
of PCRE, which had this facility before Perl, the syntax
(?(name)...) is also recognized.
Rewriting the previous example to use a named subpattern gives:
(?<OPEN> \( )? [^()]+ (?(<OPEN>) \) )
If the name used in a condition of this kind is a duplicate, the
test is applied to all subpatterns of the same name, and is true
if any one of them has matched.
Checking for Pattern Recursion
If the condition is the string (R), and there is no subpattern
with the name R, the condition is true if a recursive call to the
whole pattern or any subpattern has been made. If digits or a name
preceded by ampersand follow the letter R, for example:
(?(R3)...) or (?(R&name)...)
the condition is true if the most recent recursion is into a
subpattern whose number or name is given. This condition does not
check the entire recursion stack. If the name used in a condition
of this kind is a duplicate, the test is applied to all
subpatterns of the same name, and is true if any one of them is
the most recent recursion.
At "top-level", all these recursion test conditions are false. The
syntax for recursive patterns is described below.
Defining Subpatterns for Use By Reference Only
If the condition is the string (DEFINE), and there is no
subpattern with the name DEFINE, the condition is always false. In
this case, there can be only one alternative in the subpattern. It
is always skipped if control reaches this point in the pattern.
The idea of DEFINE is that it can be used to define "subroutines"
that can be referenced from elsewhere. (The use of subroutines is
described below.) For example, a pattern to match an IPv4 address,
such as "192.168.23.245", can be written like this (ignore
whitespace and line breaks):
(?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) ) \b (?&byte) (\.(?&byte)){3} \b
The first part of the pattern is a DEFINE group inside which is a
another group named "byte" is defined. This matches an individual
component of an IPv4 address (a number < 256). When matching takes
place, this part of the pattern is skipped, as DEFINE acts like a
false condition. The remaining pattern uses references to the
named group to match the four dot-separated components of an IPv4
address, insisting on a word boundary at each end.
Assertion Conditions
If the condition is not in any of the above formats, it must be an
assertion. This can be a positive or negative lookahead or
lookbehind assertion. Consider the following pattern, containing
non-significant whitespace, 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. That is, 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.
[;1mComments[0m
There are two ways to include comments in patterns that are
processed by PCRE. In both cases, the start of the comment must
not be in a character class, or in the middle of any other
sequence of related characters such as (?: or a subpattern name or
number. The characters that make up a comment play no part in the
pattern matching.
The sequence (?# marks the start of a comment that continues up to
the next closing parenthesis. Nested parentheses are not
permitted. If option PCRE_EXTENDED is set, an unescaped #
character also introduces a comment, which in this case continues
to immediately after the next newline character or character
sequence in the pattern. Which characters are interpreted as
newlines is controlled by the options passed to a compiling
function or by a special sequence at the start of the pattern, as
described in section Newline Conventions earlier.
Notice that the end of this type of comment is a literal newline
sequence in the pattern; escape sequences that happen to represent
a newline do not count. For example, consider the following
pattern when [;;4mextended[0m is set, and the default newline convention
is in force:
abc #comment \n still comment
On encountering character #, [;;4mpcre_compile()[0m skips along, looking
for a newline in the pattern. The sequence \n is still literal at
this stage, so it does not terminate the comment. Only a character
with code value 0x0a (the default newline) does so.
[;1mRecursive Patterns[0m
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.
For some time, Perl has provided a facility that allows regular
expressions to recurse (among other things). It does this by
interpolating Perl code in the expression at runtime, and the code
can refer to the expression itself. A Perl pattern using code
interpolation to solve the parentheses problem can be created like
this:
$re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;
Item (?p{...}) interpolates Perl code at runtime, 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 special syntax for recursion of the entire
pattern, and for individual subpattern recursion. After its
introduction in PCRE and Python, this kind of recursion was later
introduced into Perl at release 5.10.
A special item that consists of (? followed by a number > 0 and a
closing parenthesis is a recursive subroutine call of the
subpattern of the given number, if it occurs inside that
subpattern. (If not, it is a non-recursive subroutine call, which
is described in the next section.) The special item (?R) or (?0)
is a recursive call of the entire regular expression.
This PCRE pattern solves the nested parentheses problem (assume
that option [;;4mextended[0m is set so that whitespace is ignored):
\( ( [^()]++ | (?R) )* \)
First it matches an opening parenthesis. Then it matches any
number of substrings, which can either be a sequence of
non-parentheses or a recursive match of the pattern itself (that
is, a correctly parenthesized substring). Finally there is a
closing parenthesis. Notice the use of a possessive quantifier to
avoid backtracking into sequences of non-parentheses.
If this was part of a larger pattern, you would not want to
recurse the entire pattern, so instead you can use:
( \( ( [^()]++ | (?1) )* \) )
The pattern is here within parentheses so that the recursion
refers to them instead of the whole pattern.
In a larger pattern, keeping track of parenthesis numbers can be
tricky. This is made easier by the use of relative references.
Instead of (?1) in the pattern above, you can write (?-2) to refer
to the second most recently opened parentheses preceding the
recursion. That is, a negative number counts capturing parentheses
leftwards from the point at which it is encountered.
It is also possible to refer to later opened parentheses, by
writing references such as (?+2). However, these cannot be
recursive, as the reference is not inside the parentheses that are
referenced. They are always non-recursive subroutine calls, as
described in the next section.
An alternative approach is to use named parentheses instead. The
Perl syntax for this is (?&name). The earlier PCRE syntax
(?P>name) is also supported. We can rewrite the above example as
follows:
(?<pn> \( ( [^()]++ | (?&pn) )* \) )
If there is more than one subpattern with the same name, the
earliest one is used.
This particular example pattern that we have studied contains
nested unlimited repeats, and so the use of a possessive
quantifier for matching strings of non-parentheses is important
when applying the pattern to strings that do not match. For
example, when this pattern is applied to
(aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
it gives "no match" quickly. However, if a possessive quantifier
is not used, the match runs for a long time, as there are so many
different ways the + and * repeats can carve up the subject, and
all must be tested before failure can be reported.
At the end of a match, the values of capturing parentheses are
those from the outermost level. If the pattern above is matched
against
(ab(cd)ef)
the value for the inner capturing parentheses (numbered 2) is
"ef", which is the last value taken on at the top-level. If a
capturing subpattern is not matched at the top level, its final
captured value is unset, even if it was (temporarily) set at a
deeper level during the matching process.
Do not confuse item (?R) with condition (R), which tests for
recursion. Consider the following pattern, which matches text in
angle brackets, allowing for arbitrary nesting. Only digits are
allowed in nested brackets (that is, when recursing), while any
characters are permitted at the outer level.
< (?: (?(R) \d++ | [^<>]*+) | (?R)) * >
Here (?(R) is the start of a conditional subpattern, with two
different alternatives for the recursive and non-recursive cases.
Item (?R) is the actual recursive call.
Differences in Recursion Processing between PCRE and Perl
Recursion processing in PCRE differs from Perl in two important
ways. In PCRE (like Python, but unlike Perl), 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 subsequent matching failure. This can be illustrated by the
following pattern, which means to match a palindromic string
containing an odd number of characters (for example, "a", "aba",
"abcba", "abcdcba"):
^(.|(.)(?1)\2)$
The idea is that it either matches a single character, or two
identical characters surrounding a subpalindrome. In Perl, this
pattern works; in PCRE it does not work if the pattern is longer
than three characters. Consider the subject string "abcba".
At the top level, the first character is matched, but as it is not
at the end of the string, the first alternative fails, the second
alternative is taken, and the recursion kicks in. The recursive
call to subpattern 1 successfully matches the next character
("b"). (Notice that the beginning and end of line tests are not
part of the recursion.)
Back at the top level, the next character ("c") is compared with
what subpattern 2 matched, which was "a". This fails. As the
recursion is treated as an atomic group, there are now no
backtracking points, and so the entire match fails. (Perl can now
re-enter the recursion and try the second alternative.) However,
if the pattern is written with the alternatives in the other
order, things are different:
^((.)(?1)\2|.)$
This time, the recursing alternative is tried first, and continues
to recurse until it runs out of characters, at which point the
recursion fails. But this time we have another alternative to try
at the higher level. That is the significant difference: in the
previous case the remaining alternative is at a deeper recursion
level, which PCRE cannot use.
To change the pattern so that it matches all palindromic strings,
not only those with an odd number of characters, it is tempting to
change the pattern to this:
^((.)(?1)\2|.?)$
Again, this works in Perl, but not in PCRE, and for the same
reason. When a deeper recursion has matched a single character, it
cannot be entered again to match an empty string. The solution is
to separate the two cases, and write out the odd and even cases as
alternatives at the higher level:
^(?:((.)(?1)\2|)|((.)(?3)\4|.))
If you want to match typical palindromic phrases, the pattern must
ignore all non-word characters, which can be done as follows:
^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$
If run with option [;;4mcaseless[0m, this pattern matches phrases such
as "A man, a plan, a canal: Panama!" and it works well in both
PCRE and Perl. Notice the use of the possessive quantifier *+ to
avoid backtracking into sequences of non-word characters. Without
this, PCRE takes much longer (10 times or more) to match typical
phrases, and Perl takes so long that you think it has gone into a
loop.
Note:
The palindrome-matching patterns above work only if the
subject string does not start with a palindrome that is
shorter than the entire string. For example, although "abcba"
is correctly matched, if the subject is "ababa", PCRE finds
palindrome "aba" at the start, and then fails at top level, as
the end of the string does not follow. Once again, it cannot
jump back into the recursion to try other alternatives, so the
entire match fails.
The second way in which PCRE and Perl differ in their recursion
processing is in the handling of captured values. In Perl, when a
subpattern is called recursively or as a subpattern (see the next
section), it has no access to any values that were captured
outside the recursion. In PCRE these values can be referenced.
Consider the following pattern:
^(.)(\1|a(?2))
In PCRE, it matches "bab". The first capturing parentheses match
"b", then in the second group, when the back reference \1 fails to
match "b", the second alternative matches "a", and then recurses.
In the recursion, \1 does now match "b" and so the whole match
succeeds. In Perl, the pattern fails to match because inside the
recursive call \1 cannot access the externally set value.
[;1mSubpatterns as Subroutines[0m
If the syntax for a recursive subpattern call (either by number or
by name) is used outside the parentheses to which it refers, it
operates like a subroutine in a programming language. The called
subpattern can be defined before or after the reference. A
numbered reference can be absolute or relative, as in the
following examples:
(...(absolute)...)...(?2)...
(...(relative)...)...(?-1)...
(...(?+1)...(relative)...
An earlier example pointed out that the following pattern matches
"sense and sensibility" and "response and responsibility", but not
"sense and responsibility":
(sens|respons)e and \1ibility
If instead the following pattern is used, it matches "sense and
responsibility" and the other two strings:
(sens|respons)e and (?1)ibility
Another example is provided in the discussion of DEFINE earlier.
All subroutine calls, recursive or not, are always treated as
atomic groups. That is, once a subroutine 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.
Any capturing parentheses that are set during the subroutine call
revert to their previous values afterwards.
Processing options such as case-independence are fixed when a
subpattern is defined, so if it is used as a subroutine, such
options cannot be changed for different calls. For example, the
following pattern matches "abcabc" but not "abcABC", as the change
of processing option does not affect the called subpattern:
(abc)(?i:(?-1))
[;1mOniguruma Subroutine Syntax[0m
For compatibility with Oniguruma, the non-Perl syntax \g followed
by a name or a number enclosed either in angle brackets or single
quotes, is alternative syntax for referencing a subpattern as a
subroutine, possibly recursively. Here follows two of the examples
used above, rewritten using this syntax:
(?<pn> \( ( (?>[^()]+) | \g<pn> )* \) )
(sens|respons)e and \g'1'ibility
PCRE supports an extension to Oniguruma: if a number is preceded
by a plus or minus sign, it is taken as a relative reference, for
example:
(abc)(?i:\g<-1>)
Notice that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax)
are not synonymous. The former is a back reference; the latter
is a subroutine call.
[;1mBacktracking Control[0m
Perl 5.10 introduced some "Special Backtracking Control Verbs",
which are still described in the Perl documentation as
"experimental and subject to change or removal in a future version
of Perl". It goes on to say: "Their usage in production code
should be noted to avoid problems during upgrades." The same
remarks apply to the PCRE features described in this section.
The new verbs make use of what was previously invalid syntax: an
opening parenthesis followed by an asterisk. They are generally of
the form (*VERB) or (*VERB:NAME). Some can take either form,
possibly behaving differently depending on whether a name is
present. A name is any sequence of characters that does not
include a closing parenthesis. The maximum name length is 255 in
the 8-bit library and 65535 in the 16-bit and 32-bit libraries. If
the name is empty, that is, if the closing parenthesis immediately
follows the colon, the effect is as if the colon was not there.
Any number of these verbs can occur in a pattern.
The behavior of these verbs in repeated groups, assertions, and in
subpatterns called as subroutines (whether or not recursively) is
described below.
Optimizations That Affect Backtracking Verbs
PCRE contains some optimizations that are used to speed up
matching by running some checks at the start of each match
attempt. For example, it can know the minimum length of matching
subject, or that a particular character must be present. When one
of these optimizations bypasses the running of a match, any
included backtracking verbs are not processed. processed. You can
suppress the start-of-match optimizations by setting option [;;4m[0m
[;;4mno_start_optimize[0m when calling [;;4mcompile/2[0m or [;;4mrun/3[0m, or by
starting the pattern with (*NO_START_OPT).
Experiments with Perl suggest that it too has similar
optimizations, sometimes leading to anomalous results.
Verbs That Act Immediately
The following verbs act as soon as they are encountered. They must
not be followed by a name.
(*ACCEPT)
This verb causes the match to end successfully, skipping the
remainder of the pattern. However, when it is inside a subpattern
that is called as a subroutine, only that subpattern is ended
successfully. Matching then continues at the outer level. If
(*ACCEPT) is triggered in a positive assertion, the assertion
succeeds; in a negative assertion, the assertion fails.
If (*ACCEPT) is inside capturing parentheses, the data so far is
captured. For example, the following matches "AB", "AAD", or
"ACD". When it matches "AB", "B" is captured by the outer
parentheses.
A((?:A|B(*ACCEPT)|C)D)
The following verb causes a matching failure, forcing backtracking
to occur. It is equivalent to (?!) but easier to read.
(*FAIL) or (*F)
The Perl documentation states that it is probably useful only when
combined with (?{}) or (??{}). Those are Perl features that are
not present in PCRE.
A match with the string "aaaa" always fails, but the callout is
taken before each backtrack occurs (in this example, 10 times).
Recording Which Path Was Taken
The main purpose of this verb is to track how a match was arrived
at, although it also has a secondary use in with advancing the
match starting point (see (*SKIP) below).
Note:
In Erlang, there is no interface to retrieve a mark with [;;4m[0m
[;;4mrun/2,3[0m, so only the secondary purpose is relevant to the
Erlang programmer.
The rest of this section is therefore deliberately not adapted
for reading by the Erlang programmer, but the examples can
help in understanding NAMES as they can be used by (*SKIP).
(*MARK:NAME) or (*:NAME)
A name is always required with this verb. There can be as many
instances of (*MARK) as you like in a pattern, and their names do
not have to be unique.
When a match succeeds, the name of the last encountered
(*MARK:NAME), (*PRUNE:NAME), or (*THEN:NAME) on the matching path
is passed back to the caller as described in section "Extra data
for [;;4mpcre_exec()[0m" in the [;;4mpcreapi[0m documentation. In the
following example of [;;4mpcretest[0m output, the /K modifier requests
the retrieval and outputting of (*MARK) data:
re> /X(*MARK:A)Y|X(*MARK:B)Z/K
data> XY
0: XY
MK: A
XZ
0: XZ
MK: B
The (*MARK) name is tagged with "MK:" in this output, and in this
example it indicates which of the two alternatives matched. This
is a more efficient way of obtaining this information than putting
each alternative in its own capturing parentheses.
If a verb with a name is encountered in a positive assertion that
is true, the name is recorded and passed back if it is the last
encountered. This does not occur for negative assertions or
failing positive assertions.
After a partial match or a failed match, the last encountered name
in the entire match process is returned, for example:
re> /X(*MARK:A)Y|X(*MARK:B)Z/K
data> XP
No match, mark = B
Notice that in this unanchored example, the mark is retained from
the match attempt that started at letter "X" in the subject.
Subsequent match attempts starting at "P" and then with an empty
string do not get as far as the (*MARK) item, nevertheless do not
reset it.
Verbs That Act after Backtracking
The following verbs do nothing when they are encountered. Matching
continues with what follows, but if there is no subsequent match,
causing a backtrack to the verb, a failure is forced. That is,
backtracking cannot pass to the left of the verb. However, when
one of these verbs appears inside an atomic group or an assertion
that is true, its effect is confined to that group, as once the
group has been matched, there is never any backtracking into it.
In this situation, backtracking can "jump back" to the left of the
entire atomic group or assertion. (Remember also, as stated above,
that this localization also applies in subroutine calls.)
These verbs differ in exactly what kind of failure occurs when
backtracking reaches them. The behavior described below is what
occurs when the verb is not in a subroutine or an assertion.
Subsequent sections cover these special cases.
The following verb, which must not be followed by a name, causes
the whole match to fail outright if there is a later matching
failure that causes backtracking to reach it. Even if the pattern
is unanchored, no further attempts to find a match by advancing
the starting point take place.
(*COMMIT)
If (*COMMIT) is the only backtracking verb that is encountered,
once it has been passed, [;;4mrun/2,3[0m is committed to find a match at
the current starting point, or not at all, for example:
a+(*COMMIT)b
This matches "xxaab" but not "aacaab". It can be thought of as a
kind of dynamic anchor, or "I've started, so I must finish". The
name of the most recently passed (*MARK) in the path is passed
back when (*COMMIT) forces a match failure.
If more than one backtracking verb exists in a pattern, a
different one that follows (*COMMIT) can be triggered first, so
merely passing (*COMMIT) during a match does not always guarantee
that a match must be at this starting point.
Notice that (*COMMIT) at the start of a pattern is not the same as
an anchor, unless the PCRE start-of-match optimizations are turned
off, as shown in the following example:
1> re:run("xyzabc","(*COMMIT)abc",[{capture,all,list}]).
{match,["abc"]}
2> re:run("xyzabc","(*COMMIT)abc",[{capture,all,list},no_start_optimize]).
nomatch
For this pattern, PCRE knows that any match must start with "a",
so the optimization skips along the subject to "a" before applying
the pattern to the first set of data. The match attempt then
succeeds. In the second call the [;;4mno_start_optimize[0m disables the
optimization that skips along to the first character. The pattern
is now applied starting at "x", and so the (*COMMIT) causes the
match to fail without trying any other starting points.
The following verb causes the match to fail at the current
starting position in the subject if there is a later matching
failure that causes backtracking to reach it:
(*PRUNE) or (*PRUNE:NAME)
If the pattern is unanchored, the normal "bumpalong" advance to
the next starting character then occurs. Backtracking can occur as
usual to the left of (*PRUNE), before it is reached, or when
matching to the right of (*PRUNE), but if there is no match to the
right, backtracking cannot cross (*PRUNE). In simple cases, the
use of (*PRUNE) is just an alternative to an atomic group or
possessive quantifier, but there are some uses of (*PRUNE) that
cannot be expressed in any other way. In an anchored pattern,
(*PRUNE) has the same effect as (*COMMIT).
The behavior of (*PRUNE:NAME) is the not the same as
(*MARK:NAME)(*PRUNE). It is like (*MARK:NAME) in that the name is
remembered for passing back to the caller. However, (*SKIP:NAME)
searches only for names set with (*MARK).
Note:
The fact that (*PRUNE:NAME) remembers the name is useless to
the Erlang programmer, as names cannot be retrieved.
The following verb, when specified without a name, is like
(*PRUNE), except that if the pattern is unanchored, the
"bumpalong" advance is not to the next character, but to the
position in the subject where (*SKIP) was encountered.
(*SKIP)
(*SKIP) signifies that whatever text was matched leading up to it
cannot be part of a successful match. Consider:
a+(*SKIP)b
If the subject is "aaaac...", after the first match attempt fails
(starting at the first character in the string), the starting
point skips on to start the next attempt at "c". Notice that a
possessive quantifier does not have the same effect as this
example; although it would suppress backtracking during the first
match attempt, the second attempt would start at the second
character instead of skipping on to "c".
When (*SKIP) has an associated name, its behavior is modified:
(*SKIP:NAME)
When this is triggered, the previous path through the pattern is
searched for the most recent (*MARK) that has the same name. If
one is found, the "bumpalong" advance is to the subject position
that corresponds to that (*MARK) instead of to where (*SKIP) was
encountered. If no (*MARK) with a matching name is found, (*SKIP)
is ignored.
Notice that (*SKIP:NAME) searches only for names set by
(*MARK:NAME). It ignores names that are set by (*PRUNE:NAME) or
(*THEN:NAME).
The following verb causes a skip to the next innermost alternative
when backtracking reaches it. That is, it cancels any further
backtracking within the current alternative.
(*THEN) or (*THEN:NAME)
The verb name comes from the observation that it can be used for a
pattern-based if-then-else block:
( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
If the COND1 pattern matches, FOO is tried (and possibly further
items after the end of the group if FOO succeeds). On failure, the
matcher skips to the second alternative and tries COND2, without
backtracking into COND1. If that succeeds and BAR fails, COND3 is
tried. If BAZ then fails, there are no more alternatives, so there
is a backtrack to whatever came before the entire group. If
(*THEN) is not inside an alternation, it acts like (*PRUNE).
The behavior of (*THEN:NAME) is the not the same as
(*MARK:NAME)(*THEN). It is like (*MARK:NAME) in that the name is
remembered for passing back to the caller. However, (*SKIP:NAME)
searches only for names set with (*MARK).
Note:
The fact that (*THEN:NAME) remembers the name is useless to
the Erlang programmer, as names cannot be retrieved.
A subpattern that does not contain a | character is just a part of
the enclosing alternative; it is not a nested alternation with
only one alternative. The effect of (*THEN) extends beyond such a
subpattern to the enclosing alternative. Consider the following
pattern, where A, B, and so on, are complex pattern fragments that
do not contain any | characters at this level:
A (B(*THEN)C) | D
If A and B are matched, but there is a failure in C, matching does
not backtrack into A; instead it moves to the next alternative,
that is, D. However, if the subpattern containing (*THEN) is given
an alternative, it behaves differently:
A (B(*THEN)C | (*FAIL)) | D
The effect of (*THEN) is now confined to the inner subpattern.
After a failure in C, matching moves to (*FAIL), which causes the
whole subpattern to fail, as there are no more alternatives to
try. In this case, matching does now backtrack into A.
Notice that a conditional subpattern is not considered as having
two alternatives, as only one is ever used. That is, the |
character in a conditional subpattern has a different meaning.
Ignoring whitespace, consider:
^.*? (?(?=a) a | b(*THEN)c )
If the subject is "ba", this pattern does not match. As .*? is
ungreedy, it initially matches zero characters. The condition
(?=a) then fails, the character "b" is matched, but "c" is not. At
this point, matching does not backtrack to .*? as can perhaps be
expected from the presence of the | character. The conditional
subpattern is part of the single alternative that comprises the
whole pattern, and so the match fails. (If there was a backtrack
into .*?, allowing it to match "b", the match would succeed.)
The verbs described above provide four different "strengths" of
control when subsequent matching fails:
• (*THEN) is the weakest, carrying on the match at the next
alternative.
• (*PRUNE) comes next, fails the match at the current starting
position, but allows an advance to the next character (for
an unanchored pattern).
• (*SKIP) is similar, except that the advance can be more than
one character.
• (*COMMIT) is the strongest, causing the entire match to
fail.
More than One Backtracking Verb
If more than one backtracking verb is present in a pattern, the
one that is backtracked onto first acts. For example, consider the
following pattern, where A, B, and so on, are complex pattern
fragments:
(A(*COMMIT)B(*THEN)C|ABD)
If A matches but B fails, the backtrack to (*COMMIT) causes the
entire match to fail. However, if A and B match, but C fails, the
backtrack to (*THEN) causes the next alternative (ABD) to be
tried. This behavior is consistent, but is not always the same as
in Perl. It means that if two or more backtracking verbs appear in
succession, the last of them has no effect. Consider the following
example:
...(*COMMIT)(*PRUNE)...
If there is a matching failure to the right, backtracking onto
(*PRUNE) causes it to be triggered, and its action is taken. There
can never be a backtrack onto (*COMMIT).
Backtracking Verbs in Repeated Groups
PCRE differs from Perl in its handling of backtracking verbs in
repeated groups. For example, consider:
/(a(*COMMIT)b)+ac/
If the subject is "abac", Perl matches, but PCRE fails because the
(*COMMIT) in the second repeat of the group acts.
Backtracking Verbs in Assertions
(*FAIL) in an assertion has its normal effect: it forces an
immediate backtrack.
(*ACCEPT) in a positive assertion causes the assertion to succeed
without any further processing. In a negative assertion, (*ACCEPT)
causes the assertion to fail without any further processing.
The other backtracking verbs are not treated specially if they
appear in a positive assertion. In particular, (*THEN) skips to
the next alternative in the innermost enclosing group that has
alternations, regardless if this is within the assertion.
Negative assertions are, however, different, to ensure that
changing a positive assertion into a negative assertion changes
its result. Backtracking into (*COMMIT), (*SKIP), or (*PRUNE)
causes a negative assertion to be true, without considering any
further alternative branches in the assertion. Backtracking into
(*THEN) causes it to skip to the next enclosing alternative within
the assertion (the normal behavior), but if the assertion does not
have such an alternative, (*THEN) behaves like (*PRUNE).
Backtracking Verbs in Subroutines
These behaviors occur regardless if the subpattern is called
recursively. The treatment of subroutines in Perl is different in
some cases.
• (*FAIL) in a subpattern called as a subroutine has its
normal effect: it forces an immediate backtrack.
• (*ACCEPT) in a subpattern called as a subroutine causes the
subroutine match to succeed without any further processing.
Matching then continues after the subroutine call.
• (*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a
subroutine cause the subroutine match to fail.
• (*THEN) skips to the next alternative in the innermost
enclosing group within the subpattern that has alternatives.
If there is no such group within the subpattern, (*THEN)
causes the subroutine match to fail.
|