NAME¶
perlre - Perl regular expressions
DESCRIPTION¶
This page describes the syntax of regular expressions in Perl.
If you haven't used regular expressions before, a quick-start introduction is
available in perlrequick, and a longer tutorial introduction is available in
perlretut.
For reference on how regular expressions are used in matching operations, plus
various examples of the same, see discussions of "m//",
"s///", "qr//" and "??" in "Regexp
Quote-Like Operators" in perlop.
Modifiers¶
Matching operations can have various modifiers. Modifiers that relate to the
interpretation of the regular expression inside are listed below. Modifiers
that alter the way a regular expression is used by Perl are detailed in
"Regexp Quote-Like Operators" in perlop and "Gory details of
parsing quoted constructs" in perlop.
- m
- Treat string as multiple lines. That is, change "^" and
"$" from matching the start of the string's first line and the
end of its last line to matching the start and end of each line within the
string.
- s
- Treat string as single line. That is, change "." to match any
character whatsoever, even a newline, which normally it would not match.
Used together, as "/ms", they let the "." match any
character whatsoever, while still allowing "^" and "$"
to match, respectively, just after and just before newlines within the
string.
- i
- Do case-insensitive pattern matching.
If locale matching rules are in effect, the case map is taken from the
current locale for code points less than 255, and from Unicode rules for
larger code points. However, matches that would cross the Unicode
rules/non-Unicode rules boundary (ords 255/256) will not succeed. See
perllocale.
There are a number of Unicode characters that match multiple characters
under "/i". For example, "LATIN SMALL LIGATURE FI"
should match the sequence "fi". Perl is not currently able to do
this when the multiple characters are in the pattern and are split between
groupings, or when one or more are quantified. Thus
"\N{LATIN SMALL LIGATURE FI}" =~ /fi/i; # Matches
"\N{LATIN SMALL LIGATURE FI}" =~ /[fi][fi]/i; # Doesn't match!
"\N{LATIN SMALL LIGATURE FI}" =~ /fi*/i; # Doesn't match!
# The below doesn't match, and it isn't clear what $1 and $2 would
# be even if it did!!
"\N{LATIN SMALL LIGATURE FI}" =~ /(f)(i)/i; # Doesn't match!
Perl doesn't match multiple characters in a bracketed character class unless
the character that maps to them is explicitly mentioned, and it doesn't
match them at all if the character class is inverted, which otherwise
could be highly confusing. See "Bracketed Character Classes" in
perlrecharclass, and "Negation" in perlrecharclass.
- x
- Extend your pattern's legibility by permitting whitespace and comments.
Details in "/x"
- p
- Preserve the string matched such that ${^PREMATCH}, ${^MATCH}, and
${^POSTMATCH} are available for use after matching.
In Perl 5.20 and higher this is ignored. Due to a new copy-on-write
mechanism, ${^PREMATCH}, ${^MATCH}, and ${^POSTMATCH} will be available
after the match regardless of the modifier.
- a, d, l and u
- These modifiers, all new in 5.14, affect which character-set rules
(Unicode, etc.) are used, as described below in "Character set
modifiers".
- Other Modifiers
- There are a number of flags that can be found at the end of regular
expression constructs that are not generic regular expression
flags, but apply to the operation being performed, like matching or
substitution ("m//" or "s///" respectively).
Flags described further in "Using regular expressions in Perl" in
perlretut are:
c - keep the current position during repeated matching
g - globally match the pattern repeatedly in the string
Substitution-specific modifiers described in
"s/PATTERN/REPLACEMENT/msixpodualgcer" in perlop are:
e - evaluate the right-hand side as an expression
ee - evaluate the right side as a string then eval the result
o - pretend to optimize your code, but actually introduce bugs
r - perform non-destructive substitution and return the new value
Regular expression modifiers are usually written in documentation as e.g.,
"the "/x" modifier", even though the delimiter in question
might not really be a slash. The modifiers "/imsxadlup" may also be
embedded within the regular expression itself using the "(?...)"
construct, see "Extended Patterns" below.
/x
"/x" tells the regular expression parser to ignore most whitespace
that is neither backslashed nor within a bracketed character class. You can
use this to break up your regular expression into (slightly) more readable
parts. Also, the "#" character is treated as a metacharacter
introducing a comment that runs up to the pattern's closing delimiter, or to
the end of the current line if the pattern extends onto the next line. Hence,
this is very much like an ordinary Perl code comment. (You can include the
closing delimiter within the comment only if you precede it with a backslash,
so be careful!)
Use of "/x" means that if you want real whitespace or "#"
characters in the pattern (outside a bracketed character class, which is
unaffected by "/x"), then you'll either have to escape them (using
backslashes or "\Q...\E") or encode them using octal, hex, or
"\N{}" escapes. It is ineffective to try to continue a comment onto
the next line by escaping the "\n" with a backslash or
"\Q".
You can use "(?#text)" to create a comment that ends earlier than the
end of the current line, but "text" also can't contain the closing
delimiter unless escaped with a backslash.
Taken together, these features go a long way towards making Perl's regular
expressions more readable. Here's an example:
# Delete (most) C comments.
$program =~ s {
/\* # Match the opening delimiter.
.*? # Match a minimal number of characters.
\*/ # Match the closing delimiter.
} []gsx;
Note that anything inside a "\Q...\E" stays unaffected by
"/x". And note that "/x" doesn't affect space
interpretation within a single multi-character construct. For example in
"\x{...}", regardless of the "/x" modifier, there can be
no spaces. Same for a quantifier such as "{3}" or "{5,}".
Similarly, "(?:...)" can't have a space between the "(",
"?", and ":". Within any delimiters for such a construct,
allowed spaces are not affected by "/x", and depend on the
construct. For example, "\x{...}" can't have spaces because
hexadecimal numbers don't have spaces in them. But, Unicode properties can
have spaces, so in "\p{...}" there can be spaces that follow the
Unicode rules, for which see "Properties accessible through \p{} and
\P{}" in perluniprops.
Character set modifiers
"/d", "/u", "/a", and "/l", available
starting in 5.14, are called the character set modifiers; they affect the
character set rules used for the regular expression.
The "/d", "/u", and "/l" modifiers are not likely
to be of much use to you, and so you need not worry about them very much. They
exist for Perl's internal use, so that complex regular expression data
structures can be automatically serialized and later exactly reconstituted,
including all their nuances. But, since Perl can't keep a secret, and there
may be rare instances where they are useful, they are documented here.
The "/a" modifier, on the other hand, may be useful. Its purpose is to
allow code that is to work mostly on ASCII data to not have to concern itself
with Unicode.
Briefly, "/l" sets the character set to that of whatever
Locale
is in effect at the time of the execution of the pattern match.
"/u" sets the character set to
Unicode.
"/a" also sets the character set to Unicode, BUT adds several
restrictions for
ASCII-safe matching.
"/d" is the old, problematic, pre-5.14
Default character set
behavior. Its only use is to force that old behavior.
At any given time, exactly one of these modifiers is in effect. Their existence
allows Perl to keep the originally compiled behavior of a regular expression,
regardless of what rules are in effect when it is actually executed. And if it
is interpolated into a larger regex, the original's rules continue to apply to
it, and only it.
The "/l" and "/u" modifiers are automatically selected for
regular expressions compiled within the scope of various pragmas, and we
recommend that in general, you use those pragmas instead of specifying these
modifiers explicitly. For one thing, the modifiers affect only pattern
matching, and do not extend to even any replacement done, whereas using the
pragmas give consistent results for all appropriate operations within their
scopes. For example,
s/foo/\Ubar/il
will match "foo" using the locale's rules for case-insensitive
matching, but the "/l" does not affect how the "\U"
operates. Most likely you want both of them to use locale rules. To do this,
instead compile the regular expression within the scope of "use
locale". This both implicitly adds the "/l" and applies locale
rules to the "\U". The lesson is to "use locale" and not
"/l" explicitly.
Similarly, it would be better to use "use feature 'unicode_strings'"
instead of,
s/foo/\Lbar/iu
to get Unicode rules, as the "\L" in the former (but not necessarily
the latter) would also use Unicode rules.
More detail on each of the modifiers follows. Most likely you don't need to know
this detail for "/l", "/u", and "/d", and can
skip ahead to /a.
/l
means to use the current locale's rules (see perllocale) when pattern matching.
For example, "\w" will match the "word" characters of that
locale, and "/i" case-insensitive matching will match according to
the locale's case folding rules. The locale used will be the one in effect at
the time of execution of the pattern match. This may not be the same as the
compilation-time locale, and can differ from one match to another if there is
an intervening call of the
setlocale() function.
The only non-single-byte locale Perl supports is (starting in v5.20) UTF-8. This
means that code points above 255 are treated as Unicode no matter what locale
is in effect (since UTF-8 implies Unicode).
Under Unicode rules, there are a few case-insensitive matches that cross the
255/256 boundary. Except for UTF-8 locales in Perls v5.20 and later, these are
disallowed under "/l". For example, 0xFF (on ASCII platforms) does
not caselessly match the character at 0x178, "LATIN CAPITAL LETTER Y WITH
DIAERESIS", because 0xFF may not be "LATIN SMALL LETTER Y WITH
DIAERESIS" in the current locale, and Perl has no way of knowing if that
character even exists in the locale, much less what code point it is.
In a UTF-8 locale in v5.20 and later, the only visible difference between locale
and non-locale in regular expressions should be tainting (see perlsec).
This modifier may be specified to be the default by "use locale", but
see "Which character set modifier is in effect?".
/u
means to use Unicode rules when pattern matching. On ASCII platforms, this means
that the code points between 128 and 255 take on their Latin-1 (ISO-8859-1)
meanings (which are the same as Unicode's). (Otherwise Perl considers their
meanings to be undefined.) Thus, under this modifier, the ASCII platform
effectively becomes a Unicode platform; and hence, for example, "\w"
will match any of the more than 100_000 word characters in Unicode.
Unlike most locales, which are specific to a language and country pair, Unicode
classifies all the characters that are letters
somewhere in the world
as "\w". For example, your locale might not think that "LATIN
SMALL LETTER ETH" is a letter (unless you happen to speak Icelandic), but
Unicode does. Similarly, all the characters that are decimal digits somewhere
in the world will match "\d"; this is hundreds, not 10, possible
matches. And some of those digits look like some of the 10 ASCII digits, but
mean a different number, so a human could easily think a number is a different
quantity than it really is. For example, "BENGALI DIGIT FOUR"
(U+09EA) looks very much like an "ASCII DIGIT EIGHT" (U+0038). And,
"\d+", may match strings of digits that are a mixture from different
writing systems, creating a security issue. "
num()" in
Unicode::UCD can be used to sort this out. Or the "/a" modifier can
be used to force "\d" to match just the ASCII 0 through 9.
Also, under this modifier, case-insensitive matching works on the full set of
Unicode characters. The "KELVIN SIGN", for example matches the
letters "k" and "K"; and "LATIN SMALL LIGATURE
FF" matches the sequence "ff", which, if you're not prepared,
might make it look like a hexadecimal constant, presenting another potential
security issue. See <
http://unicode.org/reports/tr36> for a detailed
discussion of Unicode security issues.
This modifier may be specified to be the default by "use feature
'unicode_strings", "use locale ':not_characters'", or "use
5.012" (or higher), but see "Which character set modifier is in
effect?".
/d
This modifier means to use the "Default" native rules of the platform
except when there is cause to use Unicode rules instead, as follows:
- 1.
- the target string is encoded in UTF-8; or
- 2.
- the pattern is encoded in UTF-8; or
- 3.
- the pattern explicitly mentions a code point that is above 255 (say by
"\x{100}"); or
- 4.
- the pattern uses a Unicode name ("\N{...}"); or
- 5.
- the pattern uses a Unicode property ("\p{...}"); or
- 6.
- the pattern uses ""(?[ ])""
Another mnemonic for this modifier is "Depends", as the rules actually
used depend on various things, and as a result you can get unexpected results.
See "The "Unicode Bug"" in perlunicode. The Unicode Bug
has become rather infamous, leading to yet another (printable) name for this
modifier, "Dodgy".
Unless the pattern or string are encoded in UTF-8, only ASCII characters can
match positively.
Here are some examples of how that works on an ASCII platform:
$str = "\xDF"; # $str is not in UTF-8 format.
$str =~ /^\w/; # No match, as $str isn't in UTF-8 format.
$str .= "\x{0e0b}"; # Now $str is in UTF-8 format.
$str =~ /^\w/; # Match! $str is now in UTF-8 format.
chop $str;
$str =~ /^\w/; # Still a match! $str remains in UTF-8 format.
This modifier is automatically selected by default when none of the others are,
so yet another name for it is "Default".
Because of the unexpected behaviors associated with this modifier, you probably
should only use it to maintain weird backward compatibilities.
/a (and /aa)
This modifier stands for ASCII-restrict (or ASCII-safe). This modifier, unlike
the others, may be doubled-up to increase its effect.
When it appears singly, it causes the sequences "\d", "\s",
"\w", and the Posix character classes to match only in the ASCII
range. They thus revert to their pre-5.6, pre-Unicode meanings. Under
"/a", "\d" always means precisely the digits "0"
to "9"; "\s" means the five characters "[
\f\n\r\t]", and starting in Perl v5.18, experimentally, the vertical tab;
"\w" means the 63 characters "[A-Za-z0-9_]"; and likewise,
all the Posix classes such as "[[:print:]]" match only the
appropriate ASCII-range characters.
This modifier is useful for people who only incidentally use Unicode, and who do
not wish to be burdened with its complexities and security concerns.
With "/a", one can write "\d" with confidence that it will
only match ASCII characters, and should the need arise to match beyond ASCII,
you can instead use "\p{Digit}" (or "\p{Word}" for
"\w"). There are similar "\p{...}" constructs that can
match beyond ASCII both white space (see "Whitespace" in
perlrecharclass), and Posix classes (see "POSIX Character Classes"
in perlrecharclass). Thus, this modifier doesn't mean you can't use Unicode,
it means that to get Unicode matching you must explicitly use a construct
("\p{}", "\P{}") that signals Unicode.
As you would expect, this modifier causes, for example, "\D" to mean
the same thing as "[^0-9]"; in fact, all non-ASCII characters match
"\D", "\S", and "\W". "\b" still means
to match at the boundary between "\w" and "\W", using the
"/a" definitions of them (similarly for "\B").
Otherwise, "/a" behaves like the "/u" modifier, in that
case-insensitive matching uses Unicode rules; for example, "k" will
match the Unicode "\N{KELVIN SIGN}" under "/i" matching,
and code points in the Latin1 range, above ASCII will have Unicode rules when
it comes to case-insensitive matching.
To forbid ASCII/non-ASCII matches (like "k" with "\N{KELVIN
SIGN}"), specify the "a" twice, for example "/aai" or
"/aia". (The first occurrence of "a" restricts the
"\d", etc., and the second occurrence adds the "/i"
restrictions.) But, note that code points outside the ASCII range will use
Unicode rules for "/i" matching, so the modifier doesn't really
restrict things to just ASCII; it just forbids the intermixing of ASCII and
non-ASCII.
To summarize, this modifier provides protection for applications that don't wish
to be exposed to all of Unicode. Specifying it twice gives added protection.
This modifier may be specified to be the default by "use re '/a'" or
"use re '/aa'". If you do so, you may actually have occasion to use
the "/u" modifier explicitly if there are a few regular expressions
where you do want full Unicode rules (but even here, it's best if everything
were under feature "unicode_strings", along with the "use re
'/aa'"). Also see "Which character set modifier is in effect?".
Which character set modifier is in effect?
Which of these modifiers is in effect at any given point in a regular expression
depends on a fairly complex set of interactions. These have been designed so
that in general you don't have to worry about it, but this section gives the
gory details. As explained below in "Extended Patterns" it is
possible to explicitly specify modifiers that apply only to portions of a
regular expression. The innermost always has priority over any outer ones, and
one applying to the whole expression has priority over any of the default
settings that are described in the remainder of this section.
The "use re '/foo'" pragma can be used to set default modifiers
(including these) for regular expressions compiled within its scope. This
pragma has precedence over the other pragmas listed below that also change the
defaults.
Otherwise, "use locale" sets the default modifier to "/l";
and "use feature 'unicode_strings", or "use 5.012" (or
higher) set the default to "/u" when not in the same scope as either
"use locale" or "use bytes". ("use locale
':not_characters'" also sets the default to "/u", overriding
any plain "use locale".) Unlike the mechanisms mentioned above,
these affect operations besides regular expressions pattern matching, and so
give more consistent results with other operators, including using
"\U", "\l", etc. in substitution replacements.
If none of the above apply, for backwards compatibility reasons, the
"/d" modifier is the one in effect by default. As this can lead to
unexpected results, it is best to specify which other rule set should be used.
Character set modifier behavior prior to Perl 5.14
Prior to 5.14, there were no explicit modifiers, but "/l" was implied
for regexes compiled within the scope of "use locale", and
"/d" was implied otherwise. However, interpolating a regex into a
larger regex would ignore the original compilation in favor of whatever was in
effect at the time of the second compilation. There were a number of
inconsistencies (bugs) with the "/d" modifier, where Unicode rules
would be used when inappropriate, and vice versa. "\p{}" did not
imply Unicode rules, and neither did all occurrences of "\N{}",
until 5.12.
Regular Expressions¶
Metacharacters
The patterns used in Perl pattern matching evolved from those supplied in the
Version 8 regex routines. (The routines are derived (distantly) from Henry
Spencer's freely redistributable reimplementation of the V8 routines.) See
"Version 8 Regular Expressions" for details.
In particular the following metacharacters have their standard
egrep-ish
meanings:
\ Quote the next metacharacter
^ Match the beginning of the line
. Match any character (except newline)
$ Match the end of the string (or before newline at the end
of the string)
| Alternation
() Grouping
[] Bracketed Character class
By default, the "^" character is guaranteed to match only the
beginning of the string, the "$" character only the end (or before
the newline at the end), and Perl does certain optimizations with the
assumption that the string contains only one line. Embedded newlines will not
be matched by "^" or "$". You may, however, wish to treat
a string as a multi-line buffer, such that the "^" will match after
any newline within the string (except if the newline is the last character in
the string), and "$" will match before any newline. At the cost of a
little more overhead, you can do this by using the /m modifier on the pattern
match operator. (Older programs did this by setting $*, but this option was
removed in perl 5.10.)
To simplify multi-line substitutions, the "." character never matches
a newline unless you use the "/s" modifier, which in effect tells
Perl to pretend the string is a single line--even if it isn't.
Quantifiers
The following standard quantifiers are recognized:
* Match 0 or more times
+ Match 1 or more times
? Match 1 or 0 times
{n} Match exactly n times
{n,} Match at least n times
{n,m} Match at least n but not more than m times
(If a curly bracket occurs in any other context and does not form part of a
backslashed sequence like "\x{...}", it is treated as a regular
character. In particular, the lower quantifier bound is not optional, and a
typo in a quantifier silently causes it to be treated as the literal
characters. For example,
/o{4,a}/
compiles to match the sequence of six characters
"o { 4 , a }". It is planned to
eventually require literal uses of curly brackets to be escaped, say by
preceding them with a backslash or enclosing them within square brackets,
("\{" or "[{]"). This change will allow for future syntax
extensions (like making the lower bound of a quantifier optional), and better
error checking. In the meantime, you should get in the habit of escaping all
instances where you mean a literal "{".)
The "*" quantifier is equivalent to "{0,}", the
"+" quantifier to "{1,}", and the "?" quantifier
to "{0,1}". n and m are limited to non-negative integral values less
than a preset limit defined when perl is built. This is usually 32766 on the
most common platforms. The actual limit can be seen in the error message
generated by code such as this:
$_ **= $_ , / {$_} / for 2 .. 42;
By default, a quantified subpattern is "greedy", that is, it will
match as many times as possible (given a particular starting location) while
still allowing the rest of the pattern to match. If you want it to match the
minimum number of times possible, follow the quantifier with a "?".
Note that the meanings don't change, just the "greediness":
*? Match 0 or more times, not greedily
+? Match 1 or more times, not greedily
?? Match 0 or 1 time, not greedily
{n}? Match exactly n times, not greedily (redundant)
{n,}? Match at least n times, not greedily
{n,m}? Match at least n but not more than m times, not greedily
Normally when a quantified subpattern does not allow the rest of the overall
pattern to match, Perl will backtrack. However, this behaviour is sometimes
undesirable. Thus Perl provides the "possessive" quantifier form as
well.
*+ Match 0 or more times and give nothing back
++ Match 1 or more times and give nothing back
?+ Match 0 or 1 time and give nothing back
{n}+ Match exactly n times and give nothing back (redundant)
{n,}+ Match at least n times and give nothing back
{n,m}+ Match at least n but not more than m times and give nothing back
For instance,
'aaaa' =~ /a++a/
will never match, as the "a++" will gobble up all the "a"'s
in the string and won't leave any for the remaining part of the pattern. This
feature can be extremely useful to give perl hints about where it shouldn't
backtrack. For instance, the typical "match a double-quoted string"
problem can be most efficiently performed when written as:
/"(?:[^"\\]++|\\.)*+"/
as we know that if the final quote does not match, backtracking will not help.
See the independent subexpression ""(?>pattern)"" for
more details; possessive quantifiers are just syntactic sugar for that
construct. For instance the above example could also be written as follows:
/"(?>(?:(?>[^"\\]+)|\\.)*)"/
Note that the possessive quantifier modifier can not be be combined with the
non-greedy modifier. This is because it would make no sense. Consider the
follow equivalency table:
Illegal Legal
------------ ------
X??+ X{0}
X+?+ X{1}
X{min,max}?+ X{min}
Escape sequences
Because patterns are processed as double-quoted strings, the following also
work:
\t tab (HT, TAB)
\n newline (LF, NL)
\r return (CR)
\f form feed (FF)
\a alarm (bell) (BEL)
\e escape (think troff) (ESC)
\cK control char (example: VT)
\x{}, \x00 character whose ordinal is the given hexadecimal number
\N{name} named Unicode character or character sequence
\N{U+263D} Unicode character (example: FIRST QUARTER MOON)
\o{}, \000 character whose ordinal is the given octal number
\l lowercase next char (think vi)
\u uppercase next char (think vi)
\L lowercase until \E (think vi)
\U uppercase until \E (think vi)
\Q quote (disable) pattern metacharacters until \E
\E end either case modification or quoted section, think vi
Details are in "Quote and Quote-like Operators" in perlop.
Character Classes and other Special Escapes
In addition, Perl defines the following:
Sequence Note Description
[...] [1] Match a character according to the rules of the
bracketed character class defined by the "...".
Example: [a-z] matches "a" or "b" or "c" ... or "z"
[[:...:]] [2] Match a character according to the rules of the POSIX
character class "..." within the outer bracketed
character class. Example: [[:upper:]] matches any
uppercase character.
(?[...]) [8] Extended bracketed character class
\w [3] Match a "word" character (alphanumeric plus "_", plus
other connector punctuation chars plus Unicode
marks)
\W [3] Match a non-"word" character
\s [3] Match a whitespace character
\S [3] Match a non-whitespace character
\d [3] Match a decimal digit character
\D [3] Match a non-digit character
\pP [3] Match P, named property. Use \p{Prop} for longer names
\PP [3] Match non-P
\X [4] Match Unicode "eXtended grapheme cluster"
\C Match a single C-language char (octet) even if that is
part of a larger UTF-8 character. Thus it breaks up
characters into their UTF-8 bytes, so you may end up
with malformed pieces of UTF-8. Unsupported in
lookbehind. (Deprecated.)
\1 [5] Backreference to a specific capture group or buffer.
'1' may actually be any positive integer.
\g1 [5] Backreference to a specific or previous group,
\g{-1} [5] The number may be negative indicating a relative
previous group and may optionally be wrapped in
curly brackets for safer parsing.
\g{name} [5] Named backreference
\k<name> [5] Named backreference
\K [6] Keep the stuff left of the \K, don't include it in $&
\N [7] Any character but \n. Not affected by /s modifier
\v [3] Vertical whitespace
\V [3] Not vertical whitespace
\h [3] Horizontal whitespace
\H [3] Not horizontal whitespace
\R [4] Linebreak
- [1]
- See "Bracketed Character Classes" in perlrecharclass for
details.
- [2]
- See "POSIX Character Classes" in perlrecharclass for
details.
- [3]
- See "Backslash sequences" in perlrecharclass for details.
- [4]
- See "Misc" in perlrebackslash for details.
- [5]
- See "Capture groups" below for details.
- [6]
- See "Extended Patterns" below for details.
- [7]
- Note that "\N" has two meanings. When of the form
"\N{NAME}", it matches the character or character sequence whose
name is "NAME"; and similarly when of the form "\N{U+
hex}", it matches the character whose Unicode code point is
hex. Otherwise it matches any character but "\n".
- [8]
- See "Extended Bracketed Character Classes" in perlrecharclass
for details.
Assertions
Perl defines the following zero-width assertions:
\b Match a word boundary
\B Match except at a word boundary
\A Match only at beginning of string
\Z Match only at end of string, or before newline at the end
\z Match only at end of string
\G Match only at pos() (e.g. at the end-of-match position
of prior m//g)
A word boundary ("\b") is a spot between two characters that has a
"\w" on one side of it and a "\W" on the other side of it
(in either order), counting the imaginary characters off the beginning and end
of the string as matching a "\W". (Within character classes
"\b" represents backspace rather than a word boundary, just as it
normally does in any double-quoted string.) The "\A" and
"\Z" are just like "^" and "$", except that they
won't match multiple times when the "/m" modifier is used, while
"^" and "$" will match at every internal line boundary. To
match the actual end of the string and not ignore an optional trailing
newline, use "\z".
The "\G" assertion can be used to chain global matches (using
"m//g"), as described in "Regexp Quote-Like Operators" in
perlop. It is also useful when writing "lex"-like scanners, when you
have several patterns that you want to match against consequent substrings of
your string; see the previous reference. The actual location where
"\G" will match can also be influenced by using "pos()" as
an lvalue: see "pos" in perlfunc. Note that the rule for zero-length
matches (see "Repeated Patterns Matching a Zero-length Substring")
is modified somewhat, in that contents to the left of "\G" are not
counted when determining the length of the match. Thus the following will not
match forever:
my $string = 'ABC';
pos($string) = 1;
while ($string =~ /(.\G)/g) {
print $1;
}
It will print 'A' and then terminate, as it considers the match to be
zero-width, and thus will not match at the same position twice in a row.
It is worth noting that "\G" improperly used can result in an infinite
loop. Take care when using patterns that include "\G" in an
alternation.
Note also that "s///" will refuse to overwrite part of a substitution
that has already been replaced; so for example this will stop after the first
iteration, rather than iterating its way backwards through the string:
$_ = "123456789";
pos = 6;
s/.(?=.\G)/X/g;
print; # prints 1234X6789, not XXXXX6789
Capture groups
The bracketing construct "( ... )" creates capture groups (also
referred to as capture buffers). To refer to the current contents of a group
later on, within the same pattern, use "\g1" (or "\g{1}")
for the first, "\g2" (or "\g{2}") for the second, and so
on. This is called a
backreference.
There is no limit to the number of captured substrings that you may use. Groups
are numbered with the leftmost open parenthesis being number 1, etc. If a
group did not match, the associated backreference won't match either. (This
can happen if the group is optional, or in a different branch of an
alternation.) You can omit the "g", and write "\1", etc,
but there are some issues with this form, described below.
You can also refer to capture groups relatively, by using a negative number, so
that "\g-1" and "\g{-1}" both refer to the immediately
preceding capture group, and "\g-2" and "\g{-2}" both
refer to the group before it. For example:
/
(Y) # group 1
( # group 2
(X) # group 3
\g{-1} # backref to group 3
\g{-3} # backref to group 1
)
/x
would match the same as "/(Y) ( (X) \g3 \g1 )/x". This allows you to
interpolate regexes into larger regexes and not have to worry about the
capture groups being renumbered.
You can dispense with numbers altogether and create named capture groups. The
notation is "(?<
name>...)" to declare and
"\g{
name}" to reference. (To be compatible with .Net regular
expressions, "\g{
name}" may also be written as "\k{
name}", "\k<
name>" or
"\k'
name'".)
name must not begin with a number, nor
contain hyphens. When different groups within the same pattern have the same
name, any reference to that name assumes the leftmost defined group. Named
groups count in absolute and relative numbering, and so can also be referred
to by those numbers. (It's possible to do things with named capture groups
that would otherwise require "(??{})".)
Capture group contents are dynamically scoped and available to you outside the
pattern until the end of the enclosing block or until the next successful
match, whichever comes first. (See "Compound Statements" in
perlsyn.) You can refer to them by absolute number (using "$1"
instead of "\g1", etc); or by name via the "%+" hash,
using "$+{
name}".
Braces are required in referring to named capture groups, but are optional for
absolute or relative numbered ones. Braces are safer when creating a regex by
concatenating smaller strings. For example if you have "qr/$a$b/",
and $a contained "\g1", and $b contained "37", you would
get "/\g137/" which is probably not what you intended.
The "\g" and "\k" notations were introduced in Perl 5.10.0.
Prior to that there were no named nor relative numbered capture groups.
Absolute numbered groups were referred to using "\1",
"\2", etc., and this notation is still accepted (and likely always
will be). But it leads to some ambiguities if there are more than 9 capture
groups, as "\10" could mean either the tenth capture group, or the
character whose ordinal in octal is 010 (a backspace in ASCII). Perl resolves
this ambiguity by interpreting "\10" as a backreference only if at
least 10 left parentheses have opened before it. Likewise "\11" is a
backreference only if at least 11 left parentheses have opened before it. And
so on. "\1" through "\9" are always interpreted as
backreferences. There are several examples below that illustrate these perils.
You can avoid the ambiguity by always using "\g{}" or "\g"
if you mean capturing groups; and for octal constants always using
"\o{}", or for "\077" and below, using 3 digits padded
with leading zeros, since a leading zero implies an octal constant.
The "\
digit" notation also works in certain circumstances
outside the pattern. See "Warning on \1 Instead of $1" below for
details.
Examples:
s/^([^ ]*) *([^ ]*)/$2 $1/; # swap first two words
/(.)\g1/ # find first doubled char
and print "'$1' is the first doubled character\n";
/(?<char>.)\k<char>/ # ... a different way
and print "'$+{char}' is the first doubled character\n";
/(?'char'.)\g1/ # ... mix and match
and print "'$1' is the first doubled character\n";
if (/Time: (..):(..):(..)/) { # parse out values
$hours = $1;
$minutes = $2;
$seconds = $3;
}
/(.)(.)(.)(.)(.)(.)(.)(.)(.)\g10/ # \g10 is a backreference
/(.)(.)(.)(.)(.)(.)(.)(.)(.)\10/ # \10 is octal
/((.)(.)(.)(.)(.)(.)(.)(.)(.))\10/ # \10 is a backreference
/((.)(.)(.)(.)(.)(.)(.)(.)(.))\010/ # \010 is octal
$a = '(.)\1'; # Creates problems when concatenated.
$b = '(.)\g{1}'; # Avoids the problems.
"aa" =~ /${a}/; # True
"aa" =~ /${b}/; # True
"aa0" =~ /${a}0/; # False!
"aa0" =~ /${b}0/; # True
"aa\x08" =~ /${a}0/; # True!
"aa\x08" =~ /${b}0/; # False
Several special variables also refer back to portions of the previous match. $+
returns whatever the last bracket match matched. $& returns the entire
matched string. (At one point $0 did also, but now it returns the name of the
program.) "$`" returns everything before the matched string.
"$'" returns everything after the matched string. And $^N contains
whatever was matched by the most-recently closed group (submatch). $^N can be
used in extended patterns (see below), for example to assign a submatch to a
variable.
These special variables, like the "%+" hash and the numbered match
variables ($1, $2, $3, etc.) are dynamically scoped until the end of the
enclosing block or until the next successful match, whichever comes first.
(See "Compound Statements" in perlsyn.)
NOTE: Failed matches in Perl do not reset the match variables, which
makes it easier to write code that tests for a series of more specific cases
and remembers the best match.
WARNING: If your code is to run on Perl 5.16 or earlier, beware that once
Perl sees that you need one of $&, "$`", or "$'"
anywhere in the program, it has to provide them for every pattern match. This
may substantially slow your program.
Perl uses the same mechanism to produce $1, $2, etc, so you also pay a price for
each pattern that contains capturing parentheses. (To avoid this cost while
retaining the grouping behaviour, use the extended regular expression
"(?: ... )" instead.) But if you never use $&, "$`" or
"$'", then patterns
without capturing parentheses will not be
penalized. So avoid $&, "$'", and "$`" if you can, but
if you can't (and some algorithms really appreciate them), once you've used
them once, use them at will, because you've already paid the price.
Perl 5.16 introduced a slightly more efficient mechanism that notes separately
whether each of "$`", $&, and "$'" have been seen, and
thus may only need to copy part of the string. Perl 5.20 introduced a much
more efficient copy-on-write mechanism which eliminates any slowdown.
As another workaround for this problem, Perl 5.10.0 introduced
"${^PREMATCH}", "${^MATCH}" and "${^POSTMATCH}",
which are equivalent to "$`", $& and "$'",
except that they are only guaranteed to be defined after a successful
match that was executed with the "/p" (preserve) modifier. The use
of these variables incurs no global performance penalty, unlike their
punctuation char equivalents, however at the trade-off that you have to tell
perl when you want to use them. As of Perl 5.20, these three variables are
equivalent to "$`", $& and "$'", and "/p" is
ignored.
Backslashed metacharacters in Perl are alphanumeric, such as "\b",
"\w", "\n". Unlike some other regular expression
languages, there are no backslashed symbols that aren't alphanumeric. So
anything that looks like \\, \(, \), \[, \], \{, or \} is always interpreted
as a literal character, not a metacharacter. This was once used in a common
idiom to disable or quote the special meanings of regular expression
metacharacters in a string that you want to use for a pattern. Simply quote
all non-"word" characters:
$pattern =~ s/(\W)/\\$1/g;
(If "use locale" is set, then this depends on the current locale.)
Today it is more common to use the
quotemeta() function or the
"\Q" metaquoting escape sequence to disable all metacharacters'
special meanings like this:
/$unquoted\Q$quoted\E$unquoted/
Beware that if you put literal backslashes (those not inside interpolated
variables) between "\Q" and "\E", double-quotish backslash
interpolation may lead to confusing results. If you
need to use literal
backslashes within "\Q...\E", consult "Gory details of parsing
quoted constructs" in perlop.
"quotemeta()" and "\Q" are fully described in
"quotemeta" in perlfunc.
Extended Patterns¶
Perl also defines a consistent extension syntax for features not found in
standard tools like
awk and
lex. The syntax for most of these is
a pair of parentheses with a question mark as the first thing within the
parentheses. The character after the question mark indicates the extension.
The stability of these extensions varies widely. Some have been part of the core
language for many years. Others are experimental and may change without
warning or be completely removed. Check the documentation on an individual
feature to verify its current status.
A question mark was chosen for this and for the minimal-matching construct
because 1) question marks are rare in older regular expressions, and 2)
whenever you see one, you should stop and "question" exactly what is
going on. That's psychology....
- "(?#text)"
- A comment. The text is ignored. Note that Perl closes the comment as soon
as it sees a ")", so there is no way to put a literal
")" in the comment. The pattern's closing delimiter must be
escaped by a backslash if it appears in the comment.
See "/x" for another way to have comments in patterns.
- "(?adlupimsx-imsx)"
- "(?^alupimsx)"
- One or more embedded pattern-match modifiers, to be turned on (or turned
off, if preceded by "-") for the remainder of the pattern or the
remainder of the enclosing pattern group (if any).
This is particularly useful for dynamic patterns, such as those read in from
a configuration file, taken from an argument, or specified in a table
somewhere. Consider the case where some patterns want to be case-sensitive
and some do not: The case-insensitive ones merely need to include
"(?i)" at the front of the pattern. For example:
$pattern = "foobar";
if ( /$pattern/i ) { }
# more flexible:
$pattern = "(?i)foobar";
if ( /$pattern/ ) { }
These modifiers are restored at the end of the enclosing group. For example,
( (?i) blah ) \s+ \g1
will match "blah" in any case, some spaces, and an exact (
including the case!) repetition of the previous word, assuming the
"/x" modifier, and no "/i" modifier outside this
group.
These modifiers do not carry over into named subpatterns called in the
enclosing group. In other words, a pattern such as
"((?i)(?&NAME))" does not change the case-sensitivity of the
"NAME" pattern.
Any of these modifiers can be set to apply globally to all regular
expressions compiled within the scope of a "use re". See
"'/flags' mode" in re.
Starting in Perl 5.14, a "^" (caret or circumflex accent)
immediately after the "?" is a shorthand equivalent to
"d-imsx". Flags (except "d") may follow the caret to
override it. But a minus sign is not legal with it.
Note that the "a", "d", "l", "p",
and "u" modifiers are special in that they can only be enabled,
not disabled, and the "a", "d", "l", and
"u" modifiers are mutually exclusive: specifying one
de-specifies the others, and a maximum of one (or two "a"'s) may
appear in the construct. Thus, for example, "(?-p)" will warn
when compiled under "use warnings"; "(?-d:...)" and
"(?dl:...)" are fatal errors.
Note also that the "p" modifier is special in that its presence
anywhere in a pattern has a global effect.
- "(?:pattern)"
- "(?adluimsx-imsx:pattern)"
- "(?^aluimsx:pattern)"
- This is for clustering, not capturing; it groups subexpressions like
"()", but doesn't make backreferences as "()" does. So
@fields = split(/\b(?:a|b|c)\b/)
is like
@fields = split(/\b(a|b|c)\b/)
but doesn't spit out extra fields. It's also cheaper not to capture
characters if you don't need to.
Any letters between "?" and ":" act as flags modifiers
as with "(?adluimsx-imsx)". For example,
/(?s-i:more.*than).*million/i
is equivalent to the more verbose
/(?:(?s-i)more.*than).*million/i
Starting in Perl 5.14, a "^" (caret or circumflex accent)
immediately after the "?" is a shorthand equivalent to
"d-imsx". Any positive flags (except "d") may follow
the caret, so
(?^x:foo)
is equivalent to
(?x-ims:foo)
The caret tells Perl that this cluster doesn't inherit the flags of any
surrounding pattern, but uses the system defaults ("d-imsx"),
modified by any flags specified.
The caret allows for simpler stringification of compiled regular
expressions. These look like
(?^:pattern)
with any non-default flags appearing between the caret and the colon. A test
that looks at such stringification thus doesn't need to have the system
default flags hard-coded in it, just the caret. If new flags are added to
Perl, the meaning of the caret's expansion will change to include the
default for those flags, so the test will still work, unchanged.
Specifying a negative flag after the caret is an error, as the flag is
redundant.
Mnemonic for "(?^...)": A fresh beginning since the usual use of a
caret is to match at the beginning.
- "(?|pattern)"
- This is the "branch reset" pattern, which has the special
property that the capture groups are numbered from the same starting point
in each alternation branch. It is available starting from perl 5.10.0.
Capture groups are numbered from left to right, but inside this construct
the numbering is restarted for each branch.
The numbering within each branch will be as normal, and any groups following
this construct will be numbered as though the construct contained only one
branch, that being the one with the most capture groups in it.
This construct is useful when you want to capture one of a number of
alternative matches.
Consider the following pattern. The numbers underneath show in which group
the captured content will be stored.
# before ---------------branch-reset----------- after
/ ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
# 1 2 2 3 2 3 4
Be careful when using the branch reset pattern in combination with named
captures. Named captures are implemented as being aliases to numbered
groups holding the captures, and that interferes with the implementation
of the branch reset pattern. If you are using named captures in a branch
reset pattern, it's best to use the same names, in the same order, in each
of the alternations:
/(?| (?<a> x ) (?<b> y )
| (?<a> z ) (?<b> w )) /x
Not doing so may lead to surprises:
"12" =~ /(?| (?<a> \d+ ) | (?<b> \D+))/x;
say $+ {a}; # Prints '12'
say $+ {b}; # *Also* prints '12'.
The problem here is that both the group named "a" and the group
named "b" are aliases for the group belonging to $1.
- Look-Around Assertions
- Look-around assertions are zero-width patterns which match a specific
pattern without including it in $&. Positive assertions match when
their subpattern matches, negative assertions match when their subpattern
fails. Look-behind matches text up to the current match position,
look-ahead matches text following the current match position.
- "(?=pattern)"
- A zero-width positive look-ahead assertion. For example,
"/\w+(?=\t)/" matches a word followed by a tab, without
including the tab in $&.
- "(?!pattern)"
- A zero-width negative look-ahead assertion. For example
"/foo(?!bar)/" matches any occurrence of "foo" that
isn't followed by "bar". Note however that look-ahead and
look-behind are NOT the same thing. You cannot use this for look-behind.
If you are looking for a "bar" that isn't preceded by a
"foo", "/(?!foo)bar/" will not do what you want.
That's because the "(?!foo)" is just saying that the next thing
cannot be "foo"--and it's not, it's a "bar", so
"foobar" will match. Use look-behind instead (see below).
- "(?<=pattern)" "\K"
- A zero-width positive look-behind assertion. For example,
"/(?<=\t)\w+/" matches a word that follows a tab, without
including the tab in $&. Works only for fixed-width look-behind.
There is a special form of this construct, called "\K" (available
since Perl 5.10.0), which causes the regex engine to "keep"
everything it had matched prior to the "\K" and not include it
in $&. This effectively provides variable-length look-behind. The use
of "\K" inside of another look-around assertion is allowed, but
the behaviour is currently not well defined.
For various reasons "\K" may be significantly more efficient than
the equivalent "(?<=...)" construct, and it is especially
useful in situations where you want to efficiently remove something
following something else in a string. For instance
s/(foo)bar/$1/g;
can be rewritten as the much more efficient
s/foo\Kbar//g;
- "(?<!pattern)"
- A zero-width negative look-behind assertion. For example
"/(?<!bar)foo/" matches any occurrence of "foo"
that does not follow "bar". Works only for fixed-width
look-behind.
- "(?'NAME'pattern)"
- "(?<NAME>pattern)"
- A named capture group. Identical in every respect to normal capturing
parentheses "()" but for the additional fact that the group can
be referred to by name in various regular expression constructs (like
"\g{NAME}") and can be accessed by name after a successful match
via "%+" or "%-". See perlvar for more details on the
"%+" and "%-" hashes.
If multiple distinct capture groups have the same name then the $+{NAME}
will refer to the leftmost defined group in the match.
The forms "(?'NAME'pattern)" and
"(?<NAME>pattern)" are equivalent.
NOTE: While the notation of this construct is the same as the
similar function in .NET regexes, the behavior is not. In Perl the groups
are numbered sequentially regardless of being named or not. Thus in the
pattern
/(x)(?<foo>y)(z)/
$+{foo} will be the same as $2, and $3 will contain 'z' instead of the
opposite which is what a .NET regex hacker might expect.
Currently NAME is restricted to simple identifiers only. In other words, it
must match "/^[_A-Za-z][_A-Za-z0-9]*\z/" or its Unicode
extension (see utf8), though it isn't extended by the locale (see
perllocale).
NOTE: In order to make things easier for programmers with experience
with the Python or PCRE regex engines, the pattern
"(?P<NAME>pattern)" may be used instead of
"(?<NAME>pattern)"; however this form does not support the
use of single quotes as a delimiter for the name.
- "\k<NAME>"
- "\k'NAME'"
- Named backreference. Similar to numeric backreferences, except that the
group is designated by name and not number. If multiple groups have the
same name then it refers to the leftmost defined group in the current
match.
It is an error to refer to a name not defined by a
"(?<NAME>)" earlier in the pattern.
Both forms are equivalent.
NOTE: In order to make things easier for programmers with experience
with the Python or PCRE regex engines, the pattern "(?P=NAME)"
may be used instead of "\k<NAME>".
- "(?{ code })"
- WARNING: Using this feature safely requires that you understand its
limitations. Code executed that has side effects may not perform
identically from version to version due to the effect of future
optimisations in the regex engine. For more information on this, see
"Embedded Code Execution Frequency".
This zero-width assertion executes any embedded Perl code. It always
succeeds, and its return value is set as $^R.
In literal patterns, the code is parsed at the same time as the surrounding
code. While within the pattern, control is passed temporarily back to the
perl parser, until the logically-balancing closing brace is encountered.
This is similar to the way that an array index expression in a literal
string is handled, for example
"abc$array[ 1 + f('[') + g()]def"
In particular, braces do not need to be balanced:
s/abc(?{ f('{'); })/def/
Even in a pattern that is interpolated and compiled at run-time, literal
code blocks will be compiled once, at perl compile time; the following
prints "ABCD":
print "D";
my $qr = qr/(?{ BEGIN { print "A" } })/;
my $foo = "foo";
/$foo$qr(?{ BEGIN { print "B" } })/;
BEGIN { print "C" }
In patterns where the text of the code is derived from run-time information
rather than appearing literally in a source code /pattern/, the code is
compiled at the same time that the pattern is compiled, and for reasons of
security, "use re 'eval'" must be in scope. This is to stop
user-supplied patterns containing code snippets from being executable.
In situations where you need to enable this with "use re 'eval'",
you should also have taint checking enabled. Better yet, use the carefully
constrained evaluation within a Safe compartment. See perlsec for details
about both these mechanisms.
From the viewpoint of parsing, lexical variable scope and closures,
/AAA(?{ BBB })CCC/
behaves approximately like
/AAA/ && do { BBB } && /CCC/
Similarly,
qr/AAA(?{ BBB })CCC/
behaves approximately like
sub { /AAA/ && do { BBB } && /CCC/ }
In particular:
{ my $i = 1; $r = qr/(?{ print $i })/ }
my $i = 2;
/$r/; # prints "1"
Inside a "(?{...})" block, $_ refers to the string the regular
expression is matching against. You can also use "pos()" to know
what is the current position of matching within this string.
The code block introduces a new scope from the perspective of lexical
variable declarations, but not from the perspective of
"local" and similar localizing behaviours. So later code blocks
within the same pattern will still see the values which were localized in
earlier blocks. These accumulated localizations are undone either at the
end of a successful match, or if the assertion is backtracked (compare
"Backtracking"). For example,
$_ = 'a' x 8;
m<
(?{ $cnt = 0 }) # Initialize $cnt.
(
a
(?{
local $cnt = $cnt + 1; # Update $cnt,
# backtracking-safe.
})
)*
aaaa
(?{ $res = $cnt }) # On success copy to
# non-localized location.
>x;
will initially increment $cnt up to 8; then during backtracking, its value
will be unwound back to 4, which is the value assigned to $res. At the end
of the regex execution, $cnt will be wound back to its initial value of 0.
This assertion may be used as the condition in a
(?(condition)yes-pattern|no-pattern)
switch. If not used in this way, the result of evaluation of
"code" is put into the special variable $^R. This happens
immediately, so $^R can be used from other "(?{ code })"
assertions inside the same regular expression.
The assignment to $^R above is properly localized, so the old value of $^R
is restored if the assertion is backtracked; compare
"Backtracking".
Note that the special variable $^N is particularly useful with code blocks
to capture the results of submatches in variables without having to keep
track of the number of nested parentheses. For example:
$_ = "The brown fox jumps over the lazy dog";
/the (\S+)(?{ $color = $^N }) (\S+)(?{ $animal = $^N })/i;
print "color = $color, animal = $animal\n";
- "(??{ code })"
- WARNING: Using this feature safely requires that you understand its
limitations. Code executed that has side effects may not perform
identically from version to version due to the effect of future
optimisations in the regex engine. For more information on this, see
"Embedded Code Execution Frequency".
This is a "postponed" regular subexpression. It behaves in
exactly the same way as a "(?{ code })" code block as
described above, except that its return value, rather than being assigned
to $^R, is treated as a pattern, compiled if it's a string (or used as-is
if its a qr// object), then matched as if it were inserted instead of this
construct.
During the matching of this sub-pattern, it has its own set of captures
which are valid during the sub-match, but are discarded once control
returns to the main pattern. For example, the following matches, with the
inner pattern capturing "B" and matching "BB", while
the outer pattern captures "A";
my $inner = '(.)\1';
"ABBA" =~ /^(.)(??{ $inner })\1/;
print $1; # prints "A";
Note that this means that there is no way for the inner pattern to refer to
a capture group defined outside. (The code block itself can use $1, etc.,
to refer to the enclosing pattern's capture groups.) Thus, although
('a' x 100)=~/(??{'(.)' x 100})/
will match, it will not set $1 on exit.
The following pattern matches a parenthesized group:
$re = qr{
\(
(?:
(?> [^()]+ ) # Non-parens without backtracking
|
(??{ $re }) # Group with matching parens
)*
\)
}x;
See also "(? PARNO)" for a different, more efficient way to
accomplish the same task.
Executing a postponed regular expression 50 times without consuming any
input string will result in a fatal error. The maximum depth is compiled
into perl, so changing it requires a custom build.
- "(?PARNO)" "(?-PARNO)"
"(?+PARNO)" "(?R)" "(?0)"
- Recursive subpattern. Treat the contents of a given capture buffer in the
current pattern as an independent subpattern and attempt to match it at
the current position in the string. Information about capture state from
the caller for things like backreferences is available to the subpattern,
but capture buffers set by the subpattern are not visible to the caller.
Similar to "(??{ code })" except that it does not involve
executing any code or potentially compiling a returned pattern string;
instead it treats the part of the current pattern contained within a
specified capture group as an independent pattern that must match at the
current position. Also different is the treatment of capture buffers,
unlike "(??{ code })" recursive patterns have access to their
callers match state, so one can use backreferences safely.
PARNO is a sequence of digits (not starting with 0) whose value
reflects the paren-number of the capture group to recurse to.
"(?R)" recurses to the beginning of the whole pattern.
"(?0)" is an alternate syntax for "(?R)". If
PARNO is preceded by a plus or minus sign then it is assumed to be
relative, with negative numbers indicating preceding capture groups and
positive ones following. Thus "(?-1)" refers to the most
recently declared group, and "(?+1)" indicates the next group to
be declared. Note that the counting for relative recursion differs from
that of relative backreferences, in that with recursion unclosed groups
are included.
The following pattern matches a function foo() which may contain
balanced parentheses as the argument.
$re = qr{ ( # paren group 1 (full function)
foo
( # paren group 2 (parens)
\(
( # paren group 3 (contents of parens)
(?:
(?> [^()]+ ) # Non-parens without backtracking
|
(?2) # Recurse to start of paren group 2
)*
)
\)
)
)
}x;
If the pattern was used as follows
'foo(bar(baz)+baz(bop))'=~/$re/
and print "\$1 = $1\n",
"\$2 = $2\n",
"\$3 = $3\n";
the output produced should be the following:
$1 = foo(bar(baz)+baz(bop))
$2 = (bar(baz)+baz(bop))
$3 = bar(baz)+baz(bop)
If there is no corresponding capture group defined, then it is a fatal
error. Recursing deeper than 50 times without consuming any input string
will also result in a fatal error. The maximum depth is compiled into
perl, so changing it requires a custom build.
The following shows how using negative indexing can make it easier to embed
recursive patterns inside of a "qr//" construct for later use:
my $parens = qr/(\((?:[^()]++|(?-1))*+\))/;
if (/foo $parens \s+ \+ \s+ bar $parens/x) {
# do something here...
}
Note that this pattern does not behave the same way as the
equivalent PCRE or Python construct of the same form. In Perl you can
backtrack into a recursed group, in PCRE and Python the recursed into
group is treated as atomic. Also, modifiers are resolved at compile time,
so constructs like (?i:(?1)) or (?:(?i)(?1)) do not affect how the
sub-pattern will be processed.
- "(?&NAME)"
- Recurse to a named subpattern. Identical to "(?PARNO)"
except that the parenthesis to recurse to is determined by name. If
multiple parentheses have the same name, then it recurses to the leftmost.
It is an error to refer to a name that is not declared somewhere in the
pattern.
NOTE: In order to make things easier for programmers with experience
with the Python or PCRE regex engines the pattern "(?P>NAME)"
may be used instead of "(?&NAME)".
- "(?(condition)yes-pattern|no-pattern)"
- "(?(condition)yes-pattern)"
- Conditional expression. Matches "yes-pattern" if
"condition" yields a true value, matches "no-pattern"
otherwise. A missing pattern always matches.
"(condition)" should be one of: 1) an integer in parentheses
(which is valid if the corresponding pair of parentheses matched); 2) a
look-ahead/look-behind/evaluate zero-width assertion; 3) a name in angle
brackets or single quotes (which is valid if a group with the given name
matched); or 4) the special symbol (R) (true when evaluated inside of
recursion or eval). Additionally the R may be followed by a number, (which
will be true when evaluated when recursing inside of the appropriate
group), or by &NAME, in which case it will be true only when evaluated
during recursion in the named group.
Here's a summary of the possible predicates:
- (1) (2) ...
- Checks if the numbered capturing group has matched something.
- (<NAME>) ('NAME')
- Checks if a group with the given name has matched something.
- (?=...) (?!...) (?<=...) (?<!...)
- Checks whether the pattern matches (or does not match, for the '!'
variants).
- (?{ CODE })
- Treats the return value of the code block as the condition.
- (R)
- Checks if the expression has been evaluated inside of recursion.
- (R1) (R2) ...
- Checks if the expression has been evaluated while executing directly
inside of the n-th capture group. This check is the regex equivalent of
if ((caller(0))[3] eq 'subname') { ... }
In other words, it does not check the full recursion stack.
- (R&NAME)
- Similar to "(R1)", this predicate checks to see if we're
executing directly inside of the leftmost group with a given name (this is
the same logic used by "(?&NAME)" to disambiguate). It does
not check the full stack, but only the name of the innermost active
recursion.
- (DEFINE)
- In this case, the yes-pattern is never directly executed, and no
no-pattern is allowed. Similar in spirit to "(?{0})" but more
efficient. See below for details.
For example:
m{ ( \( )?
[^()]+
(?(1) \) )
}x
matches a chunk of non-parentheses, possibly included in parentheses themselves.
A special form is the "(DEFINE)" predicate, which never executes its
yes-pattern directly, and does not allow a no-pattern. This allows one to
define subpatterns which will be executed only by the recursion mechanism.
This way, you can define a set of regular expression rules that can be bundled
into any pattern you choose.
It is recommended that for this usage you put the DEFINE block at the end of the
pattern, and that you name any subpatterns defined within it.
Also, it's worth noting that patterns defined this way probably will not be as
efficient, as the optimizer is not very clever about handling them.
An example of how this might be used is as follows:
/(?<NAME>(?&NAME_PAT))(?<ADDR>(?&ADDRESS_PAT))
(?(DEFINE)
(?<NAME_PAT>....)
(?<ADDRESS_PAT>....)
)/x
Note that capture groups matched inside of recursion are not accessible after
the recursion returns, so the extra layer of capturing groups is necessary.
Thus $+{NAME_PAT} would not be defined even though $+{NAME} would be.
Finally, keep in mind that subpatterns created inside a DEFINE block count
towards the absolute and relative number of captures, so this:
my @captures = "a" =~ /(.) # First capture
(?(DEFINE)
(?<EXAMPLE> 1 ) # Second capture
)/x;
say scalar @captures;
Will output 2, not 1. This is particularly important if you intend to compile
the definitions with the "qr//" operator, and later interpolate them
in another pattern.
- "(?>pattern)"
- An "independent" subexpression, one which matches the substring
that a standalone "pattern" would match if anchored at
the given position, and it matches nothing other than this
substring. This construct is useful for optimizations of what would
otherwise be "eternal" matches, because it will not backtrack
(see "Backtracking"). It may also be useful in places where the
"grab all you can, and do not give anything back" semantic is
desirable.
For example: "^(?>a*)ab" will never match, since
"(?>a*)" (anchored at the beginning of string, as above) will
match all characters "a" at the beginning of string,
leaving no "a" for "ab" to match. In contrast,
"a*ab" will match the same as "a+b", since the match
of the subgroup "a*" is influenced by the following group
"ab" (see "Backtracking"). In particular,
"a*" inside "a*ab" will match fewer characters than a
standalone "a*", since this makes the tail match.
"(?>pattern)" does not disable backtracking altogether once it
has matched. It is still possible to backtrack past the construct, but not
into it. So "((?>a*)|(?>b*))ar" will still match
"bar".
An effect similar to "(?>pattern)" may be achieved by writing
"(?=(pattern))\g{-1}". This matches the same substring as a
standalone "a+", and the following "\g{-1}" eats the
matched string; it therefore makes a zero-length assertion into an
analogue of "(?>...)". (The difference between these two
constructs is that the second one uses a capturing group, thus shifting
ordinals of backreferences in the rest of a regular expression.)
Consider this pattern:
m{ \(
(
[^()]+ # x+
|
\( [^()]* \)
)+
\)
}x
That will efficiently match a nonempty group with matching parentheses two
levels deep or less. However, if there is no such group, it will take
virtually forever on a long string. That's because there are so many
different ways to split a long string into several substrings. This is
what "(.+)+" is doing, and "(.+)+" is similar to a
subpattern of the above pattern. Consider how the pattern above detects
no-match on "((()aaaaaaaaaaaaaaaaaa" in several seconds, but
that each extra letter doubles this time. This exponential performance
will make it appear that your program has hung. However, a tiny change to
this pattern
m{ \(
(
(?> [^()]+ ) # change x+ above to (?> x+ )
|
\( [^()]* \)
)+
\)
}x
which uses "(?>...)" matches exactly when the one above does
(verifying this yourself would be a productive exercise), but finishes in
a fourth the time when used on a similar string with 1000000
"a"s. Be aware, however, that, when this construct is followed
by a quantifier, it currently triggers a warning message under the
"use warnings" pragma or -w switch saying it
"matches null string many times in regex".
On simple groups, such as the pattern "(?> [^()]+ )", a
comparable effect may be achieved by negative look-ahead, as in
"[^()]+ (?! [^()] )". This was only 4 times slower on a string
with 1000000 "a"s.
The "grab all you can, and do not give anything back" semantic is
desirable in many situations where on the first sight a simple
"()*" looks like the correct solution. Suppose we parse text
with comments being delimited by "#" followed by some optional
(horizontal) whitespace. Contrary to its appearance, "#[ \t]*"
is not the correct subexpression to match the comment delimiter,
because it may "give up" some whitespace if the remainder of the
pattern can be made to match that way. The correct answer is either one of
these:
(?>#[ \t]*)
#[ \t]*(?![ \t])
For example, to grab non-empty comments into $1, one should use either one
of these:
/ (?> \# [ \t]* ) ( .+ ) /x;
/ \# [ \t]* ( [^ \t] .* ) /x;
Which one you pick depends on which of these expressions better reflects the
above specification of comments.
In some literature this construct is called "atomic matching" or
"possessive matching".
Possessive quantifiers are equivalent to putting the item they are applied
to inside of one of these constructs. The following equivalences apply:
Quantifier Form Bracketing Form
--------------- ---------------
PAT*+ (?>PAT*)
PAT++ (?>PAT+)
PAT?+ (?>PAT?)
PAT{min,max}+ (?>PAT{min,max})
- "(?[ ])"
- See "Extended Bracketed Character Classes" in
perlrecharclass.
Special Backtracking Control Verbs¶
These special patterns are generally of the form "(*VERB:ARG)". Unless
otherwise stated the ARG argument is optional; in some cases, it is forbidden.
Any pattern containing a special backtracking verb that allows an argument has
the special behaviour that when executed it sets the current package's
$REGERROR and $REGMARK variables. When doing so the following rules apply:
On failure, the $REGERROR variable will be set to the ARG value of the verb
pattern, if the verb was involved in the failure of the match. If the ARG part
of the pattern was omitted, then $REGERROR will be set to the name of the last
"(*MARK:NAME)" pattern executed, or to TRUE if there was none. Also,
the $REGMARK variable will be set to FALSE.
On a successful match, the $REGERROR variable will be set to FALSE, and the
$REGMARK variable will be set to the name of the last "(*MARK:NAME)"
pattern executed. See the explanation for the "(*MARK:NAME)" verb
below for more details.
NOTE: $REGERROR and $REGMARK are not magic variables like $1 and most
other regex-related variables. They are not local to a scope, nor readonly,
but instead are volatile package variables similar to $AUTOLOAD. Use
"local" to localize changes to them to a specific scope if
necessary.
If a pattern does not contain a special backtracking verb that allows an
argument, then $REGERROR and $REGMARK are not touched at all.
- Verbs that take an argument
- "(*PRUNE)" "(*PRUNE:NAME)"
- This zero-width pattern prunes the backtracking tree at the current point
when backtracked into on failure. Consider the pattern "A (*PRUNE)
B", where A and B are complex patterns. Until the
"(*PRUNE)" verb is reached, A may backtrack as necessary to
match. Once it is reached, matching continues in B, which may also
backtrack as necessary; however, should B not match, then no further
backtracking will take place, and the pattern will fail outright at the
current starting position.
The following example counts all the possible matching strings in a pattern
(without actually matching any of them).
'aaab' =~ /a+b?(?{print "$&\n"; $count++})(*FAIL)/;
print "Count=$count\n";
which produces:
aaab
aaa
aa
a
aab
aa
a
ab
a
Count=9
If we add a "(*PRUNE)" before the count like the following
'aaab' =~ /a+b?(*PRUNE)(?{print "$&\n"; $count++})(*FAIL)/;
print "Count=$count\n";
we prevent backtracking and find the count of the longest matching string at
each matching starting point like so:
aaab
aab
ab
Count=3
Any number of "(*PRUNE)" assertions may be used in a pattern.
See also "(?>pattern)" and possessive quantifiers for other
ways to control backtracking. In some cases, the use of
"(*PRUNE)" can be replaced with a "(?>pattern)"
with no functional difference; however, "(*PRUNE)" can be used
to handle cases that cannot be expressed using a
"(?>pattern)" alone.
- "(*SKIP)" "(*SKIP:NAME)"
- This zero-width pattern is similar to "(*PRUNE)", except that on
failure it also signifies that whatever text that was matched leading up
to the "(*SKIP)" pattern being executed cannot be part of
any match of this pattern. This effectively means that the regex
engine "skips" forward to this position on failure and tries to
match again, (assuming that there is sufficient room to match).
The name of the "(*SKIP:NAME)" pattern has special significance.
If a "(*MARK:NAME)" was encountered while matching, then it is
that position which is used as the "skip point". If no
"(*MARK)" of that name was encountered, then the
"(*SKIP)" operator has no effect. When used without a name the
"skip point" is where the match point was when executing the
(*SKIP) pattern.
Compare the following to the examples in "(*PRUNE)"; note the
string is twice as long:
'aaabaaab' =~ /a+b?(*SKIP)(?{print "$&\n"; $count++})(*FAIL)/;
print "Count=$count\n";
outputs
aaab
aaab
Count=2
Once the 'aaab' at the start of the string has matched, and the
"(*SKIP)" executed, the next starting point will be where the
cursor was when the "(*SKIP)" was executed.
- "(*MARK:NAME)" "(*:NAME)"
- This zero-width pattern can be used to mark the point reached in a string
when a certain part of the pattern has been successfully matched. This
mark may be given a name. A later "(*SKIP)" pattern will then
skip forward to that point if backtracked into on failure. Any number of
"(*MARK)" patterns are allowed, and the NAME portion may be
duplicated.
In addition to interacting with the "(*SKIP)" pattern,
"(*MARK:NAME)" can be used to "label" a pattern
branch, so that after matching, the program can determine which branches
of the pattern were involved in the match.
When a match is successful, the $REGMARK variable will be set to the name of
the most recently executed "(*MARK:NAME)" that was involved in
the match.
This can be used to determine which branch of a pattern was matched without
using a separate capture group for each branch, which in turn can result
in a performance improvement, as perl cannot optimize
"/(?:(x)|(y)|(z))/" as efficiently as something like
"/(?:x(*MARK:x)|y(*MARK:y)|z(*MARK:z))/".
When a match has failed, and unless another verb has been involved in
failing the match and has provided its own name to use, the $REGERROR
variable will be set to the name of the most recently executed
"(*MARK:NAME)".
See "(*SKIP)" for more details.
As a shortcut "(*MARK:NAME)" can be written
"(*:NAME)".
- "(*THEN)" "(*THEN:NAME)"
- This is similar to the "cut group" operator "::" from
Perl 6. Like "(*PRUNE)", this verb always matches, and when
backtracked into on failure, it causes the regex engine to try the next
alternation in the innermost enclosing group (capturing or otherwise) that
has alternations. The two branches of a
"(?(condition)yes-pattern|no-pattern)" do not count as an
alternation, as far as "(*THEN)" is concerned.
Its name comes from the observation that this operation combined with the
alternation operator ("|") can be used to create what is
essentially a pattern-based if/then/else block:
( COND (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ )
Note that if this operator is used and NOT inside of an alternation then it
acts exactly like the "(*PRUNE)" operator.
/ A (*PRUNE) B /
is the same as
/ A (*THEN) B /
but
/ ( A (*THEN) B | C ) /
is not the same as
/ ( A (*PRUNE) B | C ) /
as after matching the A but failing on the B the "(*THEN)" verb
will backtrack and try C; but the "(*PRUNE)" verb will simply
fail.
- Verbs without an argument
- "(*COMMIT)"
- This is the Perl 6 "commit pattern" "<commit>"
or ":::". It's a zero-width pattern similar to
"(*SKIP)", except that when backtracked into on failure it
causes the match to fail outright. No further attempts to find a valid
match by advancing the start pointer will occur again. For example,
'aaabaaab' =~ /a+b?(*COMMIT)(?{print "$&\n"; $count++})(*FAIL)/;
print "Count=$count\n";
outputs
aaab
Count=1
In other words, once the "(*COMMIT)" has been entered, and if the
pattern does not match, the regex engine will not try any further matching
on the rest of the string.
- "(*FAIL)" "(*F)"
- This pattern matches nothing and always fails. It can be used to force the
engine to backtrack. It is equivalent to "(?!)", but easier to
read. In fact, "(?!)" gets optimised into "(*FAIL)"
internally.
It is probably useful only when combined with "(?{})" or
"(??{})".
- "(*ACCEPT)"
- This pattern matches nothing and causes the end of successful matching at
the point at which the "(*ACCEPT)" pattern was encountered,
regardless of whether there is actually more to match in the string. When
inside of a nested pattern, such as recursion, or in a subpattern
dynamically generated via "(??{})", only the innermost pattern
is ended immediately.
If the "(*ACCEPT)" is inside of capturing groups then the groups
are marked as ended at the point at which the "(*ACCEPT)" was
encountered. For instance:
'AB' =~ /(A (A|B(*ACCEPT)|C) D)(E)/x;
will match, and $1 will be "AB" and $2 will be "B", $3
will not be set. If another branch in the inner parentheses was matched,
such as in the string 'ACDE', then the "D" and "E"
would have to be matched as well.
Backtracking¶
NOTE: This section presents an abstract approximation of regular expression
behavior. For a more rigorous (and complicated) view of the rules involved in
selecting a match among possible alternatives, see "Combining RE
Pieces".
A fundamental feature of regular expression matching involves the notion called
backtracking, which is currently used (when needed) by all regular
non-possessive expression quantifiers, namely "*", "*?",
"+", "+?", "{n,m}", and "{n,m}?".
Backtracking is often optimized internally, but the general principle outlined
here is valid.
For a regular expression to match, the
entire regular expression must
match, not just part of it. So if the beginning of a pattern containing a
quantifier succeeds in a way that causes later parts in the pattern to fail,
the matching engine backs up and recalculates the beginning part--that's why
it's called backtracking.
Here is an example of backtracking: Let's say you want to find the word
following "foo" in the string "Food is on the foo table.":
$_ = "Food is on the foo table.";
if ( /\b(foo)\s+(\w+)/i ) {
print "$2 follows $1.\n";
}
When the match runs, the first part of the regular expression
("\b(foo)") finds a possible match right at the beginning of the
string, and loads up $1 with "Foo". However, as soon as the matching
engine sees that there's no whitespace following the "Foo" that it
had saved in $1, it realizes its mistake and starts over again one character
after where it had the tentative match. This time it goes all the way until
the next occurrence of "foo". The complete regular expression
matches this time, and you get the expected output of "table follows
foo."
Sometimes minimal matching can help a lot. Imagine you'd like to match
everything between "foo" and "bar". Initially, you write
something like this:
$_ = "The food is under the bar in the barn.";
if ( /foo(.*)bar/ ) {
print "got <$1>\n";
}
Which perhaps unexpectedly yields:
got <d is under the bar in the >
That's because ".*" was greedy, so you get everything between the
first "foo" and the
last "bar". Here it's
more effective to use minimal matching to make sure you get the text between a
"foo" and the first "bar" thereafter.
if ( /foo(.*?)bar/ ) { print "got <$1>\n" }
got <d is under the >
Here's another example. Let's say you'd like to match a number at the end of a
string, and you also want to keep the preceding part of the match. So you
write this:
$_ = "I have 2 numbers: 53147";
if ( /(.*)(\d*)/ ) { # Wrong!
print "Beginning is <$1>, number is <$2>.\n";
}
That won't work at all, because ".*" was greedy and gobbled up the
whole string. As "\d*" can match on an empty string the complete
regular expression matched successfully.
Beginning is <I have 2 numbers: 53147>, number is <>.
Here are some variants, most of which don't work:
$_ = "I have 2 numbers: 53147";
@pats = qw{
(.*)(\d*)
(.*)(\d+)
(.*?)(\d*)
(.*?)(\d+)
(.*)(\d+)$
(.*?)(\d+)$
(.*)\b(\d+)$
(.*\D)(\d+)$
};
for $pat (@pats) {
printf "%-12s ", $pat;
if ( /$pat/ ) {
print "<$1> <$2>\n";
} else {
print "FAIL\n";
}
}
That will print out:
(.*)(\d*) <I have 2 numbers: 53147> <>
(.*)(\d+) <I have 2 numbers: 5314> <7>
(.*?)(\d*) <> <>
(.*?)(\d+) <I have > <2>
(.*)(\d+)$ <I have 2 numbers: 5314> <7>
(.*?)(\d+)$ <I have 2 numbers: > <53147>
(.*)\b(\d+)$ <I have 2 numbers: > <53147>
(.*\D)(\d+)$ <I have 2 numbers: > <53147>
As you see, this can be a bit tricky. It's important to realize that a regular
expression is merely a set of assertions that gives a definition of success.
There may be 0, 1, or several different ways that the definition might succeed
against a particular string. And if there are multiple ways it might succeed,
you need to understand backtracking to know which variety of success you will
achieve.
When using look-ahead assertions and negations, this can all get even trickier.
Imagine you'd like to find a sequence of non-digits not followed by
"123". You might try to write that as
$_ = "ABC123";
if ( /^\D*(?!123)/ ) { # Wrong!
print "Yup, no 123 in $_\n";
}
But that isn't going to match; at least, not the way you're hoping. It claims
that there is no 123 in the string. Here's a clearer picture of why that
pattern matches, contrary to popular expectations:
$x = 'ABC123';
$y = 'ABC445';
print "1: got $1\n" if $x =~ /^(ABC)(?!123)/;
print "2: got $1\n" if $y =~ /^(ABC)(?!123)/;
print "3: got $1\n" if $x =~ /^(\D*)(?!123)/;
print "4: got $1\n" if $y =~ /^(\D*)(?!123)/;
This prints
2: got ABC
3: got AB
4: got ABC
You might have expected test 3 to fail because it seems to a more general
purpose version of test 1. The important difference between them is that test
3 contains a quantifier ("\D*") and so can use backtracking, whereas
test 1 will not. What's happening is that you've asked "Is it true that
at the start of $x, following 0 or more non-digits, you have something that's
not 123?" If the pattern matcher had let "\D*" expand to
"ABC", this would have caused the whole pattern to fail.
The search engine will initially match "\D*" with "ABC".
Then it will try to match "(?!123)" with "123", which
fails. But because a quantifier ("\D*") has been used in the regular
expression, the search engine can backtrack and retry the match differently in
the hope of matching the complete regular expression.
The pattern really,
really wants to succeed, so it uses the standard
pattern back-off-and-retry and lets "\D*" expand to just
"AB" this time. Now there's indeed something following
"AB" that is not "123". It's "C123", which
suffices.
We can deal with this by using both an assertion and a negation. We'll say that
the first part in $1 must be followed both by a digit and by something that's
not "123". Remember that the look-aheads are zero-width
expressions--they only look, but don't consume any of the string in their
match. So rewriting this way produces what you'd expect; that is, case 5 will
fail, but case 6 succeeds:
print "5: got $1\n" if $x =~ /^(\D*)(?=\d)(?!123)/;
print "6: got $1\n" if $y =~ /^(\D*)(?=\d)(?!123)/;
6: got ABC
In other words, the two zero-width assertions next to each other work as though
they're ANDed together, just as you'd use any built-in assertions:
"/^$/" matches only if you're at the beginning of the line AND the
end of the line simultaneously. The deeper underlying truth is that
juxtaposition in regular expressions always means AND, except when you write
an explicit OR using the vertical bar. "/ab/" means match
"a" AND (then) match "b", although the attempted matches
are made at different positions because "a" is not a zero-width
assertion, but a one-width assertion.
WARNING: Particularly complicated regular expressions can take
exponential time to solve because of the immense number of possible ways they
can use backtracking to try for a match. For example, without internal
optimizations done by the regular expression engine, this will take a
painfully long time to run:
'aaaaaaaaaaaa' =~ /((a{0,5}){0,5})*[c]/
And if you used "*"'s in the internal groups instead of limiting them
to 0 through 5 matches, then it would take forever--or until you ran out of
stack space. Moreover, these internal optimizations are not always applicable.
For example, if you put "{0,5}" instead of "*" on the
external group, no current optimization is applicable, and the match takes a
long time to finish.
A powerful tool for optimizing such beasts is what is known as an
"independent group", which does not backtrack (see
""(?>pattern)""). Note also that zero-length
look-ahead/look-behind assertions will not backtrack to make the tail match,
since they are in "logical" context: only whether they match is
considered relevant. For an example where side-effects of look-ahead
might have influenced the following match, see
""(?>pattern)"".
Version 8 Regular Expressions¶
In case you're not familiar with the "regular" Version 8 regex
routines, here are the pattern-matching rules not described above.
Any single character matches itself, unless it is a
metacharacter with a
special meaning described here or above. You can cause characters that
normally function as metacharacters to be interpreted literally by prefixing
them with a "\" (e.g., "\." matches a ".", not
any character; "\\" matches a "\"). This escape mechanism
is also required for the character used as the pattern delimiter.
A series of characters matches that series of characters in the target string,
so the pattern "blurfl" would match "blurfl" in the target
string.
You can specify a character class, by enclosing a list of characters in
"[]", which will match any character from the list. If the first
character after the "[" is "^", the class matches any
character not in the list. Within a list, the "-" character
specifies a range, so that "a-z" represents all characters between
"a" and "z", inclusive. If you want either "-"
or "]" itself to be a member of a class, put it at the start of the
list (possibly after a "^"), or escape it with a backslash.
"-" is also taken literally when it is at the end of the list, just
before the closing "]". (The following all specify the same class of
three characters: "[-az]", "[az-]", and
"[a\-z]". All are different from "[a-z]", which specifies
a class containing twenty-six characters, even on EBCDIC-based character
sets.) Also, if you try to use the character classes "\w",
"\W", "\s", "\S", "\d", or
"\D" as endpoints of a range, the "-" is understood
literally.
Note also that the whole range idea is rather unportable between character
sets--and even within character sets they may cause results you probably
didn't expect. A sound principle is to use only ranges that begin from and end
at either alphabetics of equal case ([a-e], [A-E]), or digits ([0-9]).
Anything else is unsafe. If in doubt, spell out the character sets in full.
Characters may be specified using a metacharacter syntax much like that used in
C: "\n" matches a newline, "\t" a tab, "\r" a
carriage return, "\f" a form feed, etc. More generally, \
nnn, where
nnn is a string of three octal digits, matches the
character whose coded character set value is
nnn. Similarly,
\x
nn, where
nn are hexadecimal digits, matches the character
whose ordinal is
nn. The expression \c
x matches the character
control-
x. Finally, the "." metacharacter matches any
character except "\n" (unless you use "/s").
You can specify a series of alternatives for a pattern using "|" to
separate them, so that "fee|fie|foe" will match any of
"fee", "fie", or "foe" in the target string (as
would "f(e|i|o)e"). The first alternative includes everything from
the last pattern delimiter ("(", "(?:", etc. or the
beginning of the pattern) up to the first "|", and the last
alternative contains everything from the last "|" to the next
closing pattern delimiter. That's why it's common practice to include
alternatives in parentheses: to minimize confusion about where they start and
end.
Alternatives are tried from left to right, so the first alternative found for
which the entire expression matches, is the one that is chosen. This means
that alternatives are not necessarily greedy. For example: when matching
"foo|foot" against "barefoot", only the "foo"
part will match, as that is the first alternative tried, and it successfully
matches the target string. (This might not seem important, but it is important
when you are capturing matched text using parentheses.)
Also remember that "|" is interpreted as a literal within square
brackets, so if you write "[fee|fie|foe]" you're really only
matching "[feio|]".
Within a pattern, you may designate subpatterns for later reference by enclosing
them in parentheses, and you may refer back to the
nth subpattern later
in the pattern using the metacharacter \
n or \g
n. Subpatterns
are numbered based on the left to right order of their opening parenthesis. A
backreference matches whatever actually matched the subpattern in the string
being examined, not the rules for that subpattern. Therefore,
"(0|0x)\d*\s\g1\d*" will match "0x1234 0x4321", but not
"0x1234 01234", because subpattern 1 matched "0x", even
though the rule "0|0x" could potentially match the leading 0 in the
second number.
Warning on \1 Instead of $1¶
Some people get too used to writing things like:
$pattern =~ s/(\W)/\\\1/g;
This is grandfathered (for \1 to \9) for the RHS of a substitute to avoid
shocking the
sed addicts, but it's a dirty habit to get into. That's
because in PerlThink, the righthand side of an "s///" is a
double-quoted string. "\1" in the usual double-quoted string means a
control-A. The customary Unix meaning of "\1" is kludged in for
"s///". However, if you get into the habit of doing that, you get
yourself into trouble if you then add an "/e" modifier.
s/(\d+)/ \1 + 1 /eg; # causes warning under -w
Or if you try to do
s/(\d+)/\1000/;
You can't disambiguate that by saying "\{1}000", whereas you can fix
it with "${1}000". The operation of interpolation should not be
confused with the operation of matching a backreference. Certainly they mean
two different things on the
left side of the "s///".
Repeated Patterns Matching a Zero-length Substring¶
WARNING: Difficult material (and prose) ahead. This section needs a
rewrite.
Regular expressions provide a terse and powerful programming language. As with
most other power tools, power comes together with the ability to wreak havoc.
A common abuse of this power stems from the ability to make infinite loops using
regular expressions, with something as innocuous as:
'foo' =~ m{ ( o? )* }x;
The "o?" matches at the beginning of 'foo', and since the position in
the string is not moved by the match, "o?" would match again and
again because of the "*" quantifier. Another common way to create a
similar cycle is with the looping modifier "//g":
@matches = ( 'foo' =~ m{ o? }xg );
or
print "match: <$&>\n" while 'foo' =~ m{ o? }xg;
or the loop implied by
split().
However, long experience has shown that many programming tasks may be
significantly simplified by using repeated subexpressions that may match
zero-length substrings. Here's a simple example being:
@chars = split //, $string; # // is not magic in split
($whitewashed = $string) =~ s/()/ /g; # parens avoid magic s// /
Thus Perl allows such constructs, by
forcefully breaking the infinite
loop. The rules for this are different for lower-level loops given by the
greedy quantifiers "*+{}", and for higher-level ones like the
"/g" modifier or
split() operator.
The lower-level loops are
interrupted (that is, the loop is broken) when
Perl detects that a repeated expression matched a zero-length substring. Thus
m{ (?: NON_ZERO_LENGTH | ZERO_LENGTH )* }x;
is made equivalent to
m{ (?: NON_ZERO_LENGTH )* (?: ZERO_LENGTH )? }x;
For example, this program
#!perl -l
"aaaaab" =~ /
(?:
a # non-zero
| # or
(?{print "hello"}) # print hello whenever this
# branch is tried
(?=(b)) # zero-width assertion
)* # any number of times
/x;
print $&;
print $1;
prints
hello
aaaaa
b
Notice that "hello" is only printed once, as when Perl sees that the
sixth iteration of the outermost "(?:)*" matches a zero-length
string, it stops the "*".
The higher-level loops preserve an additional state between iterations: whether
the last match was zero-length. To break the loop, the following match after a
zero-length match is prohibited to have a length of zero. This prohibition
interacts with backtracking (see "Backtracking"), and so the
second best match is chosen if the
best match is of zero length.
For example:
$_ = 'bar';
s/\w??/<$&>/g;
results in
"<><b><><a><><r><>". At
each position of the string the best match given by non-greedy "??"
is the zero-length match, and the
second best match is what is
matched by "\w". Thus zero-length matches alternate with
one-character-long matches.
Similarly, for repeated "m/()/g" the second-best match is the match at
the position one notch further in the string.
The additional state of being
matched with zero-length is associated with
the matched string, and is reset by each assignment to
pos().
Zero-length matches at the end of the previous match are ignored during
"split".
Combining RE Pieces¶
Each of the elementary pieces of regular expressions which were described before
(such as "ab" or "\Z") could match at most one substring
at the given position of the input string. However, in a typical regular
expression these elementary pieces are combined into more complicated patterns
using combining operators "ST", "S|T", "S*" etc.
(in these examples "S" and "T" are regular
subexpressions).
Such combinations can include alternatives, leading to a problem of choice: if
we match a regular expression "a|ab" against "abc", will
it match substring "a" or "ab"? One way to describe which
substring is actually matched is the concept of backtracking (see
"Backtracking"). However, this description is too low-level and
makes you think in terms of a particular implementation.
Another description starts with notions of "better"/"worse".
All the substrings which may be matched by the given regular expression can be
sorted from the "best" match to the "worst" match, and it
is the "best" match which is chosen. This substitutes the question
of "what is chosen?" by the question of "which matches are
better, and which are worse?".
Again, for elementary pieces there is no such question, since at most one match
at a given position is possible. This section describes the notion of
better/worse for combining operators. In the description below "S"
and "T" are regular subexpressions.
- "ST"
- Consider two possible matches, "AB" and "A'B'",
"A" and "A'" are substrings which can be matched by
"S", "B" and "B'" are substrings which can
be matched by "T".
If "A" is a better match for "S" than "A'",
"AB" is a better match than "A'B'".
If "A" and "A'" coincide: "AB" is a better
match than "AB'" if "B" is a better match for
"T" than "B'".
- "S|T"
- When "S" can match, it is a better match than when only
"T" can match.
Ordering of two matches for "S" is the same as for "S".
Similar for two matches for "T".
- "S{REPEAT_COUNT}"
- Matches as "SSS...S" (repeated as many times as necessary).
- "S{min,max}"
- Matches as "S{max}|S{max-1}|...|S{min+1}|S{min}".
- "S{min,max}?"
- Matches as "S{min}|S{min+1}|...|S{max-1}|S{max}".
- "S?", "S*", "S+"
- Same as "S{0,1}", "S{0,BIG_NUMBER}",
"S{1,BIG_NUMBER}" respectively.
- "S??", "S*?", "S+?"
- Same as "S{0,1}?", "S{0,BIG_NUMBER}?",
"S{1,BIG_NUMBER}?" respectively.
- "(?>S)"
- Matches the best match for "S" and only that.
- "(?=S)", "(?<=S)"
- Only the best match for "S" is considered. (This is important
only if "S" has capturing parentheses, and backreferences are
used somewhere else in the whole regular expression.)
- "(?!S)", "(?<!S)"
- For this grouping operator there is no need to describe the ordering,
since only whether or not "S" can match is important.
- "(??{ EXPR })", "(?PARNO)"
- The ordering is the same as for the regular expression which is the result
of EXPR, or the pattern contained by capture group PARNO.
- "(?(condition)yes-pattern|no-pattern)"
- Recall that which of "yes-pattern" or "no-pattern"
actually matches is already determined. The ordering of the matches is the
same as for the chosen subexpression.
The above recipes describe the ordering of matches
at a given position.
One more rule is needed to understand how a match is determined for the whole
regular expression: a match at an earlier position is always better than a
match at a later position.
Creating Custom RE Engines¶
As of Perl 5.10.0, one can create custom regular expression engines. This is not
for the faint of heart, as they have to plug in at the C level. See perlreapi
for more details.
As an alternative, overloaded constants (see overload) provide a simple way to
extend the functionality of the RE engine, by substituting one pattern for
another.
Suppose that we want to enable a new RE escape-sequence "\Y|" which
matches at a boundary between whitespace characters and non-whitespace
characters. Note that "(?=\S)(?<!\S)|(?!\S)(?<=\S)" matches
exactly at these positions, so we want to have each "\Y|" in the
place of the more complicated version. We can create a module
"customre" to do this:
package customre;
use overload;
sub import {
shift;
die "No argument to customre::import allowed" if @_;
overload::constant 'qr' => \&convert;
}
sub invalid { die "/$_[0]/: invalid escape '\\$_[1]'"}
# We must also take care of not escaping the legitimate \\Y|
# sequence, hence the presence of '\\' in the conversion rules.
my %rules = ( '\\' => '\\\\',
'Y|' => qr/(?=\S)(?<!\S)|(?!\S)(?<=\S)/ );
sub convert {
my $re = shift;
$re =~ s{
\\ ( \\ | Y . )
}
{ $rules{$1} or invalid($re,$1) }sgex;
return $re;
}
Now "use customre" enables the new escape in constant regular
expressions, i.e., those without any runtime variable interpolations. As
documented in overload, this conversion will work only over literal parts of
regular expressions. For "\Y|$re\Y|" the variable part of this
regular expression needs to be converted explicitly (but only if the special
meaning of "\Y|" should be enabled inside $re):
use customre;
$re = <>;
chomp $re;
$re = customre::convert $re;
/\Y|$re\Y|/;
Embedded Code Execution Frequency¶
The exact rules for how often (??{}) and (?{}) are executed in a pattern are
unspecified. In the case of a successful match you can assume that they DWIM
and will be executed in left to right order the appropriate number of times in
the accepting path of the pattern as would any other meta-pattern. How
non-accepting pathways and match failures affect the number of times a pattern
is executed is specifically unspecified and may vary depending on what
optimizations can be applied to the pattern and is likely to change from
version to version.
For instance in
"aaabcdeeeee"=~/a(?{print "a"})b(?{print "b"})cde/;
the exact number of times "a" or "b" are printed out is
unspecified for failure, but you may assume they will be printed at least once
during a successful match, additionally you may assume that if "b"
is printed, it will be preceded by at least one "a".
In the case of branching constructs like the following:
/a(b|(?{ print "a" }))c(?{ print "c" })/;
you can assume that the input "ac" will output "ac", and
that "abc" will output only "c".
When embedded code is quantified, successful matches will call the code once for
each matched iteration of the quantifier. For example:
"good" =~ /g(?:o(?{print "o"}))*d/;
will output "o" twice.
PCRE/Python Support¶
As of Perl 5.10.0, Perl supports several Python/PCRE-specific extensions to the
regex syntax. While Perl programmers are encouraged to use the Perl-specific
syntax, the following are also accepted:
- "(?P<NAME>pattern)"
- Define a named capture group. Equivalent to
"(?<NAME>pattern)".
- "(?P=NAME)"
- Backreference to a named capture group. Equivalent to
"\g{NAME}".
- "(?P>NAME)"
- Subroutine call to a named capture group. Equivalent to
"(?&NAME)".
BUGS¶
Many regular expression constructs don't work on EBCDIC platforms.
There are a number of issues with regard to case-insensitive matching in Unicode
rules. See "i" under "Modifiers" above.
This document varies from difficult to understand to completely and utterly
opaque. The wandering prose riddled with jargon is hard to fathom in several
places.
This document needs a rewrite that separates the tutorial content from the
reference content.
SEE ALSO¶
perlrequick.
perlretut.
"Regexp Quote-Like Operators" in perlop.
"Gory details of parsing quoted constructs" in perlop.
perlfaq6.
"pos" in perlfunc.
perllocale.
perlebcdic.
Mastering Regular Expressions by Jeffrey Friedl, published by O'Reilly
and Associates.