PERLRE(1) Perl Programmers Reference Guide PERLRE(1)
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 "$" from matching the start or end of the string
to matching the start or end of any line anywhere within the string.
s Treat string as single line. That is, change "." to match any character whatsoever, even a newline, which
normally it would not match.
Used together, as "/ms", they let the "." match any character whatsoever, while still allowing "^" and "$"
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 an inverted bracketed character class, which otherwise could be
highly confusing. See "Negation" in perlrecharclass.
Another bug involves character classes that match both a sequence of multiple characters, and an initial
sub-string of that sequence. For example,
/[s\xDF]/i
should match both a single and a double "s", since "\xDF" (on ASCII platforms) matches "ss". However,
this bug ([perl #89774] <https://rt.perl.org/rt3/Ticket/Display.html?id=89774>) causes it to only match a
single "s", even if the final larger match fails, and matching the double "ss" would have succeeded.
flags affect the way the regex is used rather than the regex itself. See "Using regular expressions in
Perl" in perlretut for further explanation of the g and c modifiers.
a, d, l and u
These modifiers, all new in 5.14, affect which character-set semantics (Unicode, etc.) are used, as
described below in "Character set modifiers".
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
character class. You can use this to break up your regular expression into (slightly) more readable parts.
The "#" character is also treated as a metacharacter introducing a comment, just as in ordinary Perl code.
This also means that if you want real whitespace or "#" characters in the pattern (outside a character class,
where they are 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. Taken together, these features go a long way towards making
Perl's regular expressions more readable. Note that you have to be careful not to include the pattern
delimiter in the comment--perl has no way of knowing you did not intend to close the pattern early. See the
C-comment deletion code in perlop. Also 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 ":", but can 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 semantics 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
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.
Perl only supports single-byte locales. This means that code points above 255 are treated as Unicode no
matter what locale is in effect. Under Unicode rules, there are a few case-insensitive matches that cross the
255/256 boundary. 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.
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 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{...}")
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".
On ASCII platforms, the native rules are ASCII, and on EBCDIC platforms (at least the ones that Perl handles),
they are Latin-1.
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]"; "\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
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 explictly 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
(Older programs did this by setting $*, but this option was removed in perl 5.9.)
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.
However, in Perl v5.18, it is planned to issue a deprecation warning for all such occurrences, and in Perl
v5.20 to require literal uses of a curly bracket 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 of quantifiers. Now, a typo
in a quantifier silently causes it to be treated as the literal characters. For example,
/o{4,3}/
looks like a quantifier that matches 0 times, since 4 is greater than 3, but it really means to match the
sequence of six characters "o { 4 , 3 }".)
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
By default, 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
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:
/"(?>(?:(?>[^"\\]+)|\\.)*)"/
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 till \E (think vi)
\U uppercase till \E (think vi)
\Q quote (disable) pattern metacharacters till \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.
\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
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 $&
\N [7] Any character but \n (experimental). Not affected by
/s modifier
\v [3] Vertical whitespace
\V [3] Not vertical whitespace
\h [3] Horizontal whitespace
\H [3] Not horizontal whitespace
\R [4] Linebreak
[1] See "Bracketed Character Classes" in perlrecharclass for details.
[2] See "POSIX Character Classes" in perlrecharclass for details.
[3] See "Backslash sequences" in perlrecharclass for details.
[4] See "Misc" in perlrebackslash for details.
[5] See "Capture groups" below for details.
[6] See "Extended Patterns" below for details.
[7] Note that "\N" has two meanings. When of the form "\N{NAME}", it matches the character or character
sequence whose name is "NAME"; and similarly when of the form "\N{U+hex}", it matches the character whose
Unicode code point is hex. Otherwise it matches any character but "\n".
Assertions
Perl defines the following zero-width assertions:
\b Match a word boundary
\B Match except at a word boundary
\A Match only at beginning of string
\Z Match only at end of string, or before newline at the end
\z Match only at end of string
\G Match only at pos() (e.g. at the end-of-match position
of prior m//g)
A word boundary ("\b") is a spot between two characters that has a "\w" on one side of it and a "\W" on the
other side of it (in either order), counting the imaginary characters off the beginning and end of the string
as matching a "\W". (Within character classes "\b" represents backspace rather than a word boundary, just as
it normally does in any double-quoted string.) The "\A" and "\Z" are just like "^" and "$", 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
that include "\G" in an alternation.
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.
$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: 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
\\, \(, \), \<, \>, \{, 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. If the "/x" modifier enables whitespace formatting, a simple "#" will
suffice. Note that Perl closes the comment as soon as it sees a ")", so there is no way to put a literal
")" in the comment.
"(?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/ ) { }
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.
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", which causes the regex engine to "keep"
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: This extended regular expression feature is considered experimental, and may be changed without
notice. 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.
This zero-width assertion evaluates any embedded Perl code. It always succeeds, and its "code" is not
The "code" is properly scoped in the following sense: If the assertion is backtracked (compare
"Backtracking"), all changes introduced after "local"ization are undone, so that
$_ = 'a' x 8;
m<
(?{ $cnt = 0 }) # Initialize $cnt.
(
a
(?{
local $cnt = $cnt + 1; # Update $cnt,
# backtracking-safe.
})
)*
aaaa
(?{ $res = $cnt }) # On success copy to
# non-localized location.
>x;
will set "$res = 4". Note that after the match, $cnt returns to the globally introduced value, because
the scopes that restrict "local" operators are unwound.
This assertion may be used as 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".
For reasons of security, this construct is forbidden if the regular expression involves run-time
interpolation of variables, unless the perilous "use re 'eval'" pragma has been used (see re), or the
variables contain results of the "qr//" operator (see "qr/STRING/msixpodual" in perlop).
This restriction is due to the wide-spread and remarkably convenient custom of using run-time determined
strings as patterns. For example:
$re = <>;
chomp $re;
$string =~ /$re/;
Before Perl knew how to execute interpolated code within a pattern, this operation was completely safe
from a security point of view, although it could raise an exception from an illegal pattern. If you turn
on the "use re 'eval'", though, it is no longer secure, so you should only do so if you are also using
taint checking. Better yet, use the carefully constrained evaluation within a Safe compartment. See
perlsec for details about both these mechanisms.
WARNING: Use of lexical ("my") variables in these blocks is broken. The result is unpredictable and will
make perl unstable. The workaround is to use global ("our") variables.
WARNING: In perl 5.12.x and earlier, the regex engine was not re-entrant, so interpolated code could not
safely invoke the regex engine either directly with "m//" or "s///"), or indirectly with functions such as
"split". Invoking the regex engine in these blocks would make perl unstable.
"(??{ code })"
WARNING: This extended regular expression feature is considered experimental, and may be changed without
will match, it will not set $1.
The "code" is not interpolated. As before, the rules to determine where the "code" ends are currently
somewhat convoluted.
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.
For reasons of security, this construct is forbidden if the regular expression involves run-time
interpolation of variables, unless the perilous "use re 'eval'" pragma has been used (see re), or the
variables contain results of the "qr//" operator (see "qr/STRING/msixpodual" in perlop).
In perl 5.12.x and earlier, because the regex engine was not re-entrant, delayed code could not safely
invoke the regex engine either directly with "m//" or "s///"), or indirectly with functions such as
"split".
Recursing deeper than 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)"
Similar to "(??{ code })" except it does not involve compiling any code, instead it treats the contents of
a capture group as an independent pattern that must match at the current position. Capture groups
contained by the pattern will have the value as determined by the outermost recursion.
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
"\$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 either an integer in parentheses (which is valid if the corresponding pair of
parentheses matched), a look-ahead/look-behind/evaluate zero-width assertion, a name in angle brackets or
single quotes (which is valid if a group with the given name matched), or 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.
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 optimiser
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>....)
(?<ADRESS_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;
"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:
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})
Special Backtracking Control Verbs
WARNING: These patterns are experimental and subject to change or removal in a future version of Perl. Their
usage in production code should be noted to avoid problems during upgrades.
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:
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.
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 (*THEN) D ) /
is not the same as
/ ( A (*PRUNE) B | C (*PRUNE) D ) /
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
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.
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:
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.
"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, \xnn, where nn
are hexadecimal digits, matches the character whose ordinal is nn. The expression \cx 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 \gn. 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:
$pattern =~ s/(\W)/\\\1/g;
This is grandfathered (for \1 to \9) for the RHS of a substitute to avoid shocking the sed addicts, but it's a
dirty habit to get into. That's because in PerlThink, the righthand side of an "s///" is a double-quoted
string. "\1" in the usual double-quoted string means a control-A. The customary Unix meaning of "\1" is
kludged in for "s///". However, if you get into the habit of doing that, you get yourself into trouble if you
then add an "/e" modifier.
s/(\d+)/ \1 + 1 /eg; # causes warning under -w
comes together with the ability to wreak havoc.
A common abuse of this power stems from the ability to make infinite loops using regular expressions, with
something as innocuous as:
'foo' =~ m{ ( o? )* }x;
The "o?" matches at the beginning of 'foo', and since the position in the string is not moved by the match,
"o?" would match again and again because of the "*" quantifier. Another common way to create a similar cycle
is with the looping modifier "//g":
@matches = ( 'foo' =~ m{ o? }xg );
or
print "match: <$&>\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
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)", "(?<=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,$1) }sgex;
/\Y|$re\Y|/;
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.
perl v5.16.3 2013-03-04 PERLRE(1)
Back to main site | Back to man page index