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.
Also, Perl matching doesn't fully conform to the current Unicode
"/i" recommendations, which ask that the matching be made upon the
NFD (Normalization Form Decomposed) of the text. However, Unicode
is in the process of reconsidering and revising their
recommendations.
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.
g and c
Global matching, and keep the Current position after failed
matching. Unlike i, m, s and x, these two 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 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.
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.
On the EBCDIC platforms that Perl handles, the native character set is
equivalent to Latin-1. Thus this modifier changes behavior only when
the "/i" modifier is also specified, and it turns out it affects only
two characters, giving them full Unicode semantics: the "MICRO SIGN"
will match the Greek capital and small letters "MU", otherwise not; and
the "LATIN CAPITAL LETTER SHARP S" will match any of "SS", "Ss", "sS",
and "ss", otherwise not.
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{...}")
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
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 semantics; 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 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
. Match any character (except newline)
$ Match the end of the line (or before newline at the end)
| 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 5.9.)
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
?+ 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:
/"(?>(?:(?>[^"\\]+)|\\.)*)"/
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
\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.
\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 $&
\N [7] Any character but \n (experimental). Not affected by
/s modifier
\v [3] Vertical whitespace
\V [3] Not vertical whitespace
\h [3] Horizontal whitespace
\H [3] Not horizontal whitespace
\R [4] Linebreak
[1] See "Bracketed Character Classes" in perlrecharclass for details.
[2] See "POSIX Character Classes" in perlrecharclass for details.
[3] See "Backslash sequences" in perlrecharclass for details.
[4] See "Misc" in perlrebackslash for details.
[5] See "Capture groups" below for details.
[6] See "Extended Patterns" below for details.
[7] Note that "\N" has two meanings. When of the form "\N{NAME}", it
matches the character or character sequence whose name is "NAME";
and similarly when of the form "\N{U+hex}", it matches the
character whose Unicode code point is hex. Otherwise it matches
any character but "\n".
Assertions
Perl defines the following zero-width assertions:
\b Match a word boundary
\B Match except at a word boundary
\A Match only at beginning of string
\Z Match only at end of string, or before newline at the end
\z Match only at end of string
\G Match only at pos() (e.g. at the end-of-match position
of prior m//g)
A word boundary ("\b") is a spot between two characters that has a "\w"
on one side of it and a "\W" on the other side of it (in either order),
counting the imaginary characters off the beginning and end of the
string as matching a "\W". (Within character classes "\b" represents
backspace rather than a word boundary, just as it normally does in any
double-quoted string.) The "\A" and "\Z" are just like "^" and "$",
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 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.
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: 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. As of 5.005, $& is not so costly as the
other two.
As a workaround for this problem, Perl 5.10.0 introduces
"${^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.
Quoting metacharacters
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. 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/ ) { }
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", 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: 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 interpolated. Currently,
the rules to determine where the "code" ends are somewhat
convoluted.
This feature can be used together with the special variable $^N 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";
Inside the "(?{...})" 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" 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 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 is a "postponed" regular subexpression. The "code" is
evaluated at run time, at the moment this subexpression may match.
The result of evaluation is considered a regular expression and
matched as if it were inserted instead of this construct. Note
that this means that the contents of capture groups defined inside
an eval'ed pattern are not available outside of the pattern, and
vice versa, there is no way for the inner pattern returned from the
code block 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,
('a' x 100)=~/(??{'(.)' x 100})/
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)
\(
( # paren group 3 (contents of parens)
(?:
(?> [^()]+ ) # Non-parens without backtracking
|
(?2) # Recurse to start of paren group 2
)*
)
\)
)
)
}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 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.
(?=...) (?!...) (?<=...) (?<!...)
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 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;
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})
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:
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 $REGERROR variable will be set to the name of the most
recently executed "(*MARK:NAME)".
See "(*SKIP)" for more details.
As a shortcut "(*MARK:NAME)" can be written "(*:NAME)".
"(*THEN)" "(*THEN:NAME)"
This is similar to the "cut group" operator "::" from Perl 6.
Like "(*PRUNE)", this verb always matches, and when backtracked
into on failure, it causes the regex engine to try the next
alternation in the innermost enclosing group (capturing or
otherwise) that has alternations. The two branches of a
"(?(condition)yes-pattern|no-pattern)" do not count as an
alternation, as far as "(*THEN)" is concerned.
Its name comes from the observation that this operation combined
with the alternation operator ("|") can be used to create what
is essentially a pattern-based if/then/else block:
( COND (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ )
Note that if this operator is used and NOT inside of an
alternation then it acts exactly like the "(*PRUNE)" operator.
/ A (*PRUNE) B /
is the same as
/ A (*THEN) B /
but
/ ( A (*THEN) B | C (*THEN) D ) /
is not the same as
/ ( A (*PRUNE) B | C (*PRUNE) D ) /
as after matching the A but failing on the B the "(*THEN)" verb
will backtrack and try C; but the "(*PRUNE)" verb will simply
fail.
Verbs without an argument
"(*COMMIT)"
This is the Perl 6 "commit pattern" "<commit>" or ":::". It's a
zero-width pattern similar to "(*SKIP)", except that when
backtracked into on failure it causes the match to fail
outright. No further attempts to find a valid match by advancing
the start pointer will occur again. For example,
'aaabaaab' =~ /a+b?(*COMMIT)(?{print "$&\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 equivalent to "(?!)", but
easier to read. In fact, "(?!)" gets optimised into "(*FAIL)"
internally.
It is probably useful only when combined with "(?{})" or
"(??{})".
"(*ACCEPT)"
WARNING: This feature is highly experimental. It is not
recommended for production code.
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, \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:
$pattern =~ s/(\W)/\\\1/g;
This is grandfathered (for \1 to \9) for the RHS of a substitute to
avoid shocking the sed addicts, but it's a dirty habit to get into.
That's because in PerlThink, the righthand side of an "s///" is a
double-quoted string. "\1" in the usual double-quoted string means a
control-A. The customary Unix meaning of "\1" is kludged in for
"s///". However, if you get into the habit of doing that, you get
yourself into trouble if you then add an "/e" modifier.
s/(\d+)/ \1 + 1 /eg; # causes warning under -w
Or if you try to do
s/(\d+)/\1000/;
You can't disambiguate that by saying "\{1}000", whereas you can fix it
with "${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|/;
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)
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