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 tutorial introduction
is available in perlretut. If you know just a little about them, a
quick-start introduction is available in perlrequick.
Except for "The Basics" section, this page assumes you are familiar
with regular expression basics, like what is a "pattern", what does it
look like, and how it is basically used. For a reference on how they
are used, plus various examples of the same, see discussions of "m//",
"s///", "qr//" and "??" in "Regexp Quote-Like Operators" in perlop.
New in v5.22, "use re 'strict'" applies stricter rules than otherwise
when compiling regular expression patterns. It can find things that,
while legal, may not be what you intended.
The Basics
Regular expressions are strings with the very particular syntax and
meaning described in this document and auxiliary documents referred to
by this one. The strings are called "patterns". Patterns are used to
determine if some other string, called the "target", has (or doesn't
have) the characteristics specified by the pattern. We call this
"matching" the target string against the pattern. Usually the match is
done by having the target be the first operand, and the pattern be the
second operand, of one of the two binary operators "=~" and "!~",
listed in "Binding Operators" in perlop; and the pattern will have been
converted from an ordinary string by one of the operators in "Regexp
Quote-Like Operators" in perlop, like so:
$foo =~ m/abc/
This evaluates to true if and only if the string in the variable $foo
contains somewhere in it, the sequence of characters "a", "b", then
"c". (The "=~ m", or match operator, is described in
"m/PATTERN/msixpodualngc" in perlop.)
Patterns that aren't already stored in some variable must be
delimitted, at both ends, by delimitter characters. These are often,
as in the example above, forward slashes, and the typical way a pattern
is written in documentation is with those slashes. In most cases, the
delimitter is the same character, fore and aft, but there are a few
cases where a character looks like it has a mirror-image mate, where
the opening version is the beginning delimiter, and the closing one is
the ending delimiter, like
$foo =~ m<abc>
Most times, the pattern is evaluated in double-quotish context, but it
is possible to choose delimiters to force single-quotish, like
$foo =~ m'abc'
If the pattern contains its delimiter within it, that delimiter must be
escaped. Prefixing it with a backslash (e.g., "/foo\/bar/") serves
this purpose.
Any single character in a pattern matches that same character in the
target string, unless the character is a metacharacter with a special
meaning described in this document. A sequence of non-metacharacters
matches the same sequence in the target string, as we saw above with
"m/abc/".
Only a few characters (all of them being ASCII punctuation characters)
are metacharacters. The most commonly used one is a dot ".", which
normally matches almost any character (including a dot itself).
You can cause characters that normally function as metacharacters to be
interpreted literally by prefixing them with a "\", just like the
pattern's delimiter must be escaped if it also occurs within the
pattern. Thus, "\." matches just a literal dot, "." instead of its
normal meaning. This means that the backslash is also a metacharacter,
so "\\" matches a single "\". And a sequence that contains an escaped
metacharacter matches the same sequence (but without the escape) in the
target string. So, the pattern "/blur\\fl/" would match any target
string that contains the sequence "blur\fl".
The metacharacter "|" is used to match one thing or another. Thus
$foo =~ m/this|that/
is TRUE if and only if $foo contains either the sequence "this" or the
sequence "that". Like all metacharacters, prefixing the "|" with a
backslash makes it match the plain punctuation character; in its case,
the VERTICAL LINE.
$foo =~ m/this\|that/
is TRUE if and only if $foo contains the sequence "this|that".
You aren't limited to just a single "|".
$foo =~ m/fee|fie|foe|fum/
is TRUE if and only if $foo contains any of those 4 sequences from the
children's story "Jack and the Beanstalk".
As you can see, the "|" binds less tightly than a sequence of ordinary
characters. We can override this by using the grouping metacharacters,
the parentheses "(" and ")".
$foo =~ m/th(is|at) thing/
is TRUE if and only if $foo contains either the sequence "thisthing" or
the sequence "thatthing". The portions of the string that match the
portions of the pattern enclosed in parentheses are normally made
available separately for use later in the pattern, substitution, or
program. This is called "capturing", and it can get complicated. See
"Capture groups".
The first alternative includes everything from the last pattern
delimiter ("(", "(?:" (described later), 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.)
Besides taking away the special meaning of a metacharacter, a prefixed
backslash changes some letter and digit characters away from matching
just themselves to instead have special meaning. These are called
"escape sequences", and all such are described in perlrebackslash. A
backslash sequence (of a letter or digit) that doesn't currently have
special meaning to Perl will raise a warning if warnings are enabled,
as those are reserved for potential future use.
One such sequence is "\b", which matches a boundary of some sort.
"\b{wb}" and a few others give specialized types of boundaries. (They
are all described in detail starting at "\b{}, \b, \B{}, \B" in
perlrebackslash.) Note that these don't match characters, but the
zero-width spaces between characters. They are an example of a zero-
width assertion. Consider again,
$foo =~ m/fee|fie|foe|fum/
It evaluates to TRUE if, besides those 4 words, any of the sequences
"feed", "field", "Defoe", "fume", and many others are in $foo. By
judicious use of "\b" (or better (because it is designed to handle
natural language) "\b{wb}"), we can make sure that only the Giant's
words are matched:
$foo =~ m/\b(fee|fie|foe|fum)\b/
$foo =~ m/\b{wb}(fee|fie|foe|fum)\b{wb}/
The final example shows that the characters "{" and "}" are
metacharacters.
Another use for escape sequences is to specify characters that cannot
(or which you prefer not to) be written literally. These are described
in detail in "Character Escapes" in perlrebackslash, but the next three
paragraphs briefly describe some of them.
Various control characters can be written in C language style: "\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 native code point is nnn. You can easily
run into trouble if you don't have exactly three digits. So always use
three, or since Perl 5.14, you can use "\o{...}" to specify any number
of octal digits.
Similarly, "\xnn", where nn are hexadecimal digits, matches the
character whose native ordinal is nn. Again, not using exactly two
digits is a recipe for disaster, but you can use "\x{...}" to specify
any number of hex digits.
Besides being a metacharacter, the "." is an example of a "character
class", something that can match any single character of a given set of
them. In its case, the set is just about all possible characters.
Perl predefines several character classes besides the "."; there is a
separate reference page about just these, perlrecharclass.
You can define your own custom character classes, by putting into your
pattern in the appropriate place(s), a list of all the characters you
want in the set. You do this by enclosing the list within "[]" bracket
characters. These are called "bracketed character classes" when we are
being precise, but often the word "bracketed" is dropped. (Dropping it
usually doesn't cause confusion.) This means that the "[" character is
another metacharacter. It doesn't match anything just by itself; it is
used only to tell Perl that what follows it is a bracketed character
class. If you want to match a literal left square bracket, you must
escape it, like "\[". The matching "]" is also a metacharacter; again
it doesn't match anything by itself, but just marks the end of your
custom class to Perl. It is an example of a "sometimes metacharacter".
It isn't a metacharacter if there is no corresponding "[", and matches
its literal self:
print "]" =~ /]/; # prints 1
The list of characters within the character class gives the set of
characters matched by the class. "[abc]" matches a single "a" or "b"
or "c". But if the first character after the "[" is "^", the class
instead matches any character not in the list. Within a list, the "-"
character specifies a range of characters, 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.)
There is lots more to bracketed character classes; full details are in
"Bracketed Character Classes" in perlrecharclass.
Metacharacters
"The Basics" introduced some of the metacharacters. This section gives
them all. Most of them have the same meaning as in the egrep command.
Only the "\" is always a metacharacter. The others are metacharacters
just sometimes. The following tables lists all of them, summarizes
their use, and gives the contexts where they are metacharacters.
Outside those contexts or if prefixed by a "\", they match their
corresponding punctuation character. In some cases, their meaning
varies depending on various pattern modifiers that alter the default
behaviors. See "Modifiers".
PURPOSE WHERE
\ Escape the next character Always, except when
escaped by another \
^ Match the beginning of the string Not in []
(or line, if /m is used)
^ Complement the [] class At the beginning of []
. Match any single character except newline Not in []
(under /s, includes newline)
$ Match the end of the string Not in [], but can
(or before newline at the end of the mean interpolate a
string; or before any newline if /m is scalar
used)
| Alternation Not in []
() Grouping Not in []
[ Start Bracketed Character class Not in []
] End Bracketed Character class Only in [], and
not first
* Matches the preceding element 0 or more Not in []
times
+ Matches the preceding element 1 or more Not in []
times
? Matches the preceding element 0 or 1 Not in []
times
{ Starts a sequence that gives number(s) Not in []
of times the preceding element can be
matched
{ when following certain escape sequences
starts a modifier to the meaning of the
sequence
} End sequence started by {
- Indicates a range Only in [] interior
# Beginning of comment, extends to line end Only with /x modifier
Notice that most of the metacharacters lose their special meaning when
they occur in a bracketed character class, except "^" has a different
meaning when it is at the beginning of such a class. And "-" and "]"
are metacharacters only at restricted positions within bracketed
character classes; while "}" is a metacharacter only when closing a
special construct started by "{".
In double-quotish context, as is usually the case, you need to be
careful about "$" and the non-metacharacter "@". Those could
interpolate variables, which may or may not be what you intended.
These rules were designed for compactness of expression, rather than
legibility and maintainability. The "/x and /xx" pattern modifiers
allow you to insert white space to improve readability. And use of
"re'strict'" adds extra checking to catch some typos that might
silently compile into something unintended.
By default, the "^" character is guaranteed to match only the beginning
of the string, the "$" character only the end (or before the newline at
the end), and Perl does certain optimizations with the assumption that
the string contains only one line. Embedded newlines will not be
matched by "^" or "$". You may, however, wish to treat a string as a
multi-line buffer, such that the "^" will match after any newline
within the string (except if the newline is the last character in the
string), and "$" will match before any newline. At the cost of a
little more overhead, you can do this by using the ""/m"" modifier on
the pattern match operator. (Older programs did this by setting $*,
but this option was removed in perl 5.10.)
To simplify multi-line substitutions, the "." character never matches a
newline unless you use the "/s" modifier, which in effect tells Perl to
pretend the string is a single line--even if it isn't.
Modifiers
Overview
The default behavior for matching can be changed, using various
modifiers. Modifiers that relate to the interpretation of the pattern
are listed just below. Modifiers that alter the way a pattern is used
by Perl are detailed in "Regexp Quote-Like Operators" in perlop and
"Gory details of parsing quoted constructs" in perlop.
"m" Treat the string being matched against as multiple lines. That is,
change "^" and "$" from matching the start of the string's first
line and the end of its last line to matching the start and end of
each line within the string.
"s" Treat the 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. For example, "A" will match
"a" under "/i".
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, unless the locale is a UTF-8 one. See perllocale.
There are a number of Unicode characters that match a sequence of
multiple characters under "/i". For example, "LATIN SMALL LIGATURE
FI" should match the sequence "fi". Perl is not currently able to
do this when the multiple characters are in the pattern and are
split between groupings, or when one or more are quantified. Thus
"\N{LATIN SMALL LIGATURE FI}" =~ /fi/i; # Matches
"\N{LATIN SMALL LIGATURE FI}" =~ /[fi][fi]/i; # Doesn't match!
"\N{LATIN SMALL LIGATURE FI}" =~ /fi*/i; # Doesn't match!
# The below doesn't match, and it isn't clear what $1 and $2 would
# be even if it did!!
"\N{LATIN SMALL LIGATURE FI}" =~ /(f)(i)/i; # Doesn't match!
Perl doesn't match multiple characters in a bracketed character
class unless the character that maps to them is explicitly
mentioned, and it doesn't match them at all if the character class
is inverted, which otherwise could be highly confusing. See
"Bracketed Character Classes" in perlrecharclass, and "Negation" in
perlrecharclass.
"x" and "xx"
Extend your pattern's legibility by permitting whitespace and
comments. Details in "/x and /xx"
"p" Preserve the string matched such that "${^PREMATCH}", "${^MATCH}",
and "${^POSTMATCH}" are available for use after matching.
In Perl 5.20 and higher this is ignored. Due to a new copy-on-write
mechanism, "${^PREMATCH}", "${^MATCH}", and "${^POSTMATCH}" will be
available after the match regardless of the modifier.
"a", "d", "l", and "u"
These modifiers, all new in 5.14, affect which character-set rules
(Unicode, etc.) are used, as described below in "Character set
modifiers".
"n" Prevent the grouping metacharacters "()" from capturing. This
modifier, new in 5.22, will stop $1, $2, etc... from being filled
in.
"hello" =~ /(hi|hello)/; # $1 is "hello"
"hello" =~ /(hi|hello)/n; # $1 is undef
This is equivalent to putting "?:" at the beginning of every
capturing group:
"hello" =~ /(?:hi|hello)/; # $1 is undef
"/n" can be negated on a per-group basis. Alternatively, named
captures may still be used.
"hello" =~ /(?-n:(hi|hello))/n; # $1 is "hello"
"hello" =~ /(?<greet>hi|hello)/n; # $1 is "hello", $+{greet} is
# "hello"
Other Modifiers
There are a number of flags that can be found at the end of regular
expression constructs that are not generic regular expression
flags, but apply to the operation being performed, like matching or
substitution ("m//" or "s///" respectively).
Flags described further in "Using regular expressions in Perl" in
perlretut are:
c - keep the current position during repeated matching
g - globally match the pattern repeatedly in the string
Substitution-specific modifiers described in
"s/PATTERN/REPLACEMENT/msixpodualngcer" in perlop are:
e - evaluate the right-hand side as an expression
ee - evaluate the right side as a string then eval the result
o - pretend to optimize your code, but actually introduce bugs
r - perform non-destructive substitution and return the new value
Regular expression modifiers are usually written in documentation as
e.g., "the "/x" modifier", even though the delimiter in question might
not really be a slash. The modifiers "/imnsxadlup" may also be
embedded within the regular expression itself using the "(?...)"
construct, see "Extended Patterns" below.
Details on some modifiers
Some of the modifiers require more explanation than given in the
"Overview" above.
"/x" and "/xx"
A single "/x" tells the regular expression parser to ignore most
whitespace that is neither backslashed nor within a bracketed character
class. You can use this to break up your regular expression into more
readable parts. Also, the "#" character is treated as a metacharacter
introducing a comment that runs up to the pattern's closing delimiter,
or to the end of the current line if the pattern extends onto the next
line. Hence, this is very much like an ordinary Perl code comment.
(You can include the closing delimiter within the comment only if you
precede it with a backslash, so be careful!)
Use of "/x" means that if you want real whitespace or "#" characters in
the pattern (outside a bracketed character class, which is unaffected
by "/x"), then you'll either have to escape them (using backslashes or
"\Q...\E") or encode them using octal, hex, or "\N{}" escapes. It is
ineffective to try to continue a comment onto the next line by escaping
the "\n" with a backslash or "\Q".
You can use "(?#text)" to create a comment that ends earlier than the
end of the current line, but "text" also can't contain the closing
delimiter unless escaped with a backslash.
A common pitfall is to forget that "#" characters begin a comment under
"/x" and are not matched literally. Just keep that in mind when trying
to puzzle out why a particular "/x" pattern isn't working as expected.
Starting in Perl v5.26, if the modifier has a second "x" within it, it
does everything that a single "/x" does, but additionally non-
backslashed SPACE and TAB characters within bracketed character classes
are also generally ignored, and hence can be added to make the classes
more readable.
/ [d-e g-i 3-7]/xx
/[ ! @ " # $ % ^ & * () = ? <> ' ]/xx
may be easier to grasp than the squashed equivalents
/[d-eg-i3-7]/
/[!@"#$%^&*()=?<>']/
Taken together, these features go a long way towards making Perl's
regular expressions more readable. Here's an example:
# Delete (most) C comments.
$program =~ s {
/\* # Match the opening delimiter.
.*? # Match a minimal number of characters.
\*/ # Match the closing delimiter.
} []gsx;
Note that anything inside a "\Q...\E" stays unaffected by "/x". And
note that "/x" doesn't affect space interpretation within a single
multi-character construct. For example in "\x{...}", regardless of the
"/x" modifier, there can be no spaces. Same for a quantifier such as
"{3}" or "{5,}". Similarly, "(?:...)" can't have a space between the
"(", "?", and ":". Within any delimiters for such a construct, allowed
spaces are not affected by "/x", and depend on the construct. For
example, "\x{...}" can't have spaces because hexadecimal numbers don't
have spaces in them. But, Unicode properties can have spaces, so in
"\p{...}" there can be spaces that follow the Unicode rules, for which
see "Properties accessible through \p{} and \P{}" in perluniprops.
The set of characters that are deemed whitespace are those that Unicode
calls "Pattern White Space", namely:
U+0009 CHARACTER TABULATION
U+000A LINE FEED
U+000B LINE TABULATION
U+000C FORM FEED
U+000D CARRIAGE RETURN
U+0020 SPACE
U+0085 NEXT LINE
U+200E LEFT-TO-RIGHT MARK
U+200F RIGHT-TO-LEFT MARK
U+2028 LINE SEPARATOR
U+2029 PARAGRAPH SEPARATOR
Character set modifiers
"/d", "/u", "/a", and "/l", available starting in 5.14, are called the
character set modifiers; they affect the character set rules used for
the regular expression.
The "/d", "/u", and "/l" modifiers are not likely to be of much use to
you, and so you need not worry about them very much. They exist for
Perl's internal use, so that complex regular expression data structures
can be automatically serialized and later exactly reconstituted,
including all their nuances. But, since Perl can't keep a secret, and
there may be rare instances where they are useful, they are documented
here.
The "/a" modifier, on the other hand, may be useful. Its purpose is to
allow code that is to work mostly on ASCII data to not have to concern
itself with Unicode.
Briefly, "/l" sets the character set to that of whatever Locale is in
effect at the time of the execution of the pattern match.
"/u" sets the character set to Unicode.
"/a" also sets the character set to Unicode, BUT adds several
restrictions for ASCII-safe matching.
"/d" is the old, problematic, pre-5.14 Default character set behavior.
Its only use is to force that old behavior.
At any given time, exactly one of these modifiers is in effect. Their
existence allows Perl to keep the originally compiled behavior of a
regular expression, regardless of what rules are in effect when it is
actually executed. And if it is interpolated into a larger regex, the
original's rules continue to apply to it, and don't affect the other
parts.
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 gives 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.
Prior to v5.20, Perl did not support multi-byte locales. Starting
then, UTF-8 locales are supported. No other multi byte locales are
ever likely to be supported. However, in all locales, one can have
code points above 255 and these will always be 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. Except for UTF-8 locales in Perls v5.20
and later, these are disallowed under "/l". For example, 0xFF (on
ASCII platforms) does not caselessly match the character at 0x178,
"LATIN CAPITAL LETTER Y WITH DIAERESIS", because 0xFF may not be "LATIN
SMALL LETTER Y WITH DIAERESIS" in the current locale, and Perl has no
way of knowing if that character even exists in the locale, much less
what code point it is.
In a UTF-8 locale in v5.20 and later, the only visible difference
between locale and non-locale in regular expressions should be tainting
(see perlsec).
This modifier may be specified to be the default by "use locale", but
see "Which character set modifier is in effect?".
/u
means to use Unicode rules when pattern matching. On ASCII platforms,
this means that the code points between 128 and 255 take on their
Latin-1 (ISO-8859-1) meanings (which are the same as Unicode's).
(Otherwise Perl considers their meanings to be undefined.) Thus, under
this modifier, the ASCII platform effectively becomes a Unicode
platform; and hence, for example, "\w" will match any of the more than
100_000 word characters in Unicode.
Unlike most locales, which are specific to a language and country pair,
Unicode classifies all the characters that are letters somewhere in the
world as "\w". For example, your locale might not think that "LATIN
SMALL LETTER ETH" is a letter (unless you happen to speak Icelandic),
but Unicode does. Similarly, all the characters that are decimal
digits somewhere in the world will match "\d"; this is hundreds, not
10, possible matches. And some of those digits look like some of the
10 ASCII digits, but mean a different number, so a human could easily
think a number is a different quantity than it really is. For example,
"BENGALI DIGIT FOUR" (U+09EA) looks very much like an "ASCII DIGIT
EIGHT" (U+0038), and "LEPCHA DIGIT SIX" (U+1C46) looks very much like
an "ASCII DIGIT FIVE" (U+0035). And, "\d+", may match strings of
digits that are a mixture from different writing systems, creating a
security issue. A fraudulent website, for example, could display the
price of something using U+1C46, and it would appear to the user that
something cost 500 units, but it really costs 600. A browser that
enforced script runs ("Script Runs") would prevent that fraudulent
display. "num()" in Unicode::UCD can also be used to sort this out.
Or the "/a" modifier can be used to force "\d" to match just the ASCII
0 through 9.
Also, under this modifier, case-insensitive matching works on the full
set of Unicode characters. The "KELVIN SIGN", for example matches the
letters "k" and "K"; and "LATIN SMALL LIGATURE FF" matches the sequence
"ff", which, if you're not prepared, might make it look like a
hexadecimal constant, presenting another potential security issue. See
<http://unicode.org/reports/tr36> for a detailed discussion of Unicode
security issues.
This modifier may be specified to be the default by "use feature
'unicode_strings", "use locale ':not_characters'", or "use 5.012" (or
higher), but see "Which character set modifier is in effect?".
/d
This modifier means to use the "Default" native rules of the platform
except when there is cause to use Unicode rules instead, as follows:
1. the target string is encoded in UTF-8; or
2. the pattern is encoded in UTF-8; or
3. the pattern explicitly mentions a code point that is above 255 (say
by "\x{100}"); or
4. the pattern uses a Unicode name ("\N{...}"); or
5. the pattern uses a Unicode property ("\p{...}" or "\P{...}"); or
6. the pattern uses a Unicode break ("\b{...}" or "\B{...}"); or
7. the pattern uses ""(?[ ])""
8. the pattern uses "(*script_run: ...)"
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 (without swearing) name
for this modifier, "Dodgy".
Unless the pattern or string are encoded in UTF-8, only ASCII
characters can match positively.
Here are some examples of how that works on an ASCII platform:
$str = "\xDF"; # $str is not in UTF-8 format.
$str =~ /^\w/; # No match, as $str isn't in UTF-8 format.
$str .= "\x{0e0b}"; # Now $str is in UTF-8 format.
$str =~ /^\w/; # Match! $str is now in UTF-8 format.
chop $str;
$str =~ /^\w/; # Still a match! $str remains in UTF-8 format.
This modifier is automatically selected by default when none of the
others are, so yet another name for it is "Default".
Because of the unexpected behaviors associated with this modifier, you
probably should only explicitly use it to maintain weird backward
compatibilities.
/a (and /aa)
This modifier stands for ASCII-restrict (or ASCII-safe). This modifier
may be doubled-up to increase its effect.
When it appears singly, it causes the sequences "\d", "\s", "\w", and
the Posix character classes to match only in the ASCII range. They
thus revert to their pre-5.6, pre-Unicode meanings. Under "/a", "\d"
always means precisely the digits "0" to "9"; "\s" means the five
characters "[ \f\n\r\t]", and starting in Perl v5.18, the vertical tab;
"\w" means the 63 characters "[A-Za-z0-9_]"; and likewise, all the
Posix classes such as "[[:print:]]" match only the appropriate ASCII-
range characters.
This modifier is useful for people who only incidentally use Unicode,
and who do not wish to be burdened with its complexities and security
concerns.
With "/a", one can write "\d" with confidence that it will only match
ASCII characters, and should the need arise to match beyond ASCII, you
can instead use "\p{Digit}" (or "\p{Word}" for "\w"). There are
similar "\p{...}" constructs that can match beyond ASCII both white
space (see "Whitespace" in perlrecharclass), and Posix classes (see
"POSIX Character Classes" in perlrecharclass). Thus, this modifier
doesn't mean you can't use Unicode, it means that to get Unicode
matching you must explicitly use a construct ("\p{}", "\P{}") that
signals Unicode.
As you would expect, this modifier causes, for example, "\D" to mean
the same thing as "[^0-9]"; in fact, all non-ASCII characters match
"\D", "\S", and "\W". "\b" still means to match at the boundary
between "\w" and "\W", using the "/a" definitions of them (similarly
for "\B").
Otherwise, "/a" behaves like the "/u" modifier, in that case-
insensitive matching uses Unicode rules; for example, "k" will match
the Unicode "\N{KELVIN SIGN}" under "/i" matching, and code points in
the Latin1 range, above ASCII will have Unicode rules when it comes to
case-insensitive matching.
To forbid ASCII/non-ASCII matches (like "k" with "\N{KELVIN SIGN}"),
specify the "a" twice, for example "/aai" or "/aia". (The first
occurrence of "a" restricts the "\d", etc., and the second occurrence
adds the "/i" restrictions.) But, note that code points outside the
ASCII range will use Unicode rules for "/i" matching, so the modifier
doesn't really restrict things to just ASCII; it just forbids the
intermixing of ASCII and non-ASCII.
To summarize, this modifier provides protection for applications that
don't wish to be exposed to all of Unicode. Specifying it twice gives
added protection.
This modifier may be specified to be the default by "use re '/a'" or
"use re '/aa'". If you do so, you may actually have occasion to use
the "/u" modifier explicitly if there are a few regular expressions
where you do want full Unicode rules (but even here, it's best if
everything were under feature "unicode_strings", along with the "use re
'/aa'"). Also see "Which character set modifier is in effect?".
Which character set modifier is in effect?
Which of these modifiers is in effect at any given point in a regular
expression depends on a fairly complex set of interactions. These have
been designed so that in general you don't have to worry about it, but
this section gives the gory details. As explained below in "Extended
Patterns" it is possible to explicitly specify modifiers that apply
only to portions of a regular expression. The innermost always has
priority over any outer ones, and one applying to the whole expression
has priority over any of the default settings that are described in the
remainder of this section.
The "use re '/foo'" pragma can be used to set default modifiers
(including these) for regular expressions compiled within its scope.
This pragma has precedence over the other pragmas listed below that
also change the defaults.
Otherwise, "use locale" sets the default modifier to "/l"; and "use
feature 'unicode_strings", or "use 5.012" (or higher) set the default
to "/u" when not in the same scope as either "use locale" or "use
bytes". ("use locale ':not_characters'" also sets the default to "/u",
overriding any plain "use locale".) Unlike the mechanisms mentioned
above, these affect operations besides regular expressions pattern
matching, and so give more consistent results with other operators,
including using "\U", "\l", etc. in substitution replacements.
If none of the above apply, for backwards compatibility reasons, the
"/d" modifier is the one in effect by default. As this can lead to
unexpected results, it is best to specify which other rule set should
be used.
Character set modifier behavior prior to Perl 5.14
Prior to 5.14, there were no explicit modifiers, but "/l" was implied
for regexes compiled within the scope of "use locale", and "/d" was
implied otherwise. However, interpolating a regex into a larger regex
would ignore the original compilation in favor of whatever was in
effect at the time of the second compilation. There were a number of
inconsistencies (bugs) with the "/d" modifier, where Unicode rules
would be used when inappropriate, and vice versa. "\p{}" did not imply
Unicode rules, and neither did all occurrences of "\N{}", until 5.12.
Regular Expressions
Quantifiers
Quantifiers are used when a particular portion of a pattern needs to
match a certain number (or numbers) of times. If there isn't a
quantifier the number of times to match is exactly one. 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 non-escaped curly bracket occurs in a context other than one of
the quantifiers listed above, where it does not form part of a
backslashed sequence like "\x{...}", it is either a fatal syntax error,
or treated as a regular character, generally with a deprecation warning
raised. To escape it, you can precede it with a backslash ("\{") or
enclose it within square brackets ("[{]"). This change will allow for
future syntax extensions (like making the lower bound of a quantifier
optional), and better error checking of quantifiers).
The "*" quantifier is equivalent to "{0,}", the "+" quantifier to
"{1,}", and the "?" quantifier to "{0,1}". n and m are limited to non-
negative integral values less than a preset limit defined when perl is
built. This is usually 32766 on the most common platforms. The actual
limit can be seen in the error message generated by code such as this:
$_ **= $_ , / {$_} / for 2 .. 42;
By default, a quantified subpattern is "greedy", that is, it will match
as many times as possible (given a particular starting location) while
still allowing the rest of the pattern to match. If you want it to
match the minimum number of times possible, follow the quantifier with
a "?". Note that the meanings don't change, just the "greediness":
*? Match 0 or more times, not greedily
+? Match 1 or more times, not greedily
?? Match 0 or 1 time, not greedily
{n}? Match exactly n times, not greedily (redundant)
{n,}? Match at least n times, not greedily
{n,m}? Match at least n but not more than m times, not greedily
Normally when a quantified subpattern does not allow the rest of the
overall pattern to match, Perl will backtrack. However, this behaviour
is sometimes undesirable. Thus Perl provides the "possessive"
quantifier form as well.
*+ Match 0 or more times and give nothing back
++ Match 1 or more times and give nothing back
?+ Match 0 or 1 time and give nothing back
{n}+ Match exactly n times and give nothing back (redundant)
{n,}+ Match at least n times and give nothing back
{n,m}+ Match at least n but not more than m times and give nothing back
For instance,
'aaaa' =~ /a++a/
will never match, as the "a++" will gobble up all the "a"'s in the
string and won't leave any for the remaining part of the pattern. This
feature can be extremely useful to give perl hints about where it
shouldn't backtrack. For instance, the typical "match a double-quoted
string" problem can be most efficiently performed when written as:
/"(?:[^"\\]++|\\.)*+"/
as we know that if the final quote does not match, backtracking will
not help. See the independent subexpression ""(?>pattern)"" for more
details; possessive quantifiers are just syntactic sugar for that
construct. For instance the above example could also be written as
follows:
/"(?>(?:(?>[^"\\]+)|\\.)*)"/
Note that the possessive quantifier modifier can not be combined with
the non-greedy modifier. This is because it would make no sense.
Consider the follow equivalency table:
Illegal Legal
------------ ------
X??+ X{0}
X+?+ X{1}
X{min,max}?+ X{min}
Escape sequences
Because patterns are processed as double-quoted strings, the following
also work:
\t tab (HT, TAB)
\n newline (LF, NL)
\r return (CR)
\f form feed (FF)
\a alarm (bell) (BEL)
\e escape (think troff) (ESC)
\cK control char (example: VT)
\x{}, \x00 character whose ordinal is the given hexadecimal number
\N{name} named Unicode character or character sequence
\N{U+263D} Unicode character (example: FIRST QUARTER MOON)
\o{}, \000 character whose ordinal is the given octal number
\l lowercase next char (think vi)
\u uppercase next char (think vi)
\L lowercase until \E (think vi)
\U uppercase until \E (think vi)
\Q quote (disable) pattern metacharacters until \E
\E end either case modification or quoted section, think vi
Details are in "Quote and Quote-like Operators" in perlop.
Character Classes and other Special Escapes
In addition, Perl defines the following:
Sequence Note Description
[...] [1] Match a character according to the rules of the
bracketed character class defined by the "...".
Example: [a-z] matches "a" or "b" or "c" ... or "z"
[[:...:]] [2] Match a character according to the rules of the POSIX
character class "..." within the outer bracketed
character class. Example: [[:upper:]] matches any
uppercase character.
(?[...]) [8] Extended bracketed character class
\w [3] Match a "word" character (alphanumeric plus "_", plus
other connector punctuation chars plus Unicode
marks)
\W [3] Match a non-"word" character
\s [3] Match a whitespace character
\S [3] Match a non-whitespace character
\d [3] Match a decimal digit character
\D [3] Match a non-digit character
\pP [3] Match P, named property. Use \p{Prop} for longer names
\PP [3] Match non-P
\X [4] Match Unicode "eXtended grapheme cluster"
\1 [5] Backreference to a specific capture group or buffer.
'1' may actually be any positive integer.
\g1 [5] Backreference to a specific or previous group,
\g{-1} [5] The number may be negative indicating a relative
previous group and may optionally be wrapped in
curly brackets for safer parsing.
\g{name} [5] Named backreference
\k<name> [5] Named backreference
\K [6] Keep the stuff left of the \K, don't include it in $&
\N [7] Any character but \n. Not affected by /s modifier
\v [3] Vertical whitespace
\V [3] Not vertical whitespace
\h [3] Horizontal whitespace
\H [3] Not horizontal whitespace
\R [4] Linebreak
[1] See "Bracketed Character Classes" in perlrecharclass for details.
[2] See "POSIX Character Classes" in perlrecharclass for details.
[3] See "Unicode Character Properties" in perlunicode for details
[4] See "Misc" in perlrebackslash for details.
[5] See "Capture groups" below for details.
[6] See "Extended Patterns" below for details.
[7] Note that "\N" has two meanings. When of the form "\N{NAME}", it
matches the character or character sequence whose name is NAME; and
similarly when of the form "\N{U+hex}", it matches the character
whose Unicode code point is hex. Otherwise it matches any
character but "\n".
[8] See "Extended Bracketed Character Classes" in perlrecharclass for
details.
Assertions
Besides "^" and "$", Perl defines the following zero-width assertions:
\b{} Match at Unicode boundary of specified type
\B{} Match where corresponding \b{} doesn't match
\b Match a \w\W or \W\w boundary
\B Match except at a \w\W or \W\w 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 Unicode boundary ("\b{}"), available starting in v5.22, is a spot
between two characters, or before the first character in the string, or
after the final character in the string where certain criteria defined
by Unicode are met. See "\b{}, \b, \B{}, \B" in perlrebackslash for
details.
A word boundary ("\b") is a spot between two characters that has a "\w"
on one side of it and a "\W" on the other side of it (in either order),
counting the imaginary characters off the beginning and end of the
string as matching a "\W". (Within character classes "\b" represents
backspace rather than a word boundary, just as it normally does in any
double-quoted string.) The "\A" and "\Z" are just like "^" and "$",
except that they won't match multiple times when the "/m" modifier is
used, while "^" and "$" will match at every internal line boundary. To
match the actual end of the string and not ignore an optional trailing
newline, use "\z".
The "\G" assertion can be used to chain global matches (using "m//g"),
as described in "Regexp Quote-Like Operators" in perlop. It is also
useful when writing "lex"-like scanners, when you have several patterns
that you want to match against consequent substrings of your string;
see the previous reference. The actual location where "\G" will match
can also be influenced by using "pos()" as an lvalue: see "pos" in
perlfunc. Note that the rule for zero-length matches (see "Repeated
Patterns Matching a Zero-length Substring") is modified somewhat, in
that contents to the left of "\G" are not counted when determining the
length of the match. Thus the following will not match forever:
my $string = 'ABC';
pos($string) = 1;
while ($string =~ /(.\G)/g) {
print $1;
}
It will print 'A' and then terminate, as it considers the match to be
zero-width, and thus will not match at the same position twice in a
row.
It is worth noting that "\G" improperly used can result in an infinite
loop. Take care when using patterns that include "\G" in an
alternation.
Note also that "s///" will refuse to overwrite part of a substitution
that has already been replaced; so for example this will stop after the
first iteration, rather than iterating its way backwards through the
string:
$_ = "123456789";
pos = 6;
s/.(?=.\G)/X/g;
print; # prints 1234X6789, not XXXXX6789
Capture groups
The grouping construct "( ... )" creates capture groups (also referred
to as capture buffers). To refer to the current contents of a group
later on, within the same pattern, use "\g1" (or "\g{1}") for the
first, "\g2" (or "\g{2}") for the second, and so on. This is called a
backreference.
There is no limit to the number of captured substrings that you may
use. Groups are numbered with the leftmost open parenthesis being
number 1, etc. If a group did not match, the associated backreference
won't match either. (This can happen if the group is optional, or in a
different branch of an alternation.) You can omit the "g", and write
"\1", etc, but there are some issues with this form, described below.
You can also refer to capture groups relatively, by using a negative
number, so that "\g-1" and "\g{-1}" both refer to the immediately
preceding capture group, and "\g-2" and "\g{-2}" both refer to the
group before it. For example:
/
(Y) # group 1
( # group 2
(X) # group 3
\g{-1} # backref to group 3
\g{-3} # backref to group 1
)
/x
would match the same as "/(Y) ( (X) \g3 \g1 )/x". This allows you to
interpolate regexes into larger regexes and not have to worry about the
capture groups being renumbered.
You can dispense with numbers altogether and create named capture
groups. The notation is "(?<name>...)" to declare and "\g{name}" to
reference. (To be compatible with .Net regular expressions, "\g{name}"
may also be written as "\k{name}", "\k<name>" or "\k'name'".) name
must not begin with a number, nor contain hyphens. When different
groups within the same pattern have the same name, any reference to
that name assumes the leftmost defined group. Named groups count in
absolute and relative numbering, and so can also be referred to by
those numbers. (It's possible to do things with named capture groups
that would otherwise require "(??{})".)
Capture group contents are dynamically scoped and available to you
outside the pattern until the end of the enclosing block or until the
next successful match, whichever comes first. (See "Compound
Statements" in perlsyn.) You can refer to them by absolute number
(using "$1" instead of "\g1", etc); or by name via the "%+" hash, using
"$+{name}".
Braces are required in referring to named capture groups, but are
optional for absolute or relative numbered ones. Braces are safer when
creating a regex by concatenating smaller strings. For example if you
have "qr/$a$b/", and $a contained "\g1", and $b contained "37", you
would get "/\g137/" which is probably not what you intended.
The "\g" and "\k" notations were introduced in Perl 5.10.0. Prior to
that there were no named nor relative numbered capture groups.
Absolute numbered groups were referred to using "\1", "\2", etc., and
this notation is still accepted (and likely always will be). But it
leads to some ambiguities if there are more than 9 capture groups, as
"\10" could mean either the tenth capture group, or the character whose
ordinal in octal is 010 (a backspace in ASCII). Perl resolves this
ambiguity by interpreting "\10" as a backreference only if at least 10
left parentheses have opened before it. Likewise "\11" is a
backreference only if at least 11 left parentheses have opened before
it. And so on. "\1" through "\9" are always interpreted as
backreferences. There are several examples below that illustrate these
perils. You can avoid the ambiguity by always using "\g{}" or "\g" if
you mean capturing groups; and for octal constants always using "\o{}",
or for "\077" and below, using 3 digits padded with leading zeros,
since a leading zero implies an octal constant.
The "\digit" notation also works in certain circumstances outside the
pattern. See "Warning on \1 Instead of $1" below for details.
Examples:
s/^([^ ]*) *([^ ]*)/$2 $1/; # swap first two words
/(.)\g1/ # find first doubled char
and print "'$1' is the first doubled character\n";
/(?<char>.)\k<char>/ # ... a different way
and print "'$+{char}' is the first doubled character\n";
/(?'char'.)\g1/ # ... mix and match
and print "'$1' is the first doubled character\n";
if (/Time: (..):(..):(..)/) { # parse out values
$hours = $1;
$minutes = $2;
$seconds = $3;
}
/(.)(.)(.)(.)(.)(.)(.)(.)(.)\g10/ # \g10 is a backreference
/(.)(.)(.)(.)(.)(.)(.)(.)(.)\10/ # \10 is octal
/((.)(.)(.)(.)(.)(.)(.)(.)(.))\10/ # \10 is a backreference
/((.)(.)(.)(.)(.)(.)(.)(.)(.))\010/ # \010 is octal
$a = '(.)\1'; # Creates problems when concatenated.
$b = '(.)\g{1}'; # Avoids the problems.
"aa" =~ /${a}/; # True
"aa" =~ /${b}/; # True
"aa0" =~ /${a}0/; # False!
"aa0" =~ /${b}0/; # True
"aa\x08" =~ /${a}0/; # True!
"aa\x08" =~ /${b}0/; # False
Several special variables also refer back to portions of the previous
match. $+ returns whatever the last bracket match matched. $& returns
the entire matched string. (At one point $0 did also, but now it
returns the name of the program.) "$`" returns everything before the
matched string. "$'" returns everything after the matched string. And
$^N contains whatever was matched by the most-recently closed group
(submatch). $^N can be used in extended patterns (see below), for
example to assign a submatch to a variable.
These special variables, like the "%+" hash and the numbered match
variables ($1, $2, $3, etc.) are dynamically scoped until the end of
the enclosing block or until the next successful match, whichever comes
first. (See "Compound Statements" in perlsyn.)
NOTE: Failed matches in Perl do not reset the match variables, which
makes it easier to write code that tests for a series of more specific
cases and remembers the best match.
WARNING: If your code is to run on Perl 5.16 or earlier, beware that
once Perl sees that you need one of $&, "$`", or "$'" anywhere in the
program, it has to provide them for every pattern match. This may
substantially slow your program.
Perl uses the same mechanism to produce $1, $2, etc, so you also pay a
price for each pattern that contains capturing parentheses. (To avoid
this cost while retaining the grouping behaviour, use the extended
regular expression "(?: ... )" instead.) But if you never use $&, "$`"
or "$'", then patterns without capturing parentheses will not be
penalized. So avoid $&, "$'", and "$`" if you can, but if you can't
(and some algorithms really appreciate them), once you've used them
once, use them at will, because you've already paid the price.
Perl 5.16 introduced a slightly more efficient mechanism that notes
separately whether each of "$`", $&, and "$'" have been seen, and thus
may only need to copy part of the string. Perl 5.20 introduced a much
more efficient copy-on-write mechanism which eliminates any slowdown.
As another workaround for this problem, Perl 5.10.0 introduced
"${^PREMATCH}", "${^MATCH}" and "${^POSTMATCH}", which are equivalent
to "$`", $& and "$'", except that they are only guaranteed to be
defined after a successful match that was executed with the "/p"
(preserve) modifier. The use of these variables incurs no global
performance penalty, unlike their punctuation character equivalents,
however at the trade-off that you have to tell perl when you want to
use them. As of Perl 5.20, these three variables are equivalent to
"$`", $& and "$'", and "/p" is ignored.
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.
A question mark was chosen for this and for the minimal-matching
construct because 1) question marks are rare in older regular
expressions, and 2) whenever you see one, you should stop and
"question" exactly what is going on. That's psychology....
"(?#text)"
A comment. The text is ignored. Note that Perl closes the comment
as soon as it sees a ")", so there is no way to put a literal ")"
in the comment. The pattern's closing delimiter must be escaped by
a backslash if it appears in the comment.
See "/x" for another way to have comments in patterns.
Note that a comment can go just about anywhere, except in the
middle of an escape sequence. Examples:
qr/foo(?#comment)bar/' # Matches 'foobar'
# The pattern below matches 'abcd', 'abccd', or 'abcccd'
qr/abc(?#comment between literal and its quantifier){1,3}d/
# The pattern below generates a syntax error, because the '\p' must
# be followed immediately by a '{'.
qr/\p(?#comment between \p and its property name){Any}/
# The pattern below generates a syntax error, because the initial
# '\(' is a literal opening parenthesis, and so there is nothing
# for the closing ')' to match
qr/\(?#the backslash means this isn't a comment)p{Any}/
# Comments can be used to fold long patterns into multiple lines
qr/First part of a long regex(?#
)remaining part/
"(?adlupimnsx-imnsx)"
"(?^alupimnsx)"
Zero 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 dynamically-generated 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.
A modifier is overridden by later occurrences of this construct in
the same scope containing the same modifier, so that
/((?im)foo(?-m)bar)/
matches all of "foobar" case insensitively, but uses "/m" rules for
only the "foo" portion. The "a" flag overrides "aa" as well;
likewise "aa" overrides "a". The same goes for "x" and "xx".
Hence, in
/(?-x)foo/xx
both "/x" and "/xx" are turned off during matching "foo". And in
/(?x)foo/x
"/x" but NOT "/xx" is turned on for matching "foo". (One might
mistakenly think that since the inner "(?x)" is already in the
scope of "/x", that the result would effectively be the sum of
them, yielding "/xx". It doesn't work that way.) Similarly, doing
something like "(?xx-x)foo" turns off all "x" behavior for matching
"foo", it is not that you subtract 1 "x" from 2 to get 1 "x"
remaining.
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-imnsx".
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.
Having zero modifiers makes this a no-op (so why did you specify
it, unless it's generated code), and starting in v5.30, warns under
"use re 'strict'".
"(?:pattern)"
"(?adluimnsx-imnsx:pattern)"
"(?^aluimnsx: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/)
matches the same field delimiters as
@fields = split(/\b(a|b|c)\b/)
but doesn't spit out the delimiters themselves as extra fields
(even though that's the behaviour of "split" in perlfunc when its
pattern contains capturing groups). It's also cheaper not to
capture characters if you don't need to.
Any letters between "?" and ":" act as flags modifiers as with
"(?adluimnsx-imnsx)". For example,
/(?s-i:more.*than).*million/i
is equivalent to the more verbose
/(?:(?s-i)more.*than).*million/i
Note that any "()" constructs enclosed within this one will still
capture unless the "/n" modifier is in effect.
Like the "(?adlupimnsx-imnsx)" construct, "aa" and "a" override
each other, as do "xx" and "x". They are not additive. So, doing
something like "(?xx-x:foo)" turns off all "x" behavior for
matching "foo".
Starting in Perl 5.14, a "^" (caret or circumflex accent)
immediately after the "?" is a shorthand equivalent to "d-imnsx".
Any positive flags (except "d") may follow the caret, so
(?^x:foo)
is equivalent to
(?x-imns:foo)
The caret tells Perl that this cluster doesn't inherit the flags of
any surrounding pattern, but uses the system defaults ("d-imnsx"),
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.
Lookaround Assertions
Lookaround 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. Lookbehind matches text up to the current
match position, lookahead matches text following the current match
position.
"(?=pattern)"
"(*pla:pattern)"
"(*positive_lookahead:pattern)"
A zero-width positive lookahead assertion. For example,
"/\w+(?=\t)/" matches a word followed by a tab, without
including the tab in $&.
The alphabetic forms are experimental; using them yields a
warning in the "experimental::alpha_assertions" category.
"(?!pattern)"
"(*nla:pattern)"
"(*negative_lookahead:pattern)"
A zero-width negative lookahead assertion. For example
"/foo(?!bar)/" matches any occurrence of "foo" that isn't
followed by "bar". Note however that lookahead and lookbehind
are NOT the same thing. You cannot use this for lookbehind.
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
lookbehind instead (see below).
The alphabetic forms are experimental; using them yields a
warning in the "experimental::alpha_assertions" category.
"(?<=pattern)"
"\K"
"(*plb:pattern)"
"(*positive_lookbehind:pattern)"
A zero-width positive lookbehind assertion. For example,
"/(?<=\t)\w+/" matches a word that follows a tab, without
including the tab in $&.
Prior to Perl 5.30, it worked only for fixed-width lookbehind,
but starting in that release, it can handle variable lengths
from 1 to 255 characters as an experimental feature. The
feature is enabled automatically if you use a variable length
lookbehind assertion, but will raise a warning at pattern
compilation time, unless turned off, in the "experimental::vlb"
category. This is to warn you that the exact behavior is
subject to change should feedback from actual use in the field
indicate to do so; or even complete removal if the problems
found are not practically surmountable. You can achieve close
to pre-5.30 behavior by fatalizing warnings in this category.
There is a special form of this construct, called "\K"
(available since Perl 5.10.0), which causes the regex engine to
"keep" everything it had matched prior to the "\K" and not
include it in $&. This effectively provides non-experimental
variable-length lookbehind of any length.
And, there is a technique that can be used to handle variable
length lookbehinds on earlier releases, and longer than 255
characters. It is described in
<http://www.drregex.com/2019/02/variable-length-lookbehinds-actually.html>.
Note that under "/i", a few single characters match two or
three other characters. This makes them variable length, and
the 255 length applies to the maximum number of characters in
the match. For example "qr/\N{LATIN SMALL LETTER SHARP S}/i"
matches the sequence "ss". Your lookbehind assertion could
contain 127 Sharp S characters under "/i", but adding a 128th
would generate a compilation error, as that could match 256 "s"
characters in a row.
The use of "\K" inside of another lookaround 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;
Use of the non-greedy modifier "?" may not give you the
expected results if it is within a capturing group within the
construct.
The alphabetic forms (not including "\K" are experimental;
using them yields a warning in the
"experimental::alpha_assertions" category.
"(?<!pattern)"
"(*nlb:pattern)"
"(*negative_lookbehind:pattern)"
A zero-width negative lookbehind assertion. For example
"/(?<!bar)foo/" matches any occurrence of "foo" that does not
follow "bar".
Prior to Perl 5.30, it worked only for fixed-width lookbehind,
but starting in that release, it can handle variable lengths
from 1 to 255 characters as an experimental feature. The
feature is enabled automatically if you use a variable length
lookbehind assertion, but will raise a warning at pattern
compilation time, unless turned off, in the "experimental::vlb"
category. This is to warn you that the exact behavior is
subject to change should feedback from actual use in the field
indicate to do so; or even complete removal if the problems
found are not practically surmountable. You can achieve close
to pre-5.30 behavior by fatalizing warnings in this category.
There is a technique that can be used to handle variable length
lookbehinds on earlier releases, and longer than 255
characters. It is described in
<http://www.drregex.com/2019/02/variable-length-lookbehinds-actually.html>.
Note that under "/i", a few single characters match two or
three other characters. This makes them variable length, and
the 255 length applies to the maximum number of characters in
the match. For example "qr/\N{LATIN SMALL LETTER SHARP S}/i"
matches the sequence "ss". Your lookbehind assertion could
contain 127 Sharp S characters under "/i", but adding a 128th
would generate a compilation error, as that could match 256 "s"
characters in a row.
Use of the non-greedy modifier "?" may not give you the
expected results if it is within a capturing group within the
construct.
The alphabetic forms are experimental; using them yields a
warning in the "experimental::alpha_assertions" category.
"(?<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
$+{NAME} will refer to the leftmost defined group in the match.
The forms "(?'NAME'pattern)" and "(?<NAME>pattern)" are equivalent.
NOTE: While the notation of this construct is the same as the
similar function in .NET regexes, the behavior is not. In Perl the
groups are numbered sequentially regardless of being named or not.
Thus in the pattern
/(x)(?<foo>y)(z)/
$+{foo} will be the same as $2, and $3 will contain 'z' instead of
the opposite which is what a .NET regex hacker might expect.
Currently NAME is restricted to simple identifiers only. In other
words, it must match "/^[_A-Za-z][_A-Za-z0-9]*\z/" or its Unicode
extension (see utf8), though it isn't extended by the locale (see
perllocale).
NOTE: In order to make things easier for programmers with
experience with the Python or PCRE regex engines, the pattern
"(?P<NAME>pattern)" may be used instead of "(?<NAME>pattern)";
however this form does not support the use of single quotes as a
delimiter for the name.
"\k<NAME>"
"\k'NAME'"
Named backreference. Similar to numeric backreferences, except that
the group is designated by name and not number. If multiple groups
have the same name then it refers to the leftmost defined group in
the current match.
It is an error to refer to a name not defined by a "(?<NAME>)"
earlier in the pattern.
Both forms are equivalent.
NOTE: In order to make things easier for programmers with
experience with the Python or PCRE regex engines, the pattern
"(?P=NAME)" may be used instead of "\k<NAME>".
"(?{ code })"
WARNING: Using this feature safely requires that you understand its
limitations. Code executed that has side effects may not perform
identically from version to version due to the effect of future
optimisations in the regex engine. For more information on this,
see "Embedded Code Execution Frequency".
This zero-width assertion executes any embedded Perl code. It
always succeeds, and its return value is set as $^R.
In literal patterns, the code is parsed at the same time as the
surrounding code. While within the pattern, control is passed
temporarily back to the perl parser, until the logically-balancing
closing brace is encountered. This is similar to the way that an
array index expression in a literal string is handled, for example
"abc$array[ 1 + f('[') + g()]def"
In particular, braces do not need to be balanced:
s/abc(?{ f('{'); })/def/
Even in a pattern that is interpolated and compiled at run-time,
literal code blocks will be compiled once, at perl compile time;
the following prints "ABCD":
print "D";
my $qr = qr/(?{ BEGIN { print "A" } })/;
my $foo = "foo";
/$foo$qr(?{ BEGIN { print "B" } })/;
BEGIN { print "C" }
In patterns where the text of the code is derived from run-time
information rather than appearing literally in a source code
/pattern/, the code is compiled at the same time that the pattern
is compiled, and for reasons of security, "use re 'eval'" must be
in scope. This is to stop user-supplied patterns containing code
snippets from being executable.
In situations where you need to enable this with "use re 'eval'",
you should also have taint checking enabled. Better yet, use the
carefully constrained evaluation within a Safe compartment. See
perlsec for details about both these mechanisms.
From the viewpoint of parsing, lexical variable scope and closures,
/AAA(?{ BBB })CCC/
behaves approximately like
/AAA/ && do { BBB } && /CCC/
Similarly,
qr/AAA(?{ BBB })CCC/
behaves approximately like
sub { /AAA/ && do { BBB } && /CCC/ }
In particular:
{ my $i = 1; $r = qr/(?{ print $i })/ }
my $i = 2;
/$r/; # prints "1"
Inside a "(?{...})" block, $_ refers to the string the regular
expression is matching against. You can also use "pos()" to know
what is the current position of matching within this string.
The code block introduces a new scope from the perspective of
lexical variable declarations, but not from the perspective of
"local" and similar localizing behaviours. So later code blocks
within the same pattern will still see the values which were
localized in earlier blocks. These accumulated localizations are
undone either at the end of a successful match, or if the assertion
is backtracked (compare "Backtracking"). For example,
$_ = 'a' x 8;
m<
(?{ $cnt = 0 }) # Initialize $cnt.
(
a
(?{
local $cnt = $cnt + 1; # Update $cnt,
# backtracking-safe.
})
)*
aaaa
(?{ $res = $cnt }) # On success copy to
# non-localized location.
>x;
will initially increment $cnt up to 8; then during backtracking,
its value will be unwound back to 4, which is the value assigned to
$res. At the end of the regex execution, $cnt will be wound back
to its initial value of 0.
This assertion may be used as the condition in a
(?(condition)yes-pattern|no-pattern)
switch. If not used in this way, the result of evaluation of code
is put into the special variable $^R. This happens immediately, so
$^R can be used from other "(?{ code })" assertions inside the same
regular expression.
The assignment to $^R above is properly localized, so the old value
of $^R is restored if the assertion is backtracked; compare
"Backtracking".
Note that the special variable $^N is particularly useful with
code blocks to capture the results of submatches in variables
without having to keep track of the number of nested parentheses.
For example:
$_ = "The brown fox jumps over the lazy dog";
/the (\S+)(?{ $color = $^N }) (\S+)(?{ $animal = $^N })/i;
print "color = $color, animal = $animal\n";
"(??{ code })"
WARNING: Using this feature safely requires that you understand its
limitations. Code executed that has side effects may not perform
identically from version to version due to the effect of future
optimisations in the regex engine. For more information on this,
see "Embedded Code Execution Frequency".
This is a "postponed" regular subexpression. It behaves in exactly
the same way as a "(?{ code })" code block as described above,
except that its return value, rather than being assigned to $^R, is
treated as a pattern, compiled if it's a string (or used as-is if
its a qr// object), then matched as if it were inserted instead of
this construct.
During the matching of this sub-pattern, it has its own set of
captures which are valid during the sub-match, but are discarded
once control returns to the main pattern. For example, the
following matches, with the inner pattern capturing "B" and
matching "BB", while the outer pattern captures "A";
my $inner = '(.)\1';
"ABBA" =~ /^(.)(??{ $inner })\1/;
print $1; # prints "A";
Note that this means that there is no way for the inner pattern to
refer to a capture group defined outside. (The code block itself
can use $1, etc., to refer to the enclosing pattern's capture
groups.) Thus, although
('a' x 100)=~/(??{'(.)' x 100})/
will match, it will not set $1 on exit.
The following pattern matches a parenthesized group:
$re = qr{
\(
(?:
(?> [^()]+ ) # Non-parens without backtracking
|
(??{ $re }) # Group with matching parens
)*
\)
}x;
See also "(?PARNO)" for a different, more efficient way to
accomplish the same task.
Executing a postponed regular expression too many times without
consuming any input string will also result in a fatal error. The
depth at which that happens is compiled into perl, so it can be
changed with a custom build.
"(?PARNO)" "(?-PARNO)" "(?+PARNO)" "(?R)" "(?0)"
Recursive subpattern. Treat the contents of a given capture buffer
in the current pattern as an independent subpattern and attempt to
match it at the current position in the string. Information about
capture state from the caller for things like backreferences is
available to the subpattern, but capture buffers set by the
subpattern are not visible to the caller.
Similar to "(??{ code })" except that it does not involve executing
any code or potentially compiling a returned pattern string;
instead it treats the part of the current pattern contained within
a specified capture group as an independent pattern that must match
at the current position. Also different is the treatment of capture
buffers, unlike "(??{ code })" recursive patterns have access to
their caller's match state, so one can use backreferences safely.
PARNO is a sequence of digits (not starting with 0) whose value
reflects the paren-number of the capture group to recurse to.
"(?R)" recurses to the beginning of the whole pattern. "(?0)" is an
alternate syntax for "(?R)". If PARNO is preceded by a plus or
minus sign then it is assumed to be relative, with negative numbers
indicating preceding capture groups and positive ones following.
Thus "(?-1)" refers to the most recently declared group, and
"(?+1)" indicates the next group to be declared. Note that the
counting for relative recursion differs from that of relative
backreferences, in that with recursion unclosed groups are
included.
The following pattern matches a function "foo()" which may contain
balanced parentheses as the argument.
$re = qr{ ( # paren group 1 (full function)
foo
( # paren group 2 (parens)
\(
( # paren group 3 (contents of parens)
(?:
(?> [^()]+ ) # Non-parens without backtracking
|
(?2) # Recurse to start of paren group 2
)*
)
\)
)
)
}x;
If the pattern was used as follows
'foo(bar(baz)+baz(bop))'=~/$re/
and print "\$1 = $1\n",
"\$2 = $2\n",
"\$3 = $3\n";
the output produced should be the following:
$1 = foo(bar(baz)+baz(bop))
$2 = (bar(baz)+baz(bop))
$3 = bar(baz)+baz(bop)
If there is no corresponding capture group defined, then it is a
fatal error. Recursing deeply without consuming any input string
will also result in a fatal error. The depth at which that happens
is compiled into perl, so it can be changed with a custom build.
The following shows how using negative indexing can make it easier
to embed recursive patterns inside of a "qr//" construct for later
use:
my $parens = qr/(\((?:[^()]++|(?-1))*+\))/;
if (/foo $parens \s+ \+ \s+ bar $parens/x) {
# do something here...
}
Note that this pattern does not behave the same way as the
equivalent PCRE or Python construct of the same form. In Perl you
can backtrack into a recursed group, in PCRE and Python the
recursed into group is treated as atomic. Also, modifiers are
resolved at compile time, so constructs like "(?i:(?1))" or
"(?:(?i)(?1))" do not affect how the sub-pattern will be processed.
"(?&NAME)"
Recurse to a named subpattern. Identical to "(?PARNO)" except that
the parenthesis to recurse to is determined by name. If multiple
parentheses have the same name, then it recurses to the leftmost.
It is an error to refer to a name that is not declared somewhere in
the pattern.
NOTE: In order to make things easier for programmers with
experience with the Python or PCRE regex engines the pattern
"(?P>NAME)" may be used instead of "(?&NAME)".
"(?(condition)yes-pattern|no-pattern)"
"(?(condition)yes-pattern)"
Conditional expression. Matches yes-pattern if condition yields a
true value, matches no-pattern otherwise. A missing pattern always
matches.
"(condition)" should be one of:
an integer in parentheses
(which is valid if the corresponding pair of parentheses
matched);
a lookahead/lookbehind/evaluate zero-width assertion;
a name in angle brackets or single quotes
(which is valid if a group with the given name matched);
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.
Full syntax: "(?(1)then|else)"
"(<NAME>)" "('NAME')"
Checks if a group with the given name has matched something.
Full syntax: "(?(<name>)then|else)"
"(?=...)" "(?!...)" "(?<=...)" "(?<!...)"
Checks whether the pattern matches (or does not match, for the
"!" variants). Full syntax: "(?(?=lookahead)then|else)"
"(?{ CODE })"
Treats the return value of the code block as the condition.
Full syntax: "(?(?{ code })then|else)"
"(R)"
Checks if the expression has been evaluated inside of
recursion. Full syntax: "(?(R)then|else)"
"(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.
Full syntax: "(?(R1)then|else)"
"(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. Full syntax:
"(?(R&name)then|else)"
"(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. Full syntax:
"(?(DEFINE)definitions...)"
For example:
m{ ( \( )?
[^()]+
(?(1) \) )
}x
matches a chunk of non-parentheses, possibly included in
parentheses themselves.
A special form is the "(DEFINE)" predicate, which never executes
its yes-pattern directly, and does not allow a no-pattern. This
allows one to define subpatterns which will be executed only by the
recursion mechanism. This way, you can define a set of regular
expression rules that can be bundled into any pattern you choose.
It is recommended that for this usage you put the DEFINE block at
the end of the pattern, and that you name any subpatterns defined
within it.
Also, it's worth noting that patterns defined this way probably
will not be as efficient, as the optimizer is not very clever about
handling them.
An example of how this might be used is as follows:
/(?<NAME>(?&NAME_PAT))(?<ADDR>(?&ADDRESS_PAT))
(?(DEFINE)
(?<NAME_PAT>....)
(?<ADDRESS_PAT>....)
)/x
Note that capture groups matched inside of recursion are not
accessible after the recursion returns, so the extra layer of
capturing groups is necessary. Thus $+{NAME_PAT} would not be
defined even though $+{NAME} would be.
Finally, keep in mind that subpatterns created inside a DEFINE
block count towards the absolute and relative number of captures,
so this:
my @captures = "a" =~ /(.) # First capture
(?(DEFINE)
(?<EXAMPLE> 1 ) # Second capture
)/x;
say scalar @captures;
Will output 2, not 1. This is particularly important if you intend
to compile the definitions with the "qr//" operator, and later
interpolate them in another pattern.
"(?>pattern)"
"(*atomic: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 lookahead, 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})
Nested "(?>...)" constructs are not no-ops, even if at first glance
they might seem to be. This is because the nested "(?>...)" can
restrict internal backtracking that otherwise might occur. For
example,
"abc" =~ /(?>a[bc]*c)/
matches, but
"abc" =~ /(?>a(?>[bc]*)c)/
does not.
The alphabetic form ("(*atomic:...)") is experimental; using it
yields a warning in the "experimental::alpha_assertions" category.
"(?[ ])"
See "Extended Bracketed Character Classes" in perlrecharclass.
Note that this feature is currently experimental; using it yields a
warning in the "experimental::regex_sets" category.
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 lookahead 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 lookaheads 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 lookahead/lookbehind 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 lookahead might have influenced the following match,
see ""(?>pattern)"".
Script Runs
A script run is basically a sequence of characters, all from the same
Unicode script (see "Scripts" in perlunicode), such as Latin or Greek.
In most places a single word would never be written in multiple
scripts, unless it is a spoofing attack. An infamous example, is
paypal.com
Those letters could all be Latin (as in the example just above), or
they could be all Cyrillic (except for the dot), or they could be a
mixture of the two. In the case of an internet address the ".com"
would be in Latin, And any Cyrillic ones would cause it to be a
mixture, not a script run. Someone clicking on such a link would not
be directed to the real Paypal website, but an attacker would craft a
look-alike one to attempt to gather sensitive information from the
person.
Starting in Perl 5.28, it is now easy to detect strings that aren't
script runs. Simply enclose just about any pattern like either of
these:
(*script_run:pattern)
(*sr:pattern)
What happens is that after pattern succeeds in matching, it is
subjected to the additional criterion that every character in it must
be from the same script (see exceptions below). If this isn't true,
backtracking occurs until something all in the same script is found
that matches, or all possibilities are exhausted. This can cause a lot
of backtracking, but generally, only malicious input will result in
this, though the slow down could cause a denial of service attack. If
your needs permit, it is best to make the pattern atomic to cut down on
the amount of backtracking. This is so likely to be what you want,
that instead of writing this:
(*script_run:(?>pattern))
you can write either of these:
(*atomic_script_run:pattern)
(*asr:pattern)
(See ""(?>pattern)"".)
In Taiwan, Japan, and Korea, it is common for text to have a mixture of
characters from their native scripts and base Chinese. Perl follows
Unicode's UTS 39 (<http://unicode.org/reports/tr39/>) Unicode Security
Mechanisms in allowing such mixtures. For example, the Japanese
scripts Katakana and Hiragana are commonly mixed together in practice,
along with some Chinese characters, and hence are treated as being in a
single script run by Perl.
The rules used for matching decimal digits are slightly stricter. Many
scripts have their own sets of digits equivalent to the Western 0
through 9 ones. A few, such as Arabic, have more than one set. For a
string to be considered a script run, all digits in it must come from
the same set of ten, as determined by the first digit encountered. As
an example,
qr/(*script_run: \d+ \b )/x
guarantees that the digits matched will all be from the same set of 10.
You won't get a look-alike digit from a different script that has a
different value than what it appears to be.
Unicode has three pseudo scripts that are handled specially.
"Unknown" is applied to code points whose meaning has yet to be
determined. Perl currently will match as a script run, any single
character string consisting of one of these code points. But any
string longer than one code point containing one of these will not be
considered a script run.
"Inherited" is applied to characters that modify another, such as an
accent of some type. These are considered to be in the script of the
master character, and so never cause a script run to not match.
The other one is "Common". This consists of mostly punctuation, emoji,
and characters used in mathematics and music, the ASCII digits 0
through 9, and full-width forms of these digits. These characters can
appear intermixed in text in many of the world's scripts. These also
don't cause a script run to not match. But like other scripts, all
digits in a run must come from the same set of 10.
This construct is non-capturing. You can add parentheses to pattern to
capture, if desired. You will have to do this if you plan to use
"(*ACCEPT) (*ACCEPT:arg)" and not have it bypass the script run
checking.
This feature is experimental, and the exact syntax and details of
operation are subject to change; using it yields a warning in the
"experimental::script_run" category.
The "Script_Extensions" property as modified by UTS 39
(<http://unicode.org/reports/tr39/>) is used as the basis for this
feature.
To summarize,
o All length 0 or length 1 sequences are script runs.
o A longer sequence is a script run if and only if all of the
following conditions are met:
1. No code point in the sequence has the "Script_Extension"
property of "Unknown".
This currently means that all code points in the sequence have
been assigned by Unicode to be characters that aren't private
use nor surrogate code points.
2. All characters in the sequence come from the Common script
and/or the Inherited script and/or a single other script.
The script of a character is determined by the
"Script_Extensions" property as modified by UTS 39
(<http://unicode.org/reports/tr39/>), as described above.
3. All decimal digits in the sequence come from the same block of
10 consecutive digits.
Special Backtracking Control Verbs
These special patterns are generally of the form "(*VERB:arg)". Unless
otherwise stated the arg argument is optional; in some cases, it is
mandatory.
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. They are set in the package containing the code that
executed the regex (rather than the one that compiled it, where those
differ). If necessary, you can use "local" to localize changes to
these variables to a specific scope before executing a regex.
If a pattern does not contain a special backtracking verb that allows
an argument, then $REGERROR and $REGMARK are not touched at all.
Verbs
"(*PRUNE)" "(*PRUNE:NAME)"
This zero-width pattern prunes the backtracking tree at the
current point when backtracked into on failure. Consider the
pattern "/A (*PRUNE) B/", where A and B are complex patterns.
Until the "(*PRUNE)" verb is reached, A may backtrack as
necessary to match. Once it is reached, matching continues in B,
which may also backtrack as necessary; however, should B not
match, then no further backtracking will take place, and the
pattern will fail outright at the current starting position.
The following example counts all the possible matching strings
in a pattern (without actually matching any of them).
'aaab' =~ /a+b?(?{print "$&\n"; $count++})(*FAIL)/;
print "Count=$count\n";
which produces:
aaab
aaa
aa
a
aab
aa
a
ab
a
Count=9
If we add a "(*PRUNE)" before the count like the following
'aaab' =~ /a+b?(*PRUNE)(?{print "$&\n"; $count++})(*FAIL)/;
print "Count=$count\n";
we prevent backtracking and find the count of the longest
matching string at each matching starting point like so:
aaab
aab
ab
Count=3
Any number of "(*PRUNE)" assertions may be used in a pattern.
See also "(?>pattern)" and possessive quantifiers for other ways
to control backtracking. In some cases, the use of "(*PRUNE)"
can be replaced with a "(?>pattern)" with no functional
difference; however, "(*PRUNE)" can be used to handle cases that
cannot be expressed using a "(?>pattern)" alone.
"(*SKIP)" "(*SKIP:NAME)"
This zero-width pattern is similar to "(*PRUNE)", except that on
failure it also signifies that whatever text that was matched
leading up to the "(*SKIP)" pattern being executed cannot be
part of any match of this pattern. This effectively means that
the regex engine "skips" forward to this position on failure and
tries to match again, (assuming that there is sufficient room to
match).
The name of the "(*SKIP:NAME)" pattern has special significance.
If a "(*MARK:NAME)" was encountered while matching, then it is
that position which is used as the "skip point". If no "(*MARK)"
of that name was encountered, then the "(*SKIP)" operator has no
effect. When used without a name the "skip point" is where the
match point was when executing the "(*SKIP)" pattern.
Compare the following to the examples in "(*PRUNE)"; note the
string is twice as long:
'aaabaaab' =~ /a+b?(*SKIP)(?{print "$&\n"; $count++})(*FAIL)/;
print "Count=$count\n";
outputs
aaab
aaab
Count=2
Once the 'aaab' at the start of the string has matched, and the
"(*SKIP)" executed, the next starting point will be where the
cursor was when the "(*SKIP)" was executed.
"(*MARK:NAME)" "(*:NAME)"
This zero-width pattern can be used to mark the point reached in
a string when a certain part of the pattern has been
successfully matched. This mark may be given a name. A later
"(*SKIP)" pattern will then skip forward to that point if
backtracked into on failure. Any number of "(*MARK)" patterns
are allowed, and the NAME portion may be duplicated.
In addition to interacting with the "(*SKIP)" pattern,
"(*MARK:NAME)" can be used to "label" a pattern branch, so that
after matching, the program can determine which branches of the
pattern were involved in the match.
When a match is successful, the $REGMARK variable will be set to
the name of the most recently executed "(*MARK:NAME)" that was
involved in the match.
This can be used to determine which branch of a pattern was
matched without using a separate capture group for each branch,
which in turn can result in a performance improvement, as perl
cannot optimize "/(?:(x)|(y)|(z))/" as efficiently as something
like "/(?:x(*MARK:x)|y(*MARK:y)|z(*MARK:z))/".
When a match has failed, and unless another verb has been
involved in failing the match and has provided its own name to
use, the $REGERROR variable will be set to the name of the most
recently executed "(*MARK:NAME)".
See "(*SKIP)" for more details.
As a shortcut "(*MARK:NAME)" can be written "(*:NAME)".
"(*THEN)" "(*THEN:NAME)"
This is similar to the "cut group" operator "::" from Perl 6.
Like "(*PRUNE)", this verb always matches, and when backtracked
into on failure, it causes the regex engine to try the next
alternation in the innermost enclosing group (capturing or
otherwise) that has alternations. The two branches of a
"(?(condition)yes-pattern|no-pattern)" do not count as an
alternation, as far as "(*THEN)" is concerned.
Its name comes from the observation that this operation combined
with the alternation operator ("|") can be used to create what
is essentially a pattern-based if/then/else block:
( COND (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ )
Note that if this operator is used and NOT inside of an
alternation then it acts exactly like the "(*PRUNE)" operator.
/ A (*PRUNE) B /
is the same as
/ A (*THEN) B /
but
/ ( A (*THEN) B | C ) /
is not the same as
/ ( A (*PRUNE) B | C ) /
as after matching the A but failing on the B the "(*THEN)" verb
will backtrack and try C; but the "(*PRUNE)" verb will simply
fail.
"(*COMMIT)" "(*COMMIT:arg)"
This is the Perl 6 "commit pattern" "<commit>" or ":::". It's a
zero-width pattern similar to "(*SKIP)", except that when
backtracked into on failure it causes the match to fail
outright. No further attempts to find a valid match by advancing
the start pointer will occur again. For example,
'aaabaaab' =~ /a+b?(*COMMIT)(?{print "$&\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)" "(*FAIL:arg)"
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. You can provide an argument so that if the match
fails because of this "FAIL" directive the argument can be
obtained from $REGERROR.
It is probably useful only when combined with "(?{})" or
"(??{})".
"(*ACCEPT)" "(*ACCEPT:arg)"
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.
You can provide an argument, which will be available in the var
$REGMARK after the match completes.
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 "<>PERLRE(1)