perlre - Perl regular expressions
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.
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 delimited, at both ends, by delimiter 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 delimiter 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 "this thing" or the sequence "that thing". 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 1The 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.
"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 modifierNotice 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 $*, but this option was removed in perl 5.10.)
To simplify multi-line substitutions, the "." character never matches a newline unless you use the /s modifier, which in effect tells Perl to pretend the string is a single line--even if it isn't.
The default behavior for matching can be changed, using various modifiers. Modifiers that relate to the interpretation of the pattern are listed just below. Modifiers that alter the way a pattern is used by Perl are detailed in "Regexp Quote-Like Operators" in perlop and "Gory details of parsing quoted constructs" in perlop. Modifiers can be added dynamically; see "Extended Patterns" below.
m Treat the string being matched against as multiple lines. That is, change "^" and "$" 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 undefThis 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"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 stringSubstitution-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 valueRegular 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.
Some of the modifiers require more explanation than given in the "Overview" above.
/x and /xxA single /x tells the regular expression parser to ignore most whitespace that is neither backslashed nor within a bracketed character class, nor within the characters of a multi-character metapattern like (?i: ... ). 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{} or \p{name=...} 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 (outside a bracketed character class) 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. Inside a bracketed character class, "#" retains its non-special, literal meaning.
Starting in Perl v5.26, if the modifier has a second "x" within it, the effect of a single /x is increased. The only difference is that inside bracketed character classes, non-escaped (by a backslash) SPACE and TAB characters are not added to the class, and hence can be inserted to make the classes more readable:
/ [d-e g-i 3-7]/xx
/[ ! @ " # $ % ^ & * () = ? <> ' ]/xxmay be easier to grasp than the squashed equivalents
/[d-eg-i3-7]/
/[!@"#$%^&*()=?<>']/Note that this unfortunately doesn't mean that your bracketed classes can contain comments or extend over multiple lines. A # inside a character class is still just a literal #, and doesn't introduce a comment. And, unless the closing bracket is on the same line as the opening one, the newline character (and everything on the next line(s) until terminated by a ] will be part of the class, just as if you'd written \n.
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 (?:...) 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, all constructs using curly braces as delimiters, such as \x{...} can have blanks within but adjacent to the braces, but not elsewhere, and no non-blank space characters. An exception are Unicode properties which follow 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/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/ilwill 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/iuto 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.
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, if your perl supports taint checking (see perlsec).
This modifier may be specified to be the default by use locale, but see "Which character set modifier is in effect?".
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 https://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 v5.12 (or higher), but see "Which character set modifier is in effect?".
IMPORTANT: Because of the unpredictable behaviors this modifier causes, only use it to maintain weird backward compatibilities. Use the unicode_strings feature in new code to avoid inadvertently enabling this modifier by default.
What does this modifier do? It "Depends"!
This modifier means to use platform-native matching rules except when there is cause to use Unicode rules instead, as follows:
the target string's UTF8 flag (see below) is set; or
the pattern's UTF8 flag (see below) is set; or
the pattern explicitly mentions a code point that is above 255 (say by \x{100}); or
the pattern uses a Unicode name (\N{...}); or
the pattern uses a Unicode property (\p{...} or \P{...}); or
the pattern uses a Unicode break (\b{...} or \B{...}); or
the pattern uses "(?[ ])"
the pattern uses (*script_run: ...)
Regarding the "UTF8 flag" references above: normally Perl applications shouldn't think about that flag. It's part of Perl's internals, so it can change whenever Perl wants. /d may thus cause unpredictable results. See "The "Unicode Bug"" in perlunicode. This bug has become rather infamous, leading to yet other (without swearing) names for this modifier like "Dicey" and "Dodgy".
Here are some examples of how that works on an ASCII platform:
$str = "\xDF"; #
utf8::downgrade($str); # $str is not UTF8-flagged.
$str =~ /^\w/; # No match, since no UTF8 flag.
$str .= "\x{0e0b}"; # Now $str is UTF8-flagged.
$str =~ /^\w/; # Match! $str is now UTF8-flagged.
chop $str;
$str =~ /^\w/; # Still a match! $str retains its UTF8 flag.Under Perl's default configuration this modifier is automatically selected by default when none of the others are, so yet another name for it (unfortunately) is "Default".
Whenever you can, use the unicode_strings to cause to be the default instead.
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 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. Note that the /x modifier does NOT affect split STR patterns.
Otherwise, use locale sets the default modifier to /l; and use feature 'unicode_strings, or use v5.12 (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.
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.
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} Match at most 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 65534 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}? Match at most n times, not greedily
{n,m}? Match at least n but not more than m times, not greedilyNormally 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}+ Match at most n times and give nothing back
{n,m}+ Match at least n but not more than m times and give nothing backFor 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}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 viDetails are in "Quote and Quote-like Operators" in perlop.
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'name' [5] Named backreference
\k{name} [5] Named backreference
\K [6] Keep the stuff left of the \K, don't include it in $&
\N [7] Any character but \n. Not affected by /s modifier
\v [3] Vertical whitespace
\V [3] Not vertical whitespace
\h [3] Horizontal whitespace
\H [3] Not horizontal whitespace
\R [4] LinebreakSee "Bracketed Character Classes" in perlrecharclass for details.
See "POSIX Character Classes" in perlrecharclass for details.
See "Unicode Character Properties" in perlunicode for details
See "Misc" in perlrebackslash for details.
See "Capture groups" below for details.
See "Extended Patterns" below for details.
Note that \N has two meanings. When of the form \N{NAME}, it matches the character or character sequence whose name is NAME; and similarly when of the form \N{U+hex}, it matches the character whose Unicode code point is hex. Otherwise it matches any character but \n.
See "Extended Bracketed Character Classes" in perlrecharclass for details.
Besides "^" and "$", 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 XXXXX6789The 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
)
/xwould 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 in the same scope, whichever comes first. See "Compound Statements" in perlsyn and "Scoping Rules of Regex Variables" in perlvar for more details.
You can access the contents of a capture group 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/$x$y/, and $x contained "\g1", and $y contained "37", you would get /\g137/ which is probably not what you intended.
If you use braces, you may also optionally add any number of blank (space or tab) characters within but adjacent to the braces, like \g{ -1 }, or \k{ name }.
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
$x = '(.)\1'; # Creates problems when concatenated.
$y = '(.)\g{1}'; # Avoids the problems.
"aa" =~ /${x}/; # True
"aa" =~ /${y}/; # True
"aa0" =~ /${x}0/; # False!
"aa0" =~ /${y}0/; # True
"aa\x08" =~ /${x}0/; # True!
"aa\x08" =~ /${y}0/; # FalseSeveral 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.)
The @{^CAPTURE} array may be used to access ALL of the capture buffers as an array without needing to know how many there are. For instance
$string=~/$pattern/ and @captured = @{^CAPTURE};will place a copy of each capture variable, $1, $2 etc, into the @captured array.
Be aware that when interpolating a subscript of the @{^CAPTURE} array you must use demarcated curly brace notation:
print "${^CAPTURE[0]}";See "Demarcated variable names using braces" in perldata for more on this notation.
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.
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.
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+ \g1will 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/xxboth /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/iis equivalent to the more verbose
/(?:(?s-i)more.*than).*million/iNote 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 4Be 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 )) /xNot 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 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 $&.
(?!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).
(?<=pattern)