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PERLRE

Section: Perl Programmers Reference Guide (1)
Updated: perl 5.004, patch 04
Index

NAME

perlre - Perl regular expressions

DESCRIPTION

This page describes the syntax of regular expressions in Perl. For adescription of how to use regular expressions in matchingoperations, plus various examples of the same, see m// and s/// inthe perlop manpage.

The matching operations can have various modifiers. The modifierswhich relate to the interpretation of the regular expression insideare listed below. For the modifiers that alter the behaviour of theoperation, see the section on m// in the perlop manpage and the section on s// in the perlop manpage.

i: Do case-insensitive pattern matching.
If use locale is in effect, the case map is taken from the currentlocale. See the perllocale manpage.
m: Treat string as multiple lines. That is, change ``^'' and ``$'' from matchingat only the very start or end of the string to the start or end of anyline anywhere within the string,
s: Treat string as single line. That is, change ``.'' to match any characterwhatsoever, even a newline, which it normally would not match.
x: Extend your pattern's legibility by permitting whitespace and comments.

These are usually written as ``the /x modifier'', even though the delimiterin question might not actually be a slash. In fact, any of thesemodifiers may also be embedded within the regular expression itself usingthe new (?...) construct. See below.

The /x modifier itself needs a little more explanation. It tellsthe regular expression parser to ignore whitespace that is neitherbackslashed nor within a character class. You can use this to break upyour regular expression into (slightly) more readable parts. The #character is also treated as a metacharacter introducing a comment,just as in ordinary Perl code. This also means that if you want realwhitespace or # characters in the pattern that you'll have to eitherescape them or encode them using octal or hex escapes. Taken together,these features go a long way towards making Perl's regular expressionsmore readable. See the C comment deletion code in the perlop manpage.

Regular Expressions

The patterns used in pattern matching are regular expressions such asthose supplied in the Version 8 regexp routines. (In fact, theroutines are derived (distantly) from Henry Spencer's freelyredistributable reimplementation of the V8 routines.)See the section on Version 8 Regular Expressions for details.

In particular the following metacharacters have their standard egrep-ishmeanings:

    \   Quote the next metacharacter    ^   Match the beginning of the line    .   Match any character (except newline)    $   Match the end of the line (or before newline at the end)    |   Alternation    ()  Grouping    []  Character class

By default, the ``^'' character is guaranteed to match at only thebeginning of the string, the ``$'' character at only the end (or before thenewline at the end) and Perl does certain optimizations with theassumption that the string contains only one line. Embedded newlineswill not be matched by ``^'' or ``$''. You may, however, wish to treat astring as a multi-line buffer, such that the ``^'' will match after anynewline within the string, and ``$'' will match before any newline. At thecost of a little more overhead, you can do this by using the /m modifieron the pattern match operator. (Older programs did this by setting $*,but this practice is now deprecated.)

To facilitate multi-line substitutions, the ``.'' character never matches anewline unless you use the /s modifier, which in effect tells Perl to pretendthe string is a single line---even if it isn't. The /s modifier alsooverrides the setting of $*, in case you have some (badly behaved) oldercode that sets it in another module.

The following standard quantifiers are recognized:

    *      Match 0 or more times    +      Match 1 or more times    ?      Match 1 or 0 times    {n}    Match exactly n times    {n,}   Match at least n times    {n,m}  Match at least n but not more than m times

(If a curly bracket occurs in any other context, it is treatedas a regular character.) The ``*'' modifier is equivalent to {0,}, the ``+''modifier to {1,}, and the ``?'' modifier to {0,1}. n and m are limitedto integral values less than 65536.

By default, a quantified subpattern is ``greedy'', that is, it will match asmany times as possible (given a particular starting location) while stillallowing the rest of the pattern to match. If you want it to match theminimum number of times possible, follow the quantifier with a ``?''. Notethat the meanings don't change, just the ``greediness":

    *?     Match 0 or more times    +?     Match 1 or more times    ??     Match 0 or 1 time    {n}?   Match exactly n times    {n,}?  Match at least n times    {n,m}? Match at least n but not more than m times

Because patterns are processed as double quoted strings, the followingalso 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)    \033        octal char (think of a PDP-11)    \x1B        hex char    \c[         control char    \l          lowercase next char (think vi)    \u          uppercase next char (think vi)    \L          lowercase till \E (think vi)    \U          uppercase till \E (think vi)    \E          end case modification (think vi)    \Q          quote (disable) regexp metacharacters till \E

If use locale is in effect, the case map used by \l, \L, \uand <\U> is taken from the current locale. See the perllocale manpage.

In addition, Perl defines the following:

    \w  Match a "word" character (alphanumeric plus "_")    \W  Match a non-word character    \s  Match a whitespace character    \S  Match a non-whitespace character    \d  Match a digit character    \D  Match a non-digit character

Note that \w matches a single alphanumeric character, not a wholeword. To match a word you'd need to say \w+. If use locale is ineffect, the list of alphabetic characters generated by \w is takenfrom the current locale. See the perllocale manpage. You may use \w, \W,\s, \S, \d, and \D within character classes (though not aseither end of a range).

Perl defines the following zero-width assertions:

    \b  Match a word boundary    \B  Match a non-(word boundary)    \A  Match at only beginning of string    \Z  Match at only end of string (or before newline at the end)    \G  Match only where previous m//g left off (works only with /g)

A word boundary (\b) is defined as a spot between two characters thathas a \w on one side of it and a \W on the other side of it (ineither order), counting the imaginary characters off the beginning andend of the string as matching a \W. (Within character classes \brepresents backspace rather than a word boundary.) The \A and \Z arejust like ``^'' and ``$'' except that they won't match multiple times when the/m modifier is used, while ``^'' and ``$'' will match at every internal lineboundary. To match the actual end of the string, not ignoring newline,you can use \Z(?!\n). The \G assertion can be used to chain globalmatches (using m//g), as described inthe section on Regexp Quote-Like Operators in the perlop manpage.

It is also useful when writing lex-like scanners, when you have severalregexps which you want to match against consequent substrings of yourstring, see the previous reference.The actual location where \G will match can also be influencedby using pos() as an lvalue. See the pos entry in the perlfunc manpage.

When the bracketing construct ( ... ) is used, \<digit> matches thedigit'th substring. Outside of the pattern, always use ``$'' instead of ``\''in front of the digit. (While the \<digit> notation can on rare occasion workoutside the current pattern, this should not be relied upon. See theWARNING below.) The scope of $<digit> (and $`, $&, and $')extends to the end of the enclosing BLOCK or eval string, or to the nextsuccessful pattern match, whichever comes first. If you want to useparentheses to delimit a subpattern (e.g., a set of alternatives) withoutsaving it as a subpattern, follow the ( with a ?:.

You may have as many parentheses as you wish. If you have morethan 9 substrings, the variables $10, $11, ... refer to thecorresponding substring. Within the pattern, \10, \11, etc. refer backto substrings if there have been at least that many left parentheses beforethe backreference. Otherwise (for backward compatibility) \10 is thesame as \010, a backspace, and \11 the same as \011, a tab. And soon. (\1 through \9 are always backreferences.)

$+ returns whatever the last bracket match matched. $& returns theentire matched string. ($0 used to return the same thing, but not anymore.) $` returns everything before the matched string. $' returnseverything after the matched string. Examples:

    s/^([^ ]*) *([^ ]*)/$2 $1/;     # swap first two words

    if (/Time: (..):(..):(..)/) {        $hours = $1;        $minutes = $2;        $seconds = $3;    }

Once perl sees that you need one of $&, $` or $' anywhere inthe program, it has to provide them on each and every pattern match.This can slow your program down. The same mechanism that handlesthese provides for the use of $1, $2, etc., so you pay the same pricefor each regexp that contains capturing parentheses. But if you neveruse $&, etc., in your script, then regexps without capturingparentheses won't be penalized. So avoid $&, $', and $` if you can,but if you can't (and some algorithms really appreciate them), onceyou've used them once, use them at will, because you've already paidthe price.

You will note that all backslashed metacharacters in Perl arealphanumeric, such as \b, \w, \n. Unlike some other regularexpression languages, there are no backslashed symbols that aren'talphanumeric. So anything that looks like \\, $, $, \<, \>,\{, or \} is always interpreted as a literal character, not ametacharacter. This was once used in a common idiom to disable orquote the special meanings of regular expression metacharacters in astring that you want to use for a pattern. Simply quote all thenon-alphanumeric characters:

    $pattern =~ s/(\W)/\\$1/g;

Now it is much more common to see either the quotemeta() function orthe \Q escape sequence used to disable the metacharacters specialmeanings like this:

    /$unquoted\Q$quoted\E$unquoted/

Perl defines a consistent extension syntax for regular expressions.The syntax is a pair of parentheses with a question mark as the firstthing within the parentheses (this was a syntax error in olderversions of Perl). The character after the question mark gives thefunction of the extension. Several extensions are already supported:

(?#text)

A comment. The text is ignored. If the /x switch is used to enablewhitespace formatting, a simple # will suffice.

(?:regexp)

This groups things like ``()'' but doesn't make backreferences like ``()'' does. So

    split(/\b(?:a|b|c)\b/)

is like

    split(/\b(a|b|c)\b/)

but doesn't spit out extra fields.

(?=regexp)

A zero-width positive lookahead assertion. For example, /\w+(?=\t)/matches a word followed by a tab, without including the tab in $&.

(?!regexp)

A zero-width negative lookahead assertion. For example /foo(?!bar)/matches any occurrence of ``foo'' that isn't followed by ``bar''. Notehowever that lookahead and lookbehind are NOT the same thing. You cannotuse this for lookbehind: /(?!foo)bar/ will not find an occurrence of``bar'' that is preceded by something which is not ``foo''. That's becausethe (?!foo) is just saying that the next thing cannot be ``foo''---andit's not, it's a ``bar'', so ``foobar'' will match. You would have to dosomething like /(?!foo)...bar/ for that. We say ``like'' because there'sthe case of your ``bar'' not having three characters before it. You couldcover that this way: /(?:(?!foo)...|^..?)bar/. Sometimes it's stilleasier just to say:

    if (/foo/ && $` =~ /bar$/)

(?imsx)

One or more embedded pattern-match modifiers. This is particularlyuseful for patterns that are specified in a table somewhere, some ofwhich want to be case sensitive, and some of which don't. The caseinsensitive ones need to include merely (?i) at the front of thepattern. For example:

    $pattern = "foobar";    if ( /$pattern/i )

    # more flexible:

    $pattern = "(?i)foobar";    if ( /$pattern/ )

The specific choice of question mark for this and the new minimalmatching construct was because 1) question mark is pretty rare in olderregular expressions, and 2) whenever you see one, you should stopand ``question'' exactly what is going on. That's psychology...

Backtracking

A fundamental feature of regular expression matching involves the notioncalled backtracking. which is used (when needed) by all regularexpression quantifiers, namely *, *?, +, +?, {n,m}, and{n,m}?.

For a regular expression to match, the entire regular expression mustmatch, not just part of it. So if the beginning of a pattern containing aquantifier succeeds in a way that causes later parts in the pattern tofail, the matching engine backs up and recalculates the beginningpart---that's why it's called backtracking.

Here is an example of backtracking: Let's say you want to find theword 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'sno whitespace following the ``Foo'' that it had saved in $1, it realizes itsmistake and starts over again one character after where it had thetentative match. This time it goes all the way until the next occurrenceof ``foo''. The complete regular expression matches this time, and you getthe expected output of ``table follows foo.''

Sometimes minimal matching can help a lot. Imagine you'd like to matcheverything between ``foo'' and ``bar''. Initially, you write somethinglike 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 thefirst ``foo'' and the last ``bar''. In this case, it's more effectiveto 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 endof a string, and you also want to keep the preceding part 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 thewhole string. As \d* can match on an empty string the completeregular 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 aregular expression is merely a set of assertions that gives a definitionof success. There may be 0, 1, or several different ways that thedefinition might succeed against a particular string. And if there aremultiple 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 eventricker. Imagine you'd like to find a sequence of non-digits notfollowed 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. Itclaims that there is no 123 in the string. Here's a clearer picture ofwhy it 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 moregeneral purpose version of test 1. The important difference betweenthem is that test 3 contains a quantifier (\D*) and so can usebacktracking, whereas test 1 will not. What's happening isthat you've asked ``Is it true that at the start of $x, following 0 or morenon-digits, you have something that's not 123?'' If the pattern matcher hadlet \D* expand to ``ABC'', this would have caused the whole pattern tofail.The search engine will initially match \D* with ``ABC''. Then it willtry to match (?!123 with ``123'' which, of course, fails. But becausea quantifier (\D*) has been used in the regular expression, thesearch engine can backtrack and retry the match differentlyin the hope of matching the complete regular expression.

Well now,the pattern really, really wants to succeed, so it uses thestandard regexp back-off-and-retry and lets \D* expand to just ``AB'' thistime. Now there's indeed something following ``AB'' that is not``123''. It's in fact ``C123'', which suffices.

We can deal with this by using both an assertion and a negation. We'llsay that the first part in $1 must be followed by a digit, and in fact, itmust also be followed by something that's not ``123''. Remember that thelookaheads are zero-width expressions---they only look, but don't consumeany of the string in their match. So rewriting this way produces whatyou'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 likethey're ANDed together, just as you'd use any builtin assertions: /^$/matches only if you're at the beginning of the line AND the end of theline simultaneously. The deeper underlying truth is that juxtaposition inregular expressions always means AND, except when you write an explicit ORusing 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.

One warning: particularly complicated regular expressions can takeexponential time to solve due to the immense number of possible ways theycan use backtracking to try match. For example this will take a very longtime to run

    /((a{0,5}){0,5}){0,5}/

And if you used *'s instead of limiting it to 0 through 5 matches, thenit would take literally forever---or until you ran out of stack space.

Version 8 Regular Expressions

In case you're not familiar with the ``regular'' Version 8 regexproutines, here are the pattern-matching rules not described above.

Any single character matches itself, unless it is a metacharacterwith a special meaning described here or above. You can causecharacters which normally function as metacharacters to be interpretedliterally by prefixing them with a ``\'' (e.g., ``\.'' matches a ``.'', not anycharacter; ``\\'' matches a ``\"). A series of characters matches thatseries of characters in the target string, so the pattern blurflwould match ``blurfl'' in the target string.

You can specify a character class, by enclosing a list of charactersin [], which will match any one of the characters in the list. If thefirst character after the ``['' is ``^'', the class matches any character notin the list. Within a list, the ``-'' character is used to specify arange, so that a-z represents all the characters between ``a'' and ``z'',inclusive. If you want ``-'' itself to be a member of a class, put itat the start or end of the list, or escape it with a backslash. (Thefollowing all specify the same class of three characters: [-az],[az-], and [a\-z]. All are different from [a-z], whichspecifies a class containing twenty-six characters.)

Characters may be specified using a metacharacter syntax much like thatused in C: ``\n'' matches a newline, ``\t'' a tab, ``\r'' a carriage return,``\f'' a form feed, etc. More generally, \nnn, where nnn is a stringof octal digits, matches the character whose ASCII value is nnn.Similarly, \xnn, where nn are hexadecimal digits, matches thecharacter whose ASCII value is nn. The expression \cx matches theASCII character control-x. Finally, the ``.'' metacharacter matches anycharacter except ``\n'' (unless you use /s).

You can specify a series of alternatives for a pattern using ``|'' toseparate them, so that fee|fie|foe will match any of ``fee'', ``fie'',or ``foe'' in the target string (as would f(e|i|o)e). Note that thefirst alternative includes everything from the last pattern delimiter("('', ``['', or the beginning of the pattern) up to the first ``|'', andthe last alternative contains everything from the last ``|'' to the nextpattern delimiter. For this reason, it's common practice to includealternatives in parentheses, to minimize confusion about where theystart and end. Note however that ``|'' is interpreted as a literal withsquare brackets, so if you write [fee|fie|foe] you're really onlymatching [feio|].

Within a pattern, you may designate subpatterns for later reference byenclosing them in parentheses, and you may refer back to the nthsubpattern later in the pattern using the metacharacter \n.Subpatterns are numbered based on the left to right order of theiropening parenthesis. Note that a backreference matches whateveractually matched the subpattern in the string being examined, not therules for that subpattern. Therefore, (0|0x)\d*\s\1\d* willmatch ``0x1234 0x4321'',but not ``0x1234 01234'', because subpattern 1actually matched ``0x'', even though the rule 0|0x couldpotentially match the leading 0 in the second number.

WARNING on \1 vs $1

Some people get too used to writing things like

    $pattern =~ s/(\W)/\\\1/g;

This is grandfathered for the RHS of a substitute to avoid shocking thesed addicts, but it's a dirty habit to get into. That's because inPerlThink, the righthand side of a s/// is a double-quoted string. \1 inthe usual double-quoted string means a control-A. The customary Unixmeaning of \1 is kludged in for s///. However, if you get into the habitof doing that, you get yourself into trouble if you then add an /emodifier.

    s/(\d+)/ \1 + 1 /eg;

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. Basically, the operation of interpolation should not be confusedwith the operation of matching a backreference. Certainly they mean twodifferent things on the left side of the s///.

SEE ALSO

``Mastering Regular Expressions'' (see the perlbook manpage) by Jeffrey Friedl.

This document was created byman2html,using the manual pages.