Section: Perl Programmers Reference Guide (1)
Updated: 2007-12-18
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perlreref - Perl Regular Expressions Reference  


This is a quick reference to Perl's regular expressions. For full information see perlre and perlop, as well as the ``SEE ALSO'' section in this document.  


"=~" determines to which variable the regex is applied. In its absence, $_ is used.

    $var =~ /foo/;

"!~" determines to which variable the regex is applied, and negates the result of the match; it returns false if the match succeeds, and true if it fails.

    $var !~ /foo/;

"m/pattern/msixpogc" searches a string for a pattern match, applying the given options.

    m  Multiline mode - ^ and $ match internal lines
    s  match as a Single line - . matches \n
    i  case-Insensitive
    x  eXtended legibility - free whitespace and comments
    p  Preserve a copy of the matched string -
       ${^PREMATCH}, ${^MATCH}, ${^POSTMATCH} will be defined.
    o  compile pattern Once
    g  Global - all occurrences
    c  don't reset pos on failed matches when using /g

If 'pattern' is an empty string, the last successfully matched regex is used. Delimiters other than '/' may be used for both this operator and the following ones. The leading "m" can be omitted if the delimiter is '/'.

"qr/pattern/msixpo" lets you store a regex in a variable, or pass one around. Modifiers as for "m//", and are stored within the regex.

"s/pattern/replacement/msixpogce" substitutes matches of 'pattern' with 'replacement'. Modifiers as for "m//", with one addition:

    e  Evaluate 'replacement' as an expression

'e' may be specified multiple times. 'replacement' is interpreted as a double quoted string unless a single-quote ("'") is the delimiter.

"?pattern?" is like "m/pattern/" but matches only once. No alternate delimiters can be used. Must be reset with reset().  


   \       Escapes the character immediately following it
   .       Matches any single character except a newline (unless /s is used)
   ^       Matches at the beginning of the string (or line, if /m is used)
   $       Matches at the end of the string (or line, if /m is used)
   *       Matches the preceding element 0 or more times
   +       Matches the preceding element 1 or more times
   ?       Matches the preceding element 0 or 1 times
   {...}   Specifies a range of occurrences for the element preceding it
   [...]   Matches any one of the characters contained within the brackets
   (...)   Groups subexpressions for capturing to $1, $2...
   (?:...) Groups subexpressions without capturing (cluster)
   |       Matches either the subexpression preceding or following it
   \1, \2, \3 ...           Matches the text from the Nth group
   \g1 or \g{1}, \g2 ...    Matches the text from the Nth group
   \g-1 or \g{-1}, \g-2 ... Matches the text from the Nth previous group
   \g{name}     Named backreference
   \k<name>     Named backreference
   \k'name'     Named backreference
   (?P=name)    Named backreference (python syntax)



These work as in normal strings.

   \a       Alarm (beep)
   \e       Escape
   \f       Formfeed
   \n       Newline
   \r       Carriage return
   \t       Tab
   \037     Any octal ASCII value
   \x7f     Any hexadecimal ASCII value
   \x{263a} A wide hexadecimal value
   \cx      Control-x
   \N{name} A named character

   \l  Lowercase next character
   \u  Titlecase next character
   \L  Lowercase until \E
   \U  Uppercase until \E
   \Q  Disable pattern metacharacters until \E
   \E  End modification

For Titlecase, see ``Titlecase''.

This one works differently from normal strings:

   \b  An assertion, not backspace, except in a character class



   [amy]    Match 'a', 'm' or 'y'
   [f-j]    Dash specifies "range"
   [f-j-]   Dash escaped or at start or end means 'dash'
   [^f-j]   Caret indicates "match any character _except_ these"

The following sequences work within or without a character class. The first six are locale aware, all are Unicode aware. See perllocale and perlunicode for details.

   \d      A digit
   \D      A nondigit
   \w      A word character
   \W      A non-word character
   \s      A whitespace character
   \S      A non-whitespace character
   \h      An horizontal white space
   \H      A non horizontal white space
   \v      A vertical white space
   \V      A non vertical white space
   \R      A generic newline           (?>\v|\x0D\x0A)

   \C      Match a byte (with Unicode, '.' matches a character)
   \pP     Match P-named (Unicode) property
   \p{...} Match Unicode property with long name
   \PP     Match non-P
   \P{...} Match lack of Unicode property with long name
   \X      Match extended Unicode combining character sequence

POSIX character classes and their Unicode and Perl equivalents:

   alnum   IsAlnum              Alphanumeric
   alpha   IsAlpha              Alphabetic
   ascii   IsASCII              Any ASCII char
   blank   IsSpace  [ \t]       Horizontal whitespace (GNU extension)
   cntrl   IsCntrl              Control characters
   digit   IsDigit  \d          Digits
   graph   IsGraph              Alphanumeric and punctuation
   lower   IsLower              Lowercase chars (locale and Unicode aware)
   print   IsPrint              Alphanumeric, punct, and space
   punct   IsPunct              Punctuation
   space   IsSpace  [\s\ck]     Whitespace
           IsSpacePerl   \s     Perl's whitespace definition
   upper   IsUpper              Uppercase chars (locale and Unicode aware)
   word    IsWord   \w          Alphanumeric plus _ (Perl extension)
   xdigit  IsXDigit [0-9A-Fa-f] Hexadecimal digit

Within a character class:

    POSIX       traditional   Unicode
    [:digit:]       \d        \p{IsDigit}
    [:^digit:]      \D        \P{IsDigit}



All are zero-width assertions.

   ^  Match string start (or line, if /m is used)
   $  Match string end (or line, if /m is used) or before newline
   \b Match word boundary (between \w and \W)
   \B Match except at word boundary (between \w and \w or \W and \W)
   \A Match string start (regardless of /m)
   \Z Match string end (before optional newline)
   \z Match absolute string end
   \G Match where previous m//g left off

   \K Keep the stuff left of the \K, don't include it in $&



Quantifiers are greedy by default --- match the longest leftmost.

   Maximal Minimal Possessive Allowed range
   ------- ------- ---------- -------------
   {n,m}   {n,m}?  {n,m}+     Must occur at least n times
                              but no more than m times
   {n,}    {n,}?   {n,}+      Must occur at least n times
   {n}     {n}?    {n}+       Must occur exactly n times
   *       *?      *+         0 or more times (same as {0,})
   +       +?      ++         1 or more times (same as {1,})
   ?       ??      ?+         0 or 1 time (same as {0,1})

The possessive forms (new in Perl 5.10) prevent backtracking: what gets matched by a pattern with a possessive quantifier will not be backtracked into, even if that causes the whole match to fail.

There is no quantifier {,n} --- that gets understood as a literal string.  


   (?#text)          A comment
   (?:...)           Groups subexpressions without capturing (cluster)
   (?pimsx-imsx:...) Enable/disable option (as per m// modifiers)
   (?=...)           Zero-width positive lookahead assertion
   (?!...)           Zero-width negative lookahead assertion
   (?<=...)          Zero-width positive lookbehind assertion
   (?<!...)          Zero-width negative lookbehind assertion
   (?>...)           Grab what we can, prohibit backtracking
   (?|...)           Branch reset
   (?<name>...)      Named capture
   (?'name'...)      Named capture
   (?P<name>...)     Named capture (python syntax)
   (?{ code })       Embedded code, return value becomes $^R
   (??{ code })      Dynamic regex, return value used as regex
   (?N)              Recurse into subpattern number N
   (?-N), (?+N)      Recurse into Nth previous/next subpattern
   (?R), (?0)        Recurse at the beginning of the whole pattern
   (?&name)          Recurse into a named subpattern
   (?P>name)         Recurse into a named subpattern (python syntax)
   (?(cond)yes)      Conditional expression, where "cond" can be:
                     (N)       subpattern N has matched something
                     (<name>)  named subpattern has matched something
                     ('name')  named subpattern has matched something
                     (?{code}) code condition
                     (R)       true if recursing
                     (RN)      true if recursing into Nth subpattern
                     (R&name)  true if recursing into named subpattern
                     (DEFINE)  always false, no no-pattern allowed



   $_    Default variable for operators to use

   $`    Everything prior to matched string
   $&    Entire matched string
   $'    Everything after to matched string

   ${^PREMATCH}   Everything prior to matched string
   ${^MATCH}      Entire matched string
   ${^POSTMATCH}  Everything after to matched string

The use of "$`", $& or "$'" will slow down all regex use within your program. Consult perlvar for "@-" to see equivalent expressions that won't cause slow down. See also Devel::SawAmpersand. Starting with Perl 5.10, you can also use the equivalent variables "${^PREMATCH}", "${^MATCH}" and "${^POSTMATCH}", but for them to be defined, you have to specify the "/p" (preserve) modifier on your regular expression.

   $1, $2 ...  hold the Xth captured expr
   $+    Last parenthesized pattern match
   $^N   Holds the most recently closed capture
   $^R   Holds the result of the last (?{...}) expr
   @-    Offsets of starts of groups. $-[0] holds start of whole match
   @+    Offsets of ends of groups. $+[0] holds end of whole match
   %+    Named capture buffers
   %-    Named capture buffers, as array refs

Captured groups are numbered according to their opening paren.  


   lc          Lowercase a string
   lcfirst     Lowercase first char of a string
   uc          Uppercase a string
   ucfirst     Titlecase first char of a string

   pos         Return or set current match position
   quotemeta   Quote metacharacters
   reset       Reset ?pattern? status
   study       Analyze string for optimizing matching

   split       Use a regex to split a string into parts

The first four of these are like the escape sequences "\L", "\l", "\U", and "\u". For Titlecase, see ``Titlecase''.  



Unicode concept which most often is equal to uppercase, but for certain characters like the German ``sharp s'' there is a difference.  


Iain Truskett. Updated by the Perl 5 Porters.

This document may be distributed under the same terms as Perl itself.  


perlretut for a tutorial on regular expressions.
perlrequick for a rapid tutorial.
perlre for more details.
perlvar for details on the variables.
perlop for details on the operators.
perlfunc for details on the functions.
perlfaq6 for FAQs on regular expressions.
perlrebackslash for a reference on backslash sequences.
perlrecharclass for a reference on character classes.
The re module to alter behaviour and aid debugging.
``Debugging regular expressions'' in perldebug
perluniintro, perlunicode, charnames and perllocale for details on regexes and internationalisation.
Mastering Regular Expressions by Jeffrey Friedl ( for a thorough grounding and reference on the topic.


David P.C. Wollmann, Richard Soderberg, Sean M. Burke, Tom Christiansen, Jim Cromie, and Jeffrey Goff for useful advice.