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
- \N{U+263D} A Unicode character by hex ordinal
- \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 (except \N
) 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 whitespace
- \H A non horizontal whitespace
- \N A non newline (when not followed by '{NAME}'; experimental; not
- valid in a character class; equivalent to [^\n]; it's like '.'
- without /s modifier)
- \v A vertical whitespace
- \V A non vertical whitespace
- \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 name longer than 1 character
- \PP Match non-P
- \P{...} Match lack of Unicode property with name longer than 1 char
- \X Match Unicode extended grapheme cluster
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 and 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's interpreted 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|no)
- (?(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.
perluniintro, perlunicode, charnames and perllocale for details on regexes and internationalisation.
Mastering Regular Expressions by Jeffrey Friedl (http://oreilly.com/catalog/9780596528126/) 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.