Difference between revisions of "Perl regex"
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* '''+''' = 1 or more of previous character | * '''+''' = 1 or more of previous character | ||
* '''?''' after '''+''', '''*''', or '''{}''' indicates non-greedy behavior, i.e. match the fewest characters, not the most | * '''?''' after '''+''', '''*''', or '''{}''' indicates non-greedy behavior, i.e. match the fewest characters, not the most | ||
+ | ** Apparently it can also mean "either zero or one of the preceding group or character", e.g. "the?" matches both "th" and "the" | ||
* <u>a</u>'''-'''<u>b</u> = range of characters from <u>a</u> to <u>b</u>, e.g. "t-w" means any of t,u,v,w in that position | * <u>a</u>'''-'''<u>b</u> = range of characters from <u>a</u> to <u>b</u>, e.g. "t-w" means any of t,u,v,w in that position | ||
* '''?=''' = lookahead (need explanation of how this works) <u>a</u>'''(?='''<u>b</u>''')''' returns "<u>a</u>, but only if it's followed by a <u>b</u>"; the <u>a</u> becomes part of the matched sequence, but the <u>b</u> does not | * '''?=''' = lookahead (need explanation of how this works) <u>a</u>'''(?='''<u>b</u>''')''' returns "<u>a</u>, but only if it's followed by a <u>b</u>"; the <u>a</u> becomes part of the matched sequence, but the <u>b</u> does not | ||
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** $string =~ s|^.*\.([^\.]*)$|$1|; | ** $string =~ s|^.*\.([^\.]*)$|$1|; | ||
==Links== | ==Links== | ||
+ | ===Reference=== | ||
+ | * [http://php.net/pcre Regular Expressions (Perl-Compatible)] in [[PHP]] | ||
+ | ** PHP also supports [http://php.net/regex POSIX regex] via the [http://php.net/ereg ereg*()] series of functions, but these are apparently deprecated; to convert code from ereg* to PCRE, just add slashes around the string: /<u>regex expression</u>/ | ||
+ | ===Articles=== | ||
* '''2007-10-24''' [http://pugs.blogs.com/pugs/2007/10/a-graphical-tra.html A graphical tracer for Perl 6 regexes based on PCR]: code for a graphical regex tracer written in [[Perl 6]], with links to some demo sites | * '''2007-10-24''' [http://pugs.blogs.com/pugs/2007/10/a-graphical-tra.html A graphical tracer for Perl 6 regexes based on PCR]: code for a graphical regex tracer written in [[Perl 6]], with links to some demo sites |
Latest revision as of 13:34, 16 July 2009
computing: programming: Perl: regex
Overview
This article explains regular expressions in terms understandable to mere mortals, and also how to use them in Perl.
Related Articles
- regex: manpage documentation
Details
Special characters in regex:
- . = any character except newline (\n) (if /s option is included, then \n is also matched)
- * = 0 or more of previous character
- ^ = following string begins the line (except [^...] means "not these characters")
- $ = preceding string ends the line
- [] = list of characters which can satisfy the match at this position
- Note: cannot use special characters like "." as part of list
- {} = # of repetitions of previous character:
- {x} -> exactly x repetitions
- {x,y} -> minimum of x repetitions, maximum of y repetitions
- | = alternatives
- + = 1 or more of previous character
- ? after +, *, or {} indicates non-greedy behavior, i.e. match the fewest characters, not the most
- Apparently it can also mean "either zero or one of the preceding group or character", e.g. "the?" matches both "th" and "the"
- a-b = range of characters from a to b, e.g. "t-w" means any of t,u,v,w in that position
- ?= = lookahead (need explanation of how this works) a(?=b) returns "a, but only if it's followed by a b"; the a becomes part of the matched sequence, but the b does not
- ?<= = reverse lookahead (need explanation of how this works)
Operators used to invoke regex:
- =~ returns TRUE if pattern matches
- !~ returns FALSE if pattern matches
- m/ searches a string for a pattern match; returns true/false scalar and an array of matches (if () are used)
- c don't reset pos on failed matches when using /g
- g (Global) all occurrences – repeat the pattern search until there are no more matches
- i case-Insensitive
- m Multiline mode - ^ and $ match internal lines
- o compile pattern Once
- s match as a Single line - . (dot) matches \n
- x eXtended legibility - free whitespace and comments
- s/pattern/replacement/gi; replaces pattern with replacement
- all m/ options are available
- e Evaluate replacement as an expression. May be specified multiple times. 'replacement' is interpreted as a double quoted string unless a single-quote (') is the delimiter.
- y/searchlist/replacelist/d: replaces each character found in searchlist with the corresponding character in replacelist
- d just deletes matching characters
- tr/ is the same as y/
Examples
These examples have been tested only briefly.
- Replace "thingy" with "stuffs" in $string:
- $string =~ s/thingy/stuffs/;
- Keep only the part of $string before the final "/" (using "|" as the delimiter instead of "/"):
- $string =~ s|(.*)/[^/]*|$1|;
- ...after the final "/":
- $string =~ s| ^.*/([^/]*)$|$1|;
- ...before the final "-":
- $string =~ s|(.*)-[^-]*|$1|;
- ...before the final ".":
- $string =~ s|(.*)\.[^\.]*|$1|;
- ...after the final "." (both of these return the full string if no "." is found):
- $string =~ s|^.+\.(.+$)|$1|;
- $string =~ s|^.*\.([^\.]*)$|$1|;
Links
Reference
- Regular Expressions (Perl-Compatible) in PHP
- PHP also supports POSIX regex via the ereg*() series of functions, but these are apparently deprecated; to convert code from ereg* to PCRE, just add slashes around the string: /regex expression/
Articles
- 2007-10-24 A graphical tracer for Perl 6 regexes based on PCR: code for a graphical regex tracer written in Perl 6, with links to some demo sites