Perl regex

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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|;

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