Basics of Regular Expressions

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#ERE#Regular Expressions

Overview

An introduction to the basics of regular expressions. This post summarizes commonly used syntax in ERE (Extended Regular Expressions).

Character Classes

[Characters]

  • Matches any single character within the brackets.
  • Ex:
    • [きつね]
      • きつねたぬきねこ
        • き,つ,ね,き,ね

[^Characters]

  • Matches any single character not within the brackets.
  • Ex:
    • [^きつね]
      • きつねたぬきねこ
        • た,ね,こ

[Character-Character]

  • Matches any single character within the specified range.
  • Ex:
    • [あ-ん]
      • きつねたぬきcat
        • き,つ,ね,た,ぬ,き

\d

  • Matches any decimal digit.
  • Ex:
    • \d
      • りんごが10個
        • 1, 0

\D

  • Matches any character that is not a decimal digit.
  • Ex:
    • \D
      • りんごが10個
        • り,ん,ご,が,個

\w

  • Matches any alphanumeric character or underscore.
  • Ex:
    • \w
      • abc_*
        • a,b,c,_

\W

  • Matches any character that is not alphanumeric or an underscore.
  • Ex:
    • \W
      • abc_*

\s

  • Matches any whitespace character.
  • Ex:
    • \s
      • a b c
        • Matches two spaces (between a and b, and b and c).

\S

  • Matches any non-whitespace character.
  • Ex:
    • \S
      • a b c
        • a,b,c

Anchors

^

  • Matches the beginning of a line.
  • Ex:
    • ^ありがとう
      • ありがとう友よ
      • 昨日はありがとう ✗

$

  • Matches the end of a line.
  • Ex:
    • ありがとう$
      • ありがとう友よ ✗
      • 昨日はありがとう ○

Grouping Constructs

(Subexpression)

  • Captures the substring that matches the subexpression.
  • Ex:
    • (りり){2}
      • ありりりりがとう ○
      • ありりがとう ✗

Quantifiers

*

  • Matches the preceding element zero or more times (greedy).
  • Ex:
    • ab*
      • ab
        • ab
      • abab
        • ab, ab
      • aabb
        • ab
      • abbb
        • abbb
      • a
        • a
      • ba
        • a

+

  • Matches the preceding element one or more times (greedy).
  • Ex:
    • ab+
      • ab
        • ab
      • abab
        • ab,ab
      • aabb
        • abb
      • abbb
        • abbb
      • a
        • No match
      • ba
        • No match

?

  • Matches the preceding element zero or one time (greedy).
  • Ex:
    • ab?
      • ab
        • ab
      • abab
        • ab,ab
      • aabb
        • a, ab
      • abbb
        • ab
      • a
        • a
      • ba
        • a

*?

  • Matches the preceding element zero or more times (lazy).
  • Ex:
    • ab*?
      • ab
        • a
      • abab
        • a,a
      • aabb
        • a,a
      • abbb
        • a
      • a
        • a
      • ba
        • a

+?

  • Matches the preceding element one or more times (lazy).
  • Ex:
    • ab+?
      • ab
        • ab
      • abab
        • ab,ab
      • aabb
        • ab
      • abbb
        • ab
      • a
        • No match
      • ba
        • No match

??

  • Matches the preceding element zero or one time (lazy).
  • Ex:
    • ab??
      • ab
        • a
      • abab
        • a,a
      • aabb
        • a, a
      • abbb
        • a
      • a
        • a
      • ba
        • a

{n}

  • Matches the preceding element exactly n times.
  • Ex:
    • b{2}
      • abba
        • bb

{n,}

  • Matches the preceding element n or more times.
  • Ex:
    • b{2,}
      • abbba
        • bbb

{n, m}

  • Matches the preceding element between n and m times.
  • Ex:
    • b{2,4}
      • abbba
        • bbb
      • abbbba
        • bbbb

Alternation Constructs

|

  • Matches any one of the separated alternatives.
  • Ex:
    • ab|cd
      • abcd
        • ab,cd
      • aaccd
        • cd

References