Overview

Basics of regular expressions. This summarizes commonly used notations in ERE (Extended Regular Expression).

Character Classes

[string]

• Matches any single character in the string.
• Ex:
  • [fox]
    • fox raccoon cat
      • f, o, x

[^string]

• Matches any single character not in the string.
• Ex:
  • [^fox]
    • fox raccoon cat
      • r, a, c, o, n

[string-string]

• Matches any single character in the specified range.
• Ex:
  • [a-z]
    • fox raccoon cat
      • f, o, x, r, a, c, o, n

\d

• Matches decimal digits.
• Ex:
  • \d
    • There are 10 apples
      • 1, 0

\D

• Matches any character that is not a decimal digit.
• Ex:
  • \D
    • There are 10 apples
      • T, h, e, r, e, a, p, l, e, s

\w

• Matches all alphanumeric characters and underscores.
• Ex:
  • \w
    • abc_*
      • a, b, c, _

\W

• Matches all characters that are not alphanumeric or underscores.
• Ex:
  • \W
    • abc_*

\s

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

\S

• Matches any character that is not a whitespace.
• Ex:
  • \S
    • a b c
      • a, b, c

Anchors

^

• Matches the beginning of a line.
• Ex:
  • ^Thank you
    • Thank you, my friend
    • Yesterday, thank you ✗

$

• Matches the end of a line.
• Ex:
  • thank you$
    • Thank you, my friend ✗
    • Yesterday, thank you ○

Grouping Constructs

(subexpression)

• Captures the string that matches the subexpression.
• Ex:
  • (ri){2}
    • aririri thank you ○
    • ariri thank you ✗

Quantifiers

*

• Matches when the preceding element is repeated 0 or more times.
• Greedy quantifier.
• Ex:
  • ab*
    • ab
      • ab
    • abab
      • ab, ab
    • aabb
      • ab
    • abbb
      • abbb
    • a
      • a
    • ba
      • a

+

• Matches when the preceding element is repeated 1 or more times.
• Greedy quantifier.
• Ex:
  • ab+
    • ab
      • ab
    • abab
      • ab, ab
    • aabb
      • abb
    • abbb
      • abbb
    • a
      • No match
    • ba
      • No match

?

• Matches when the preceding element is repeated 0 or 1 time.
• Greedy quantifier.
• Ex:
  • ab?
    • ab
      • ab
    • abab
      • ab, ab
    • aabb
      • a, ab
    • abbb
      • ab
    • a
      • a
    • ba
      • a

*?

• Matches when the preceding element is repeated 0 or more times.
• Lazy quantifier.
• Ex:
  • ab*?
    • ab
      • a
    • abab
      • a, a
    • aabb
      • a, a
    • abbb
      • a
    • a
      • a
    • ba
      • a

+?

• Matches when the preceding element is repeated 1 or more times.
• Lazy quantifier.
• Ex:
  • ab+?
    • ab
      • ab
    • abab
      • ab, ab
    • aabb
      • ab
    • abbb
      • ab
    • a
      • No match
    • ba
      • No match

??

• Matches when the preceding element is repeated 0 or 1 time.
• Lazy quantifier.
• Ex:
  • ab??
    • ab
      • a
    • abab
      • a, a
    • aabb
      • a, a
    • abbb
      • a
    • a
      • a
    • ba
      • a

{n}

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

{n,}

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

{n, m}

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

Alternation Constructs

|

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

References

• Regular Expression Language - Quick Reference
• Regular Expression Notes
• Qiita - Regular Expressions Usable in Any UNIX Command
• SE Academy - Regular Expressions