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Regex

Regular expressions are a powerful tool for pattern matching and text processing. They are used in many programming languages and tools like grep, sed, awk, and more.

Resources​

Syntax​

/pattern/modifiers

Ranges:​

  • [abc] : Matches a, b, or c
  • [^abc] : Matches any character except a, b, or c
  • [a-z] : Matches any lowercase letter
  • [A-Z] : Matches any uppercase letter
  • [a-zA-Z] : Matches any letter
  • [0-9] : Matches any digit
  • [a-zA-Z0-9] : Matches any letter or digit

Quantifiers:​

  • a : Matches the string a
  • a+ : Matches the string with one or more as like a, aa, aaa, aaaa, aaaaa, ....
  • a* : Matches the string with zero or more as like "", a, aa, aaa, aaaa, aaaaa, ....
  • a? : Matches the string with zero or one as like "", a
  • a{3} : Matches the string aaa
  • abc : Matches the string abc
  • [ ] = []{1} : Matches exactly one occurrence
  • [ ]? : Matches zero or one occurrence
  • [ ]+ : Matches one or more occurrences
  • [ ]* : Matches zero or more occurrences
  • [ ]{n} : Matches exactly n occurrences
  • [ ]{n,} : Matches n or more occurrences
  • [ ]{n,m} : Matches between n and m occurrences

Metacharacters (shortcuts):​

  • \d : Matches any digit, [0-9]
  • \D : Matches any non-digit, [^0-9]
  • \w : Matches any word character, [a-zA-Z0-9_]
  • \W : Matches any non-word character, [^a-zA-Z0-9_]
  • \s : Matches any whitespace character, [\t\n\f\r\p{Z}]
  • \S : Matches any non-whitespace character, [^\t\n\f\r\p{Z}]

Anchors:​

  • ^ : Matches the start of the string
  • $ : Matches the end of the string
  • \b : Matches a word boundary
  • \B : Matches a non-word boundary

Groups:​

  • ( ) : Used for grouping
  • (this|that) : Matches this or that

Order of Grouping:​

\1 : Matches the same text as most recently matched by the 1st capturing group \2 : Matches the same text as most recently matched by the 2nd capturing group

Example:

To match ha-ha,haa-haa:

(ha)-\1,(haa)-\2

Lookarounds:​

  • (?=...) : Positive lookahead
  • (?!...) : Negative lookahead
  • (?<=...) : Positive lookbehind
  • (?<!...) : Negative lookbehind

Mixed:​

  • . : Matches any character except newline
  • ..a : Matches any three-character string ending with a