Regular Expressions

• Simple characters

  • abc

• Quantifiers

  • * 0 or more
  • + 1 or more
  • ? 0 or 1
  • {2} exactly 2
  • {2-4} 2, 3 or 4
  • {2,} 2 or more

• More characters

  • . wildcard, any character except line break
  • \d a digit, \D not a digit
  • \w word character, \W not a word character
  • \s whitespace character, \S not a whitespace character
  • a|b logical OR

• Character classes

  • [abc] character classes
  • [a-zA-Z] ranged character class
  • [^abc] negated character class

• Anchors (zero length)

  • ^ start of line
  • $ end of line
  • \b word boundary
  • \a start of string
  • \z end of string (including trailing \n)
  • \Z end of string (excluding trailing \n)

• Capture groups

  • (abc) standard capture group
  • (?:abc) non capturing group
  • \1, \2 captured groups
  • Ordered by the open brackets

• Look arounds

  • a(?=b) positive lookahead, match an a that is followed by a b (but don't match the b)
  • a(?!b) negative lookahead, match an a that is NOT followed by a b (but don't match what follows the a)
  • (?<=a)b positive lookbehind, match a b that is proceeded by a a (but don't match the a)
  • (?<!a)b negative lookbehind, match a b that is NOT proceeded by a a (but don't match what proceeds the b)

• Modifiers

  • Case-insensitivity (/i), global (/g), multiline (/m), etc.
  • There are more (ungreedy, line endings, etc.)
  • Often better to look at the tool/language as they vary a lot (/i, (?i), etc.)

• Tools

  • http://rubular.com/
  • http://regexr.com/
  • http://emailregex.com/
  • vim (with :set hls)
  • Remember some tools need some symbols escaped!