A regular expression (regex) (sometimes called a rational expression) is a sequence of characters that define a search pattern, mainly for use in pattern matching with strings, or string matching.

Cheat sheet

Character classes

  • . - any character except newline
  • \w \d \s - word, digit, whitespace
  • \W \D \S - not word, digit, whitespace
  • [abc] - any of a, b, or c
  • [^abc] - not a, b, or c
  • [a-g] - character between a & g

Anchors

  • ^abc$ - start / end of the string
  • \b - word boundary

Escaped characters

  • \t\n\r - escaped special characters
  • \. \* \\ - tab, linefeed, carriage return
  • \u00A9 - unicode escaped ©

Groups & Lookaround

  • (abc) - capture group
  • \1 - backreference to group #1
  • (?:abc) - non-capturing group
  • (?=abc) - positive lookahead
  • (?!abc) - negative lookahead

Quantifiers & Alternation

  • a* a+ a? - 0 or more, 1 or more, 0 or 1
  • a{5} a{2,} - exactly five, two or more
  • a{1,3} - between one & three
  • a+? a{2,}? - match as few as possible
  • ab|cd - match ab or cd

Examples

Single line comment

  • \/\/.*$ - will match all single line (// ...) comments

Multiline momment

  • \/\*[\s\S]*?\*\/ - will match all multiline (/* ... */) comments

Replace the line containing the Regex

  • ^.*xyz.*$ - will match the lines containing the specified (“xyz” in this case) pattern
  • .*xyz.* - in Java the regex does not default to DOTALL (single line mode) so we have to remove the ^ and $
  • .*@@.*(\r?\n|\r)? - full regex that does not leave blank lines

Email

^([A-Z|a-z|0-9](\.|_){0,1})+[A-Z|a-z|0-9]\@([A-Z|a-z|0-9])+((\.){0,1}[A-Z|a-z|0-9]){2}\.[a-z]{2,3}$

Acceptable

  • abc1.1@domain.com
  • abc.abc@domain.com
  • abc_41.15768@domain.com
  • 12345@domain.com
  • 12.12.23@domain.com
  • ABC.ABC.ABC.ABC0046@domain.com
  • abc_123_4595@domain.com
  • abc@asdas.co.in
  • abc@insta123.com
  • abc@inta.co.in

Non-Acceptable

  • abc@@insta.com.com.com
  • abc@insta..com
  • abc@insta%.com
  • abc@insta/.com
  • abc@insta.com
  • abc..abc@domain.com
  • abc__abc@domain.com
  • abc.@insta.com
  • abc..de@insta.com
  • abc-@insta.com
  • abc-def@insta.com
  • abc+@insta.com
  • abc123..45@insta.com
  • abc%@insta.com
  • abc\@insta.com
  • abc/@insta.com
  • abc@insta-.com
  • ABC.ABC.ABC.ABC0046@domain.com.com.com

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