Regular expressions (regex) let you search for a particular pattern. Regex consist of literal characters, and special characters, also called meta characters. Regex syntax has 2 modes, one inside of character classes, which is inside [ ] and one outsided of character classes.

Character classes []:
  Regex inside [] are in different mode and have different syntax. It matches a
  single character specified as a list, range or class shortcut.

  03[-./]19[-./]76
  Will match the above date delimited by a dot or a hyphen.
  Inside [] the dot is no longer a meta-character, its matched literally!

  Negation Operator ^
  f[^u]
  matches something with f, followed by any character other then u

  Character classes shortcuts
  d [0-9] digit
  D non digit
  w [a-zA-Z0-9_] part of word
  W non word
  s [ tnrfv] whitespace
  S non whitespace

Alternation:
  | operator is 'or'

  Jeff(re|er|izz)y
  => Jeffrey, Jeffery, Jeffizzy

Conditionals:
  (?if then | else)
  if the "if" part is true, "then" expression is attmpted, otherwise
  "else" is attempted

  (?if then)
  else condition is optional, so is the pipe character prefixing it

Anchors:
  ^  beg of line
  $  end of line
  b word boundary
  B non-word boundary

  Ruby:
  A begining of a string
  Z end of string (or before newline at the end)
  z end of string

Optional Items:
  ? optionally matches a character before it, or you can specify a group() of
  characters before it:

  colou?r
  will match color and colour

  4(th)?
  will match 4 and 4th

Repetitions:
  +            s1 or more times
  *            0 or more times
  ?            1 or 0 times
  {n}          match exactly n times
  {min, max}   min to max times
  {min,}       at least min
  {,max}       at most max (ruby)

Backreferences:
  First matched group ():
    sed, vi: 1
    ruby: $1
    php: $m[1]
    python, java: m.group(1)
    c#: m.Groups[1]
  Group zero is typically the entire match

Escape:
  . will escape dot meta character

More Info:
  http://www.regular-expressions.info/

This entry was posted on Monday, December 18th, 2006 at 3:37 pm and is filed under Programming, Regular Expressions (regex). You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.