A brief introduction to regular expressions

Awk matches text in terms of regular expressions, which are enclosed in forward slashes: /pattern/. Here is a brief synopsis of some of the basics:

• An ordinary character matches itself. Ordinary characters include letters, digits, and spaces. For example, /banana/ matches lines containing banana and bananas.
• A metacharacter matches one or more characters. Some metacharacters include:
  • The period (.), which matches any single character. For example, the pattern /h.t/ matches lines containing hat, hotter, and huts.
  • The asterisk (*), which matches zero or more occurrences of of the character preceding it. For example, the pattern /met*/ matches lines containing meteorology, mean, and mettle.
  • The circumflex (^), which matches lines that begin with the indicated pattern. For example, /^#/ matches lines beginning with the # character.
  • The dollar sign ($), which matches lines that end with the indicated pattern. For example, /xyz$/ matches lines whose last three characters are xyz.
• Characters in square brackets match a single character from the indicated selection. For example:
  • /[Aa]rmy/ matches lines containing army and Army.
  • /[A-Z]/ matches lines containing any capital letter.
  • /xy[0-9]/ matches lines containing any of xy0, xy1, ..., xy9.
• All these elements can be combined in reasonable ways. For example,
  • /^[0-9][0-9]/ matches lines that begin with two consecutive digits.
  • /[0-9][0-9]*/ matches lines containing one or more consecutive digits.
  • /^[A-Z].*[0-9]$/ matches any line that begins with a capital letter and ends with a numeric digit. (The .* indicates that any number of other characters may intervene.)