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Regular Expressions

A regular expression, or regexp, is a way of describing a set of strings. Because regular expressions are such a fundamental part of awk programming, their format and use deserve a separate chapter.

A regular expression enclosed in slashes (/) is an awk pattern that matches every input record whose text belongs to that set. The simplest regular expression is a sequence of letters, numbers, or both. Such a regexp matches any string that contains that sequence. Thus, the regexp foo matches any string containing foo. Therefore, the pattern /foo/ matches any input record containing the three characters foo anywhere in the record. Other kinds of regexps let you specify more complicated classes of strings.

Initially, the examples in this chapter are simple. As we explain more about how regular expressions work, we will present more complicated instances.

• Regexp Usage: How to Use Regular Expressions.
• Escape Sequences: How to write nonprinting characters.
• Regexp Operators: Regular Expression Operators.
• Character Lists: What can go between [...].
• GNU Regexp Operators: Operators specific to GNU software.
• Case-sensitivity: How to do case-insensitive matching.
• Leftmost Longest: How much text matches.
• Computed Regexps: Using Dynamic Regexps.