Effective awk Programming, 4th Edition

Chapter 3. 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.’ Thus, the pattern /foo/ matches any input record containing the three adjacent 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 present more complicated instances.

How to Use Regular Expressions

A regular expression can be used as a pattern by enclosing it in slashes. Then the regular expression is tested against the entire text of each record. (Normally, it only needs to match some part of the text in order to succeed.) For example, the following prints the second field of each record where the string ‘li’ appears anywhere in the record:

$ awk '/li/ { print $2 }' mail-list
555-5553
555-0542
555-6699
555-3430

Regular expressions can also be used in matching expressions. These expressions allow you to specify the string to match against; it need not be the entire current ...