Summary

If you manipulate data files in shell scripts, you'll need to become familiar with regular expressions. Regular expressions are implemented in Linux utilities, programming languages, and applications using regular expression engines. A host of different regular expression engines are available in the Linux world. The two most popular are the POSIX Basic Regular Expression (BRE) engine and the POSIX Extended Regular Expression (ERE) engine. The sed editor conforms mainly to the BRE engine, while the gawk program utilizes most features found in the ERE engine.

A regular expression defines a pattern template that's used to filter text in a data stream. The pattern consists of a combination of standard text characters and special characters. The special characters are used by the regular expression engine to match a series of one or more characters, similarly to how wildcard characters work in other applications.