(Extracted from the Free Software Foundation's Gnu Emacs Info page on regular expressions)Regular-expression matching allows you to test whether a string fits into a specific syntactic shape. You can also search a string for a substring that fits a pattern.
A regular expression describes a set of strings. The simplest case is one that describes a particular string; for example, the string foo when regarded as a regular expression matches foo and nothing else. Nontrivial regular expressions use certain special constructs so that they can match more than one string. For example, the regular expression foo|bar matches either the string foo or the string bar; the regular expression c[ad]*r matches any of the strings cr, car, cdr, caar, cadddar, and all other such strings with any number of as and ds.