This chapter informally describes "XML regular expressions" as supported by Rx.
A regular expression describes a textual pattern: a string of characters either matches (fits the pattern) or does not match (does not fit the pattern). Regular expressions have many applications associated with searching, editing, and parsing text. Typically, a program allows users to specify a regular expression, then it searches for text that matches that expression. In XML Schema, regular expressions are used to define "lexical spaces" (the set of valid literals for a datatype).