A regular expression is a mechanism for identifying a restricting string values (note that a regular expression is tested against the normalized string, regardless of the simple type's base). XML Schema regular expressions conform to a specific grammar. The Schema Recommendation suggests that an XML validator should implement "Level 1" regular expressions as defined in the Unicode Regular Expression Guidelines.
In this document, the term expression (without "regular") indicates a regular expression snippet, or a subset of a regular expression. An expression may match one or many characters. An expression may comprise an entire regular expression.
Also this article presents the term XML instance to mean well-formed XML (file, stream, or otherwise) being validated against an XML Schema.
XML Schema regular expressions are similar to other well-known regular expressions, such as might be found in UNIX or Perl.