The W3C XML Schema standard defines its own regular expression rules for use in pattern facet which is based on Perl. The rules are different to all other versions of regular expression engines and are quite limited in functionality.
The main difference is that it only validates entire pattern matches, rather than breaking the input down into multiple partial matches. For most other regular expression engines, this would be like adding ^ and $ to the start and end of the pattern, i.e. ^pattern$
Other differences include: the . character does not match line breaks; patterns are always case sensitive; and the absence of special characters ^, $, \b, \x, \u.