Regular expressions are covered by several standard bodies, including IEEE standard 1003.1 (so called Posix): http://pubs.opengroup.org/onlinepubs/000095399/basedefs/xbd_chap09.html
However, there are plenty of other approaches to regular expression syntax, the other popular one being Perl (PCRE). For a nice overview of all major regular expression implementations, including extensions, check out this useful resource: http://www.regular-expressions.info/tools.html