The Perl Compatible Regular Expressions (PCRE) library that is used by Watson Explorer Engine implements most of the regular expression syntax supported by Perl version 5. See the PCRE entry on Wikipedia for a list of the differences between PCRE and Perl 5 regular expressions.
Watson Explorer Engine's use of PCRE is primarily focused on its support for UTF-8 characters as a match for a single wildcard character ('.'). See sites such as Regular-Expressions.info for more detailed comparisons of POSIX and PCRE regular expression capabilities.
Watson Explorer Engine functions such as viv:test enable you to identify the type of pattern matching that you want to use when comparing a string and a regular expression pattern by specifying an optional parameter. When creating a custom parser for a Watson Explorer Engine source, regex, perl-regex, case-insensitive-regex, and case-insensitive-perl-regex are all valid types for identifying the regular expression syntax used by your parser.