Souped-Up Regular Expressions
The Perl regular expression--and its associated functions--remains one of the most powerful and useful features of the overall language.
A Convention
The words "Regular Expression" from here on may be shortened to "RE."
All of the older Perl4 RE functionality remains in Perl5, and several capabilities have been added.
Possibly the most interesting and usable of the newer features is the extension syntax that is now available for regular expressions. The extensions that are currently available with Perl5 enable you to embed comments in your pattern, do grouping without backreferences, perform zero-width positive lookahead assertions, and even have embedded pattern-match modifiers. See PERLRE for details on all the powerful new RE features.