D.8 Additional Regular Expression Features

Regular expressions can contain extended syntax (where whitespace is optional, so a regular expression can be split over multiple lines, and can contain regular Perl comments), and can have positive and negative lookahead. The syntax is a bit ugly, so rather than scare you off here, go look in Programming Perl, or see the perlre documentation. Jeffrey Friedl's book, Mastering Regular Expressions (published by O'Reilly & Associates), explains all of this and much more.