The (? ... ) syntax is for "extended patterns". In this case, it makes the rest of the regular expression case insensitive. Read through http://perldoc.perl.org

The (? ... ) syntax is for "extended patterns". In this case, it makes the rest of the regular expression case insensitive. Read through http://perldoc.perl.org/perlre.html#Extended-Patterns http://perldoc.perl.org/perlre.html#Extended-Patterns for more information on extended patterns.