The original regular expressions application, grep, is a part of Darwin and is right at your fingertips via the Terminal. grep is also included in Unix and MacOS X-based text editors and even some word processors. BBEdit and its free little brother, Text Wrangler, are just two such apps. There are many others. If you were just a bit more specific how you want to use the regular expressions editor, we could be much more helpful in advising you on the tool that best fits your needs.