Regex is a regular-expression testing tool that is easy to use.

Features

• Immediate feedback. Dynamic highlighting of matches as you modify the regular expression or sample text
• Automatic escaping of special characters when copying a regular expression
• Automatic unescaping of special characters when pasting a regular expression
• Supports capture groups
• Supports matching options such as case-insensitive matches
• Supports positive lookahead, negative lookahead
• Supports positive lookbehind, negative lookbehind
• Supports backreferences
• Fast and dead-simple to use

See the regular expressions tutorial at http://motionobj.com/articles/regex-macos-tutorial.