The Rosie Pattern Language (RPL) is intended to replace regular expressions (regex) in situations where:

many regex are used in an application, project, or organization; or
some regex are used by many people, or by a few people over a long period of time; or
regex are used in production systems, where the cost of run-time errors is high.

Free PCRE-based regular expression debugger with real time explanation, error detection and highlighting.

RegExr is an online tool to learn, build, & test Regular Expressions (RegEx / RegExp).

This page compares regular expressions that validate e-mail addresses in order to find the best one.

Online Regular Expression Testing Tool