Expressions is described as 'minimal, distraction-free tool for experimenting with regular expressions' and is an app in the Development category. There are more than 25 alternatives to Expressions for a variety of platforms, including Windows, Online / Web-based, Linux, Mac and Self-Hosted solutions. The best alternative is RegExr, which is both free and Open Source. Other great apps like Expressions are regular expressions 101 (Free), Expresso (Free), RegexBuddy (Paid) and iHateRegex (Free, Open Source).
The award-winning Expresso editor is equally suitable as a teaching tool for the beginning user of regular expressions or as a full-featured development environment for the experienced programmer or web designer with an extensive knowledge of regular expressions.
jRegExAnalyser is an interactive tool written in Java to write, test, debug, analyse and profile regular expressions. Its free and Open Source software, licensed under the terms of GPLv2 with Class Path Exception.