Cool tool for trying to understand complex regex https://jex.im/regulex/. It's a regex visualizer. - Source: Reddit / 18 days ago
RegExr - RegExr.com is an online tool to learn, build, and test Regular Expressions.
Expresso - 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 with an extensive knowledge of regular expressions.
RegexBuddy - RegexBuddy is your perfect companion for working with regular expressions.
Regex Hero - Online Silverlight regular expression tester with instantaneous highlighting, C# and VB.NET code generation, code completion, regex analysis, benchmarking, and more.
RegexMagic - Use RegexMagic to generate complete regular expressions to your specifications by using RegexMagic's flexible patterns, instead of the cryptic regex syntax.