Regular Expression can be very complicated to read and maintain, these two sites can help to make working with regex easier.

Regex101

One of the largest regex testing services in the world, let’s you run regular expressions against test strings, giving you an explanation of your regex, and detailed matching information breaking down the results in a clear way. Also has a quick reference guide so you can look up pattern commands. You can create an account, so that you can store your regex while working on them, or for future reference.

Regulex

A JavaScript Regular Expression Visualizer - for any regex you enter a diagram is generated which gives you a visual explanation of the expression. For instance:

Excellent for documenting any complicated regular expressions you create.