Regex101.com
Regex101.com is a free of charge regex testing service where you can easily create expressions while you simultaneously have real time error detection, highlighting and explanation of your regex. Thanks to the permalink feature, it is a great reference which may even be used in code where you need to explain your regular expressions to co-workers.
Regex Debugger
The regex debugger allows you to see exactly how the regular expression engine is interpreting and trying to match your regular expression.
The debugger is affected by two global variables in the editor:
- The flags adjust the behavior of the debugger. If the global flag is set, the debugger will find all matches. Otherwise stop at the first.
- You can select text in the textarea and have the debugger match your regular expression only against that portion of the string
You can also adjust custom modifiers within the debugger itself:
- You can show/hide which regex token currently is being matched
- You can disable the internal engine optimizations which are ran at the start of every match. For example, if your regular expression was /a.*?c/ and your string did not contain a c, the match would fail outright without any debugging information being output. Disabling this setting would help you see why it did not match.
The default behavior of the debugger will be to debug until it can no longer find any matches.