Debuggex: A visual regex helper
Regexes (regular expressions) are a mainstay of data mining/extraction.
Debuggex is a regex debugger with visual cues to help you with writing/debugging regular expressions.
The webpage reports full JS regexes are not yet supported.
If you need a fuller alternative, consider debugging regex expressions in Emacs.
M - x regexp-builder
which shows matches as you type.
Be aware that regex languages vary (no real surprise).
One helpful resource: Regular Expression Flavor Comparison