Yeah debuggex's output looks slightly better, on top of having a lot more options. Regulex however is open-source with nicely written javascript: https://github.com/JexCheng/regulex

Here's a side by side comparison with the regexp I wrote to parse URLs [0]

Debuggex: https://www.debuggex.com/r/yPBRv77Rxl0Ji3Pi

Regulex: http://jex.im/regulex/#!embed=true&re=^%28https%3F\%3A%29\%2...

[0] http://stackoverflow.com/a/21553982/542156

My explanation：https://news.ycombinator.com/item?id=8665721