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...