[hclee]

Not bad. You don't have to jump around your regexp reference and editor. It does not exactly tell you why your exp & string does not match. It just show what typed reg exp will do.

[Lindrian]

I think this shows quite clearly what is going on and why I get the result I get: http://regex101.com/r/kU3cJ6/#debugger (imgur link: http://imgur.com/H2IkNGy)

If you don't agree with me, could you perhaps suggest an improvement?

[hclee]

I mean it does its job and it is good. I would use it. But look at this case; test string inout [123:0] Asdfg

Exp (made it wrong in purpose) ^inout\s+\[[0-9]\: It just display "No Matches".

It would be cool if it kind of guide what exp you want to use to match test string. What you put in Test string is what you want to get.