A nice idea, but I wonder if the average user would understand such a complex system? For me, I think I would need a help file open every time I tried to use regular expressions.
Regular expressions would be off by default. A new toggle button entitled "Use Regular Expressions" would need to be added to the right of the Highlight button. And to the right of that new button would need to be a small rightward-pointing triangle button that pops up a list of character classes and quantifiers, with an entry in that list to view the complete description of whatever flavor of regular expressions K-Meleon would use.