Cool. Glad it worked for you.
I thank you for the compliment, but I only consider my regex-fu to be passingly fair... there are some regex
gods out there. I am an egg.
But helping others helps me improve my skills. I actually find rexexp construction addictive in a slightly weird way.
As for "why so many?" and which is "better," I don't really have an answer. Most of their difference are fairly minor and the average user might never come across them. I
prefer PCRE (Sigil), but mostly because of its widespread use (PHP, Apache, etc) and open source origins. PCRE might have a slight edge in capabilities over JGSoft in that PCRE can do recursion. And that "\K" you were initially using is a biggie as well. It allows you to get around the "fixed-length-only lookbehind hurdle."
Keep plugging away at it. A little bit will stick each time!