in reply to Efficient run determination.
I donít think regexp will help improve efficiency in this case. The requirement is very straight forward, and I believe everyone agrees that, any solution has to iterate thru the string (if you use regexp, regexp will do this), doesnít matter how quick or slow it is. Theoretically, in this case, for any reasonably efficient algorithm, including the best solution anyone would be able to find, t ~ o(l) stands true for all solutions (t is the time spent, and the length of the string is l, and t ~ o(l) means t is a linear function of l).
Considering regexp is more general, and most likely would do more than you want in a specific case. It is reasonable that a specific function you wrote would be faster than use regexp.