mirc's regex libraries already support the {N,M} quantifier!
//var %regex = /lo{1,5}l/ | echo -a $regex(looool,%regex)
i suspect you're having trouble with that comma, trying to plug it into the regex directly:
//echo -a $regex(looool,/lo{1,5}l/)
reason that doesn't work is pretty much the same effect as putting a comma directly into any other identifier. in the above, $regex() is passed 3 parameters, name = looool, string = /lo{1 and regex = 5}l/
there's a workaround, you can put the comma inside a group of parentheses:
//echo -a $regex(looool,/(?:lo{1,5}l)/)
if enclosed in (), the comma is no longer seen as a parameter separator :>