Character classes match a single character (hence their name), not a string.
[5.91] matches "5" or "." or "9" or "1"
(?:5\.91) matches "5.91"
You'd have to use something like
Code:
/^mIRC v(?:5\.91|6\.(?:0[1-3]?|1[12]?)) Khaled Mardam-Bey$/