Specifies there is a pattern within. I've also seen #'s use, but / being more common.
This looks for the text "(BF2:11)". Regex uses things like hypens, parenthesis, and backslashes for its syntax. As such, the parenthesis must be escaped using a backslash (same for hyphen, etc.)
The  are grouping brackets, and specify to look for "items contained within", meaning "in these braces are the only characters you should be looking for". The : was a possible character, and so was a space (the \s represents space characters). The trailing * means is may or may not be present. You could also use + to represent there must be atleast one of these characters.
The outer parenthesis specify this is a group we are looking for, and the contained items will be the value of our $regml result(s).
The inner parenthesis (because the first characters are "?:") are non-grouping. Don't seperate this set of parethesis off as its own return value, only use it for sake of containment. And what is it containing? The pattern for <number><space> (\d is digit, \s is again space). The +'s (as i mentioned) mean 1 or more. This set of parenthesis are trailed by another plus meaning repeat this pattern; look for a string of <number><space>.
The i flag stands for case-insensitive. mainly for the BF found in the pattern.