See the example pictured below. In Regex Buddy (a common tool to experiment with regex operations) a simple regex does the trick. To do the same thing in BRU, I have to waste time with brackets like ((.*)%..)* or something, and re-constitute the entire string with /1/2 etc.... I've dropped BRU for that reason. Every other regex rename utility works in the standard way (see Regex Renamer, Advanced Renamer).
I do like the all-in-one interface of BRU, but cannot use it for regex operations for this reason. So please consider revamping how regex replace works, or at least provide a tick-box option for "standard mode / BRU mode". People want to rename files quickly, not waste time working out how to match an entire string of patterns, when it's not the standard way, and other apps are available which do use that standard, which is far easier.
BRU:
