This [regexp] looks for the word "fader" followed immediately by an integer, then a space and finally another number which could be a float or int. It remembers the numbers as backreferences and outputs them as a list. If you want a step by step of what each character is doing then just ask.
The regexp that doesn't work for you will match a /slash/ followed by any positive integer followed by a space followed by any positive number. However the string you are entering doesn't match this format, it has "/MP" in the middle which the regex can't match. You might find it easier downloading [OSC-route] from the CNMAT website instead of writing your own regexps.