Need to reverse php regex and see what needs to be matched

I am not very good in RegEx and saying that I am editing a website written in php. One of it's functions are failing because of this regex:

preg_match("/^minus\((\-?[\d\.]+)\)$/i",$val,$m)

I know it should be something like minus() but it doesn't seems to find any matches as I can't figure it out what needs to be inside the brackets.