[_- ]
is a range match specification. It matches any one character that is between '_' and ' ' (space), inclusive. And it happens that you can specify the range backwards, so for example [d-a] is the same as [a-d] both matching any of 'a', 'b', 'c', 'd'.
The characters that are in the range of space to underscore are ' !"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_ ' -- however you used regexpi so it is case insensitive, so it also matches lower-case alphabetics.
If you wanted to match any one of '_', '-' or space, then you use a syntax oddity: '-' is not recognized as a range indication if in any of the forms [LETTERS-] or [-LETTERS] or [^LETTERS-] or [^-LETTERS] so you could code [_ -] instead of [_- ]
