Not sure if you need the "(.*)" part in the RegEx, and the "$$1" in the Results. If I understood correctly he simply wants to list all the .xspf files, not have any restrictions on the name. So perhaps some wildcard in the Locate32 paramters to list all .xspf files is required instead of the $$1. Just '*' I assume.

This regexp:

(\d+(?:[+-/*^]\d+)+)


Allows simple expressions, not using any parenthesis but that could be easily fixed I assume. Not much time now :) This way you can use +, -, *, /, and ^. Just add more to the regexp.

Dont put spaces between the numbers and operators, regexp doesn't allow that yet but again, can be added. Gotta go now, mb I'll enhance it later tonight.

--EDIT

Slightly improved:
(\(?\s?\d+(?:\s?[+-/*^]\s?\(?\s?\d+\s?\)?)+).

You can import it directly using this:

This allows both parenthesis and spacing between numbers and operators. For example, this is all valid input:
1 + ( 5 * 5 )
1 + (5 * 5)
1 + (5*5)
1+(5*5)

etc.

You can ofcourse expand the equation with more operands and operators. Please note I currently do not allow for more than 1 space, but it's easily adjusted by editing each \s? (whitespace, 1 or 0 times), to \s* (whitespace, zero or more times).

This should be sufficient for most input imho, I hope I am not forgetting something here :). Feel free to improve or (dramatically) change. Not gonna spend a lot more time on this; you can probably increase the amount of available operators (I have only included 5 now) :)