Storing negative values in MySQL

[eluser]megabyte[/eluser]
I am building an inventory table. It needs to store addition of inventory and substraction of inventory.

Now I realize you can store a negative number but this is something I've never done before, so I'm trying to think about pros and cons.

If I allow the user to enter a negative value in a text field, I could check that the value only contains a '-.0-9' (I'llhave to play with this because I'm not a regular expressions expert) now is their a way to make sure in a regex their is only one instance of the '-'

And has anyone had issue allowing users to store negative numbers that later need to be calculated on? This is my biggest worry of course.