Validating custom member fields - expressionengine registration form

I've setup an ExpressionEngine site with a custom member field. This field is showing up on the registration form but I need to validate it and can't find any documentation on how to do this.
The field is a company code and is used to assign the user to a company, so it basically needs to check against the company channel entries to make sure that the custom member field matches one of the entries code, is this possible?