Thread:Extension talk:Semantic Forms Inputs/Conditional Regular Expression

Hi I'm sure I'm missing something simple, but here's the problem: I have some optional fields that a user can elect to fill out or not, but when they are filled out they need to be limited to the pattern they can enter. But if I set a regular expression for that field it shows an error if it is not filled out. The mandatory setting in Semantic Forms recognizes if statements -- is there away with semantic inputs to write a regular expression that's conditional? I'm sure I'm over looking something simple, because that's generally the case, but any guidance here would be helpful. Thanks