InfoPath Dev

I have text box in Info Path form and i would like to user to enter in this format only. it should not allow other format. 1) first 3 character should be in text format 2) then put "-" 3) after that four digit character Ex: ABC-1234 Please help me on this.

Here's a sample with a validation rule - save the file locally, right click and select design to open in design mode. From there you can preview, check rules, etc.