Why do you care what the format is in if it's a textbox? Example, whether someone enters
.06
or
6%
it is still the same thing.
What you do with the entry afterwards is really what matters.
Expecting a user to input a text with a format you want will only lead to disappointment; users will do everything except what you want them to.
What you should care about is if they enter the kind of base data you want, example, a number within a range of numbers, or whatever text you have in mind.