You are correct, but Form builder have Validation Toolkit. We have a document that explains how to add validation to input field using Validation Toolkit. Please visit this url:
and under the How TOs click on the Applying CSS styles to form elements with Validation Toolkit (PDF) pdf. This will explain how to add validation to an input field. You could skip the css styling portion of the document.
Let me know if you have any questions.
In the document it says Select WA Validation Toolkit > Server Validations. from the server behaviors panel, but in your case, you should already have a validation toolkit entry under your server behaviors panel. So you could just double click it to add your input field.