One TextBox can only be controlled by one RadInputManager setting because each of the RadInputManager settings applies for different behavior. In your case you could use RegExpTextBoxSetting with Validation-IsRequired property set to “true”. This will fire both required field validation and regular expression validation, however the error message will be only one. The other option is to use RadTextBox/TextBox with asp validators.
I am attaching a sample project illustrating the both cases.
the Telerik team