UserID & Password Validation

Dear Friends,

I am developing one application using module pool programming.

The initial screen has User ID and Password fields.

I want to keep the User ID field strictly alphanumeric and have subroutines in place to check that.

What I want is that the cursor should remain at the User ID field whenever I press TAB or Click on the Password input field after entering a non alphanumerical User ID.

How can I achieve this?