How To Check Characters / Digits In Textbox

i am trying to check a character with another character which are in the same text box.when we are entering date,date in the first text box and month in another text box and year in another text box.now i want when i am trying to enter '0'in the character position when there is '0' at first character position it must raise an alert box as well as when i am trying to enter a digit greater than '1' at 2nd character position it must raise an alert box when there is '3' in the first character position.