how can use condtional format to ask the user enters wrong character in a field

[fuadmab]

how to use conditional expresion to prompt the user if enters wrong characters in field for example i have field of 6 digits first digit must 5 second digits to be 1 or x or and the total characters are 6 (5 then 1 or a or x then ????).
Thanks

[debasisdas]

better to write a validation function for the purpose.

call the function on the lost focus / change event of the control.