How do you restrict a field to contain alphapleric character?

THIS An Exple of Code That Uses Regular Expressions To Limit A Field's Entry. Put this in a field's input value formula:

@IF (@matches; " {0-9}"); @success;

@Matches (serial_num; " {a-za-z}"); @success;

@DO (@Prompt ([OK];

"Invalid Serial Number";

"The Unit Serial Number Contains An Illegal Character, Probably a Carriage Return. Please remove the illegal character and save again.");

@Return (""))))