st: Removing trailing letter from string

["Paul O'Brien" <paul.obrien@afe2.org.uk>]

How do I remove the last letter from a string which ends with a letter but does nothing if the string ends in a digit? The string is either 3 or 4 characters long:

EC4A
EC4P
EC4Y
DA15
SE10
SE2
SE3
W1A


Thanks

Paul

Dr Paul O'Brien
Westside Contraceptive Services
Raymede Clinic, Exmoor St, London
W10 6DZ
020 8962 4450 - tel
020 8962 4451 - fax
07900 495 118 - mob