Here is one more way:
create table Demo (Col1 varchar(10))
go
insert into Demo(Col1) values ('123453'),('124344ME'),('123453'),('124344ME'),('323390'),('3233MS')
select Col1, substring(Col1,1, CASE WHEN Col1 like '%[aA-zZ]' THEN len(Col1)-2 else len(Col1) end)
from Demo
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To know how to ask questions and increase the chances of getting asnwers:
http://www.sqlservercentral.com/articles/Best+Practices/61537/
For better answers on performance questions, click on the following...
http://www.sqlservercentral.com/articles/SQLServerCentral/66909/