Emulate REGEXP like behaviour in DB2 SQL
Since this old question has been dug up anyway, I'll mention that you can use the built-in XQuery support in DB2 for regular expression matching, something along the lines of 

select whatever
from users
where
   xmlcast(
     xmlquery('fn:matches($USER_NAME,"^a[aofdmep][a-z][a-z0-9]{4}[sidbfkfpo]")') 
     as integer) = 1


XMLQUERY above calls the XQuery matches function against the column USER_NAME. The result is an XML boolean, so the XMLCAST is used to convert it to an SQL data type.