The simple answer is you can't. But if you are very insistent you can acheive that.
A lot of RDBSes have built in support for regular expressions, why doesn't DB2 has one? I have no idea.
In MySQL you do REGEXP, NOT REGEXP or RLIKE. here is another link.
In Microsoft SQL Server 2000 you call xp_pcre. In Microsoft SQL Server 2005 you call integrated .NET Regex classes from .
In PostgreSQL you call the regexp functions, here is another link.
For Oracle here we have a bunch of resources: First, second and also on regular-expressions.info
If you'll do a search on DB2 regexp you will stumbled upon this IBM article:
Which I found very interesting.