You may THINK you want to do a Scalar UDF, but you probably don't. I wrote a chapter for the SQL Server MVP Deep Dives 2 book entitled "Death by UDF". It is a very aptly named missive. Please do everything in your power to avoid the UDF and inline the code where you need it.
Oh, I haven't tested it, but I wonder if UDFs can call UDFs ...
Best,
Kevin G. Boles
SQL Server Consultant
SQL MVP 2007-2012
TheSQLGuru on googles mail service