i think this is what you might be after:
declare @MyDomainLogin varchar(30)
--SET @MyDomainlogin = SUSER_NAME()
--the -1 and +1 are to remov ethe char '\' itself fromt eh results
SUBSTRING(@MyDomainLogin,1, CHARINDEX('\',@MyDomainLogin) -1) As TheDomain,
--find the location of the slash, grab substring of the right of THAT.
SUBSTRING(@MyDomainLogin,CHARINDEX('\',@MyDomainLogin) + 1,30) As TheUser
WHERE CHARINDEX('\',@MyDomainLogin) > 0--don't crash on a non-domain name
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