looking for a function doing what the difference one does but using soundex values as source

hi,

I'm looking for a function which count the number of matching characters in a string, like the difference function do it.

http://msdn.microsoft.com/en-us/library/ms188753.aspx

In my case the input strings are soundex values! so the difference function doesnt works here.

so looking for something like:

select CustomDifference('G660','G560')

return = 3 (3 matching characters)

so is there a fast function available? (I have to scan million of records...)

thanks.