Well you can find that specific string fairly easy using a CONTAINS() function:
CONTAINS( [Alternative Codes], 'xlM6R' )
And you could set this up as a parameter-driven calculation:
CONTAINS( [Alternative Codes], [My Parameter] )
But it get more complicated when you start wanting to isolate other values.
Try this blog, you have bunch of string calculations.
I'm not sure I was clear enough, as neither of these options are any good, or maybe I am asking too much of Tableau?
I want to find all lines of data, that include any of the items in my parameter list, the "CONTAINS( [Alternative Codes], [My Parameter] )" formula, only seemed to give me a response for the first item in the parameter.
Have I done something wrong, or am I asking too much of Tableau?