match strings between delimiters

uni's solution would work assuming that the literals you have specified are as specified.

Couldn't you just enclose your contains string with commas?

if {$al_ctrl_parameter contains ",$switch_app_pool," }

But, that would assume that all the items in al_ctrl_parameter be delimited on both sides by commas but that doesn't look like it's the case in your example. If you can't rely on that, I'd do something with one or maybe two split statements to first break the string and extract the "myURI_A1,myURI,myURI_A2,myURI_A3" and then do another split with a comma being a delimiter. Then you can iterate through that list to see if you find a match.

Let us know if the first solution works of if you need help implementing another option.

-Joe