I’ve seen posts from Microsoft and others, saying that XLANG/s doesn’t (directly) support substrings in expression shapes. The advice is to create a helper object. Yes, but frankly I don’t want to code such a ridiculously simple helper class and remember to deploy it to every BizTalk environment.
Add a reference to Microsoft.VisualBasic. In your expression shape, reference Microsoft.VisualBasic.Strings. You now have access to Left, Mid, and Right.