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Pattern match exactly 0 or 1 times?
Is there a way to specify a pattern which will match exactly zero or one times? I find myself wanting somethinglike the standard regular expression '?' operator, but can't see a way to do this with Mathematica's pattern features. The closest I've been able to come is something like: x___ /; x=="a" which will match zero or more copies of a specific value. I'd like to match no more than one though. Is there anything like a general Regular Expression package for Mathematica anywhere? I did a quick search and didn't find anything like this, so I'm wondering if my desire for such a thing simply indicates that there's something fundamental about Mathematica that I don't get yet which would eliminate the need for this. Thanks, G.