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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.



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