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Pattern match exactly 0 or 1 times?

• To: mathgroup at smc.vnet.net
• Subject: [mg26383] Pattern match exactly 0 or 1 times?
• From: gavin at allegro.com (Gavin Scott)
• Date: Sat, 16 Dec 2000 02:40:13 -0500 (EST)
• Organization: Globix Corp.
• Sender: owner-wri-mathgroup at wolfram.com

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.

• Follow-Ups:
  • Re: Pattern match exactly 0 or 1 times?
    • From: Ken Levasseur <Kenneth_Levasseur@uml.edu>