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MathGroup Archive 2000

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Re: Pattern match exactly 0 or 1 times?

  • To: mathgroup at
  • Subject: [mg26411] Re: Pattern match exactly 0 or 1 times?
  • From: Jens-Peer Kuska <kuska at>
  • Date: Wed, 20 Dec 2000 00:21:35 -0500 (EST)
  • Organization: Universitaet Leipzig
  • References: <91f70k$>
  • Sender: owner-wri-mathgroup at


found = False;
lst = {"b", "c", "a", "a", "q"} /. 
    x___ /; x == "a" :> If[! found, found = True; alpha, x]


ReplaceFirst[expr_, pat_, new_] :=
    pos = Position[expr , pat];
    If[{} =!= pos, ReplacePart[expr, new, First[pos]], expr]



Gavin Scott wrote:
> 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.

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