Re: Pattern match exactly 0 or 1 times?

• To: mathgroup at smc.vnet.net
• Subject: [mg26411] Re: Pattern match exactly 0 or 1 times?
• From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
• Date: Wed, 20 Dec 2000 00:21:35 -0500 (EST)
• Organization: Universitaet Leipzig
• References: <91f70k\$548@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```Hi,

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

??

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

???

Regards
Jens

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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