       Re: Pattern matching question and answer

• To: mathgroup at smc.vnet.net
• Subject: [mg63062] Re: Pattern matching question and answer
• From: dh <dh at metrohm.ch>
• Date: Tue, 13 Dec 2005 03:40:43 -0500 (EST)
• References: <dnir79\$9ur\$1@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```Hi Matt,
your problem is that you want to use the machanism of pattern matching
but you specify your condition not as a pattern, but as a string-pattern.
One way to solve this problem would be to use pattern matching, but
attaching a condition to the pattern. This condition can then contain
the string-pattern.
E.g.

Cases[expr, _ x_ /; StringMatchQ[
ToString[x], CharacterRange["x", "z"] ~~ "0"]]

Daniel

Matt wrote:
> Hello,
>   I recently was going to post this question:
>
> *******QUESTION START*******
>
> Is there a way to specify a pattern within an expression that can
> specify certain alpha-numeric combinations?
>
> e.g.
>
> expr = y0*y^2 + z0*z^2 + x0*x^2 + a1*a^2 + b1*b^2;
>
> If I wanted to use Cases (or whatever is deemed appropriate to
> accomplish the task) to return a list of only the parts of expr that
> are multiplied by a variable of the form 'letter'zero (i.e. x0,y0,z0),
> how would I do this?
>
> I could do this:
> Cases[expr, Times[x0,_]]
> Cases[expr, Times[y0,_]]
> Cases[expr, Times[z0,_]]
>
> But what I'm looking for would be a pattern that specifies a single
> character followed by a zero.  In regular expression syntax, if I were
> dealing with a text file, I might use something like this:
> [A-Za-z]0
>
> or like this
>
> [xyz]0
>
> So, my Cases statement would be able to grab all three matches like
> this (if it were supported):
> Cases[expr, Times[[xyz]0,_]]
>
> *******QUESTION END*******
>
> But before I got around to posting, I came up with a solution.  First
> off, once again, I'm impressed with Mathematica's flexibility, but I was
> wondering if there's better or more intuitive way to accomplish the
>
> Solution:
> Remove[expr, x0, x, y0, y, z0, z]
> expr = x0*x^2 + x00*x0^2 + y0*y^2 + z0*z^2 + x0*Sin[x]
> Cases[expr, _^_*(x:_Symbol?(StringMatchQ[ToString[#1],
> RegularExpression["[xy]0"]] & ))]
>
> I could also use this:
> Cases[expr, _*(x:_Symbol?(StringLength[ToString[#1]] == 2 & ))]
>
> if I were interested in identifiers of length 2.
>
> Thanks,
>
> Matt
>

```

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