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Re: Why does this pattern fail to match?

• To: mathgroup at smc.vnet.net
• Subject: [mg114175] Re: Why does this pattern fail to match?
• From: Andrea <btlgs2000 at gmail.com>
• Date: Fri, 26 Nov 2010 05:27:53 -0500 (EST)
• References: <iclfej$lb7$1@smc.vnet.net>

On Nov 25, 11:57 am, kj <no.em... at please.post> wrote:
> I'm tearing my hair out over this one.  Could someone please explain
> to me why all the following MatchQ expressions
>
> MatchQ[Times[ -1, Power[s, -1]      ],
>        Times[___, Power[___],    ___]]
>
> MatchQ[Times[ -1, Power[s, -1]],
>        Times[___, Power[___]  ]]
>
> MatchQ[Times[ -1,  Power[s, -1]      ],
>        Times[___, _Power        , ___]]
>
> return False?  (In all cases, s is undefined).  And yet, this succeed=
s:
>
> MatchQ[Times[ -1,  Power[s, -1]],
>        Times[___, _Power       ]]
>
> Is there a systematic way to debug/troubleshoot such pattern matching
> problems? Trace is useless here.
>
> Also, whatever the reason is for the failure of this pattern to
> match, where is this reason documented?  I've gone blind poring
> over the documentation trying to find an answer, without success.
>
> TIA!
>
> ~kj

Patterns evaluate as other expression.
So as

Times[a, Power[a],   a]]   ----->   a^3

so

Times[___, Power[___],    ___]]    ------>    ___^3

as you can try.
Trace is not useless. If you execute

MatchQ[Times[-1, Power[s, -1]], Times[___, Power[___], ___]] // Trace

the resul contains the evaluation of the pattern.
If you don't want to evaluate a pattern you have to use HoldPattern.

MatchQ[Times[-1, Power[s, -1]],
 HoldPattern@Times[___, Power[___], ___]]

works fine.
Andrea