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Re: Pattern matches only when named

• To: mathgroup at smc.vnet.net
• Subject: [mg43878] Re: Pattern matches only when named
• From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
• Date: Fri, 10 Oct 2003 03:05:47 -0400 (EDT)
• Organization: Universitaet Leipzig
• References: <bm2ugv$6so$1@smc.vnet.net>
• Reply-to: kuska at informatik.uni-leipzig.de
• Sender: owner-wri-mathgroup at wolfram.com

Hi,

and 
_ + _  is simplifyed to 2 _ and this does not match a
sum. While u_ + _ can not simplifyed and matches your sum.
You must use

f + g /. HoldPattern[ _ + _] -> h

and all works as expected.

Regards
  Jens

Ken Morgan wrote:
> 
> Just a curiosity:
> 
> In[1]:=   f + g /. u_ + _ -> h
> Out[1]:=  h
> 
> but
> 
> In[2]:=   f + g /. _ + _ -> h
> Out[2]:=  f + g
> 
> Why does the pattern match only when one of the Blanks has a name?
> 
> Thanks,
> Ken