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RE: Pattern matches only when named

• To: mathgroup at smc.vnet.net
• Subject: [mg43894] RE: [mg43873] Pattern matches only when named
• From: "David Park" <djmp at earthlink.net>
• Date: Fri, 10 Oct 2003 03:06:04 -0400 (EDT)
• Sender: owner-wri-mathgroup at wolfram.com

Ken,

Because the lhs of the rule is evaluated by Mathematica giving 2_.

_ + _ -> h
2 _ -> h

And that doesn't match anything in your expression.

Use HoldPattern to prevent evaluation.

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

David Park
djmp at earthlink.net
http://home.earthlink.net/~djmp/ 



From: Ken Morgan [mailto:kemorgan at vt.edu]
To: mathgroup at smc.vnet.net

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