Re: perplexed by blank sequence in pattern replacement

• To: mathgroup at smc.vnet.net
• Subject: [mg68708] Re: perplexed by blank sequence in pattern replacement
• From: albert <awnl at arcor.de>
• Date: Thu, 17 Aug 2006 04:18:14 -0400 (EDT)
• References: <ebuj5d\$6f5\$1@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```Hi Blake,

> In[1]:=Replace[a*b*c,Times[mysequence__]:>{mysequence}]
>
> Out[1]={a b c}
>
> I expected Out[1]={a,b,c}, from a naieve reading of the full form of
> a*b*c
>
> In[2]:=FullForm[a*b*c]
>
> Out[2]//FullForm=Times[a,b,c]
>
> Will someone PLEASE tell me why In[1] does not yield the results I
> expected? (I can readily use a work-around, what I am concerned with is
> a correct understanding of pattern matching).

your understanding of pattern matching is alright, the matter here is that
the LHS of your rule is evaluated before the pattern matching even begins.
Since Times[something] is evaluated to something the actual rule that
Replace sees is:

mysequence__:>{mysequence}

You can check this by either using Trace[] or just look at the fullform of:

In[4]:= Times[mysequence___]:>{mysequence}//FullForm

Out[4]//FullForm= RuleDelayed[Pattern[mysequence, BlankNullSequence[]],
List[mysequence]]

To avoid this, use HoldPattern on the LHS:

In[5]:= Replace[a*b*c,HoldPattern[Times[mysequence__]]:>{mysequence}]

Out[5]= {a, b, c}

hth,

albert

```

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