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Re: question on Cases with elements that might be strings

  • To: mathgroup at smc.vnet.net
  • Subject: [mg86554] Re: question on Cases with elements that might be strings
  • From: Szabolcs <szhorvat at gmail.com>
  • Date: Thu, 13 Mar 2008 20:50:56 -0500 (EST)
  • References: <frasc3$1hm$1@smc.vnet.net>

On Mar 13, 10:32 am, ramiro <ramiro.barran... at uvm.edu> wrote:
> Hi,
>
> The following is a valid pattern match:
>
> In[96]:= StringMatchQ["s1", StringExpression["s", NumberString]]
>
> Out[96]= True
>
> HOWEVER, the following does not work:
> In[89]:= Cases[{{1 -> "s1", 1}, {2 -> "s2", 2}}, {_ ->
> StringExpression["s", NumberString], _}]
>
> Out[89]= {}
>
> Even though this works:
> In[95]:= Cases[{{1 -> "s1", 1}, {2 -> "s", 2}}, {_ ->
> StringExpression["s"], _}]
>
> Out[95]= {{2 -> "s", 2}}
>
> How come cases does not work on ln[89]?
>
> Ramiro

StringExpression cannot be used with Cases.  Generally, string
patterns and expression patterns cannot be mixed.  In[95] doesn't
"work" either.  It returns the result you expected only because
StringExpression["s"] was immediately evaluated to "s".  So, in
In[85], change the StringExpression[ ... ] to s_String /;
StringMatchQ[s, "s" ~~ NumberString], and it'll work fine.


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