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MathGroup Archive 2004

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Re: Extract substrings using metacharacters

  • To: mathgroup at smc.vnet.net
  • Subject: [mg48469] Re: Extract substrings using metacharacters
  • From: Paul Abbott <paul at physics.uwa.edu.au>
  • Date: Tue, 1 Jun 2004 03:02:45 -0400 (EDT)
  • Organization: The University of Western Australia
  • References: <c99dks$k5f$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

In article <c99dks$k5f$1 at smc.vnet.net>,
 Marcus Stollsteimer <marcus314 at yahoo.com> wrote:

> I do not have much experience with Mathematica string functions,
> so this may be trivial:
> 
> When I use a string pattern with metacharacters
> (like the patterns used for StringMatchQ[] or FileNames[]),
> is there a simple way to extract the substring that
> matches the metacharacters?
> (something similar to regular expressions and m// in perl?)
> 
> I have a lot of similar but different file names that I
> read in with FileNames[pattern], and I would like to extract
> the different parts.
> 
> At the moment I have the following workaround:
> 
> files = {"data040523.dat","data040527.dat","data040528.dat"};
> Do[
>   datestr = files[[i]] // StringDrop[#, StringLength["data"]]& // 
>       StringDrop[#, -StringLength[".dat"]]&;
>   Print[datestr],
>   {i, 1, Length[files]}
> ]
> 
> I am looking for a function StringPatternTake[str,"data*.dat"]
> that works like StringTake, but not with the positions, but
> with patterns instead.

For your list of files

  files = {"data040523.dat","data040527.dat","data040528.dat"};

you can delete a specific string as follows:

  StringDelete[str_String, del_String] :=   
      First[StringDrop[str, #] & /@ StringPosition[str, del]]

It is better to use StringPosition than StringDrop and StringLength for 
you don't need to assume the structure of your string.

Making this function Listable (avoiding the need for Do loops),

  SetAttributes[StringDelete, Listable]

you can obtain your date strings as folows:
  
  StringDelete[StringDelete[files, "data"], ".dat"]

Since string-matching is somewhat limited, here is another way using 
"ordinary" pattern-matching, replacing the meta-characters "*" and "@" 
by their corresponding symbolic patterns:

  StringPatternTake[str_String, pat_String] := 
   If[StringMatchQ[str, pat], Characters[str] /.    
      (Characters[pat] /. {"*" -> x___,  "@" -> x__} ) :>   
         StringJoin[x], str]

Make this function Listable:

  SetAttributes[StringPatternTake, Listable]

Then the following works:

  StringPatternTake[files,"data*.dat"]

Cheers,
Paul

-- 
Paul Abbott                                   Phone: +61 8 9380 2734
School of Physics, M013                         Fax: +61 8 9380 1014
The University of Western Australia      (CRICOS Provider No 00126G)         
35 Stirling Highway
Crawley WA 6009                      mailto:paul at physics.uwa.edu.au 
AUSTRALIA                            http://physics.uwa.edu.au/~paul


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