in reply to refine regex

The process of unifying two regular expressions can be done in the following steps:

  1. Start with an empty regex : m!!
  2. Extract the common prefix : m! NOT !
  3. Extract the alternating parts A and B from both regexes, and put it in between (?:A|B) : m! NOT (?:".*?"\[MESH\]|\(.*?\))
  4. Go to step 2 if there is more of the regex(es) left : m! NOT (?:".*?"\[MESH\]|\(.*?\)) ?!
  5. Compare your new regex against the old regular expressions to confirm that it matches exactly what the old ones matched.

That way, I end up with m! NOT (?:".*?"\[MESH\]|\(.*?\)) ?!.

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Re^2: refine regex
by fglock (Vicar) on Nov 26, 2004 at 11:50 UTC

    step 6. Test it! - it was necessary to add a small fix:

    #/usr/bin/perl

use strict;
use warnings;

my $string_original  =<< "END";
("Immunologic and Biological Factors"[MESH] OR "Immunosuppressive Agen
+ts"[MESH] OR "Transplantation Immunology"[MESH] OR "Allergy and Immun
+ology"[MESH] OR "Graft vs Host Disease"[MESH]) NOT ("Foo"[MESH] OR "B
+ar"[MESH]) AND ("Kidney Transplantation"[MESH] OR "Liver Transplantat
+ion"[MESH] OR "Heart Transplantation"[MESH]) NOT ("My Term"[MESH] OR 
+"Blah"[MESH]) NOT "foobar"[MESH]
END

# original

{
my $string = $string_original;

$string =~ s/ NOT ".*?"\[MESH\] ?//g;
$string =~ s/ NOT \(.*?\) ?//g;

print $string, "\n";
}

# Corion's

{
my $string = $string_original;

$string =~ s! NOT (?:".*?"\[MESH\]|\(.*?\)) ?!!g;

print $string, "\n";
}

# fixed

{
my $string = $string_original;

$string =~ s! ?NOT (?:".*?"\[MESH\]|\(.*?\)) ?!!g;

print $string, "\n";
}

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[reply]
[d/l]

      Don't need the trailing ' ?'.
      $string =~ s! ?NOT (?:".*?"\[MESH\]|\(.*?\))!!g;
      works fine.

[reply]
[d/l]
[select]
      yeah, i picked that up to when i tested :)
      Cheers.
[reply]
Re^2: refine regex
by rsiedl (Friar) on Nov 26, 2004 at 11:40 UTC
    Thanks Corion. Great explanation!
[reply]
Re^2: refine regex
by exussum0 (Vicar) on Nov 27, 2004 at 01:20 UTC
    Or you can convert it to an NFA and remove useless transitions etc ;) But let's not get into language theory. Good job.

    ----
    Then B.I. said, "Hov' remind yourself nobody built like you, you designed yourself"

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