abcdecgf

A single regexp would be too convoluted I guess (I say that only because such a regexp is beyond my skills :-) update: or laziness :-)

#!/usr/bin/perl -nl
if (length == 8) {
  $c = substr ($_, 2, 1);
  if (substr ($_, 5, 1) eq $c) {
    my %h;
    @h{split//,$_} = (1) x 8;
    print if keys %h == 7; 
  }
}
__END__
perl match.pl /usr/share/dict/words
Abednego
abscised
Acadians
Acadia's
Acalia's
acerbest
Adaiha's
Adalia's
Adelbert

...

whirling
whisking
Wieche's
wielders
Winton's
worker's
writhing
Yahweh's
Yeargain
Ygerne's
Yorker's
Zarger's
Ziegfeld
[download]

update: changed to -nl to apply on /usr/share/dict/words

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}

Thanks again.

I tried your code on a dict file of words with length 8. Here're some of the results:

...
abetters (not ok, t is repeated too)
abigails (not ok)
abillity (not ok, l is repeated too)
abscises (not ok, s repeated more than twice)
abscisin (not ok, i is repeated too)
abscisse (not ok)
acaudate (not ok)
...
[download]

The result list is about 3300 words. I scanned through about 200 and couldn't find one that fits the regex...Maybe the word doesn't exist in the list...

Did you run exactly the code I posted? You must have some cut'n'paste error. My result list is 776 words and none of those you report are included.

--shmem

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}

abcdecgf

There are 6 unique letters, one is repeated at positions 3 and 6.

There must be 7! (No, not seven factorial...)

I don't know what the word is, so the letter "c" is only an example.

Are the positions fixed too? I'm assuming they are, since the problem is slightly more complex like that. Of course there are tons of ways to do it. And as shmem wrote, probably not best done with a single regex - although it may be possible, perhaps by means of one of those funky extensions still marked as "experimental". One possible way that springs to my mind is:

#!/usr/bin/perl

use strict;
use warnings;

$_=<<'.';
bEjhMELGUaL smtMDEYSxyDvuQiUfAbJfYMPnfJAqaPnKL VWZWSdfYRSaSGlXOyPfxusC
dtRAHabcdecgf taNdvtKdBlJcnFryVXObEDvawRyviWO hwlKiBpDWYeBPYhlpKFvrSeQ
ksWmkXqQdLQPIzvKFE Jqrclq mPqQbMvkAx LtVuFMehKirSATuqlFzqwRknocsrcKXAE
FNbOivdvkRonEkg apuPyHpTlssvVs BbwiHBvhfrSFwVkhwHkvoYjaGgntzFbEvPCIttD
IAlYqoLUjtxsYvbwUBHIoMYmPJbGeXymuwERkHwSyKbE XMCcFgsYPzmJbVUsOwfDTgUiJ
.

while (/(?=([a-zA-Z]{8}))/g) {
    my @l = (my $found=$1) =~ /./g;
    print $found, "\n" if $l[2] eq $l[5] and do {
        my %h; @h{@l}=(); 7 == keys %h;
    }
}
  
__END__
[download]

Update: or, in a slightly more agile way:

for (/(?=([A-zA-Z]{8}))/g) {
    my @l=/./g;
    print $_, "\n" if $l[2] eq $l[5] and do {
        my %h; @h{@l}=(); 7 == keys %h;
    }
}
[download]