#! perl -slw
use strict;

my $spec = shift or die 'No spec supplied!';

my $re = '';
my %tally;
my $i = 1;
for my $c ( split '', $spec ) {
    if( exists $tally{ $c } ) {
        $re .= '\\' . $tally{ $c };
    }
    else {
        if( $i == 1 ) {
            $re .= '(.)';
        }
        else {
            $re .= '(?!' 
                . join( '|', map{ '\\' . $_ } values %tally ) 
                . ')(.)';
        }
        $tally{ $c } = $i++;
    }
}
$re = qr[^$re$];
print $re;

m[$re] and printf $_  while <>;
__END__
C:\test>wordSolver ABCDECFG words
(?-xism:^(.)(?!\1)(.)(?!\1|\2)(.)(?!\1|\3|\2)(.)(?!\1|\4|\3|\2)(.)\3(?!\1|\4|\3|\5|\2)(.)(?!\6|\1|\4|\3|\5|\2)(.)$)
abednego
abscised
airborne
...
whisking
worker's
writhing
ziegfeld