/^(.{4}).*\1$/     # ^ and $ match beginning and end of string respectively

##</code><code>##

/^(.{4}).*\1.*\1$/

##</code><code>##

ABCD # pattern

ABCD # match
ACDB # no match
ABED # match
BBCD # match
ABC  # no match

##</code><code>##

sub makepattern
{
   my $str = shift;
   my @res;
   for( $i = 0; $i < length( $str ); $i++ ) { 
      push @res, substr( $str, 0, $i ) . "." . substr( $str, $i + 1, length( $str ) - $i ); 
   } 
   return join( "|", @res );
}

# example
my $imprecisematch = "ABCD";
my $pattern = makepattern( $imprecisematch );
/^($pattern).*\1.*\1$/     #match $_ against three occurrences of this pattern