For any word:

my $set='bat';
my $regexp;
for (my $i=1;$i<=length($set);$i++){
 my $x=join '|', map {"\\$_"} 1..$i;
 $regexp.="([$set])(?!$x)";
}
if (/^$regexp/){
  # do something
}
[download]

Terrible... :-)