You are *so* the monk!

The regex works because if the variable ength of the space (0-full line) before matching the backreference. And lookaheads can be variable length.

This is essentially the converse approach to what I was thinking trying to solve the problem. Someday...

while (<DATA>) {
   chomp;
   if ( /^(?:([smtwhfa])(?!.*\1))*$/ ) {
      print "$_ : OK\n";
   } else {
      print "$_ : Not OK\n";
   }
}

__DATA__
swma
smqa
smsa
fhtm
ttma
t2ms

__END__
swma : OK
smqa : Not OK
smsa : Not OK
fhtm : OK
ttma : Not OK
t2ms : Not OK
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