my $re = assemble_regex(\%hash);
my $old_pos = pos;
while (m/\G$re/){
   $pos = $old_pos; # reset match position
   # ... extract name of matched regex in $matched_re here ...
   delete $hash{$matched_re};
   # re-generate regex, without the one that previously matched:
   $re = assemble_regex(\%hash);
}