in reply to regex backtracking question

You can think of any regular expression in term of some nested for loops.

Please, consider:
for my $i (0 .. $eos) { for my $j ($min .. $max) { last if $j > $eos-$i; print substr($str, $i, $j),"\n"; } }
The following variables specifies the meaning of "ABCDEF" =~ /(\w{2,}?)(?{print "$1\n"})(?!)/;
my $min = 2; # minimum match length my $max = 32767; # maximum match length my $str = "ABCDEF"; my $eos = length($str);

Using the same nested for loops, we have another specification for "ABCDEF" =~ /([A-Z]{3})(?{print "$1\n"})(?!)/;
my $min = 3; # minimum match length my $max = 3; # maximum match length my $str = "ABCDEF"; my $eos = length($str);

By executing the code, it gives us the same output as the corresponding regular expressions do. I hope this will give you a basic intuition on how backtracking works.