Question

Can anybody show with some examples the exact difference between .. and ... operator?

From the perlop man page:

If you don’t want it to test the right operand until the next
evaluation, as in sed, just use three dots (“…”) instead of two.

But what exactly this mean? I don’t understand the perlop’s example:

@lines = ("   - Foo",
          "01 - Bar",
          "1  - Baz",
          "   - Quux"
);
foreach (@lines) {
    if (/0/ .. /1/) {
        print "$_\n";
    }
}

with ... will print the Baz – but why? More precisely, why is Baz not printed with two dots and only with ...?

Answer 1

«...» doesn’t do a flop check immediately after a true flip check.

With «..»,

1. " - Foo"
   1. /0/ returns false.
   2. .. returns false.
2. "01 - Bar"
   1. /0/ returns true. Flip!
   2. /1/ returns true. Flop! ⇐
   3. .. returns true (since the first check was true).
3. "1 - Baz"
   1. /0/ returns false.
   2. .. returns false.
4. " - Quux"
   1. /0/ returns false.
   2. .. returns false.

With «...»,

1. " - Foo"
   1. /0/ returns false.
   2. ... returns false.
2. "01 - Bar"
   1. /0/ returns true. Flip!
   2. ... returns true.
3. "1 - Baz"
   1. /1/ returns true. Flop!
   2. ... returns true.
4. " - Quux"
   1. /0/ returns false.
   2. ... returns false.