Unlike the two previous posters, I won't make the mistake of assuming you are using 'a', 'b' and 'c' here in another role as placeholders. In particular, I presume 'c' here to be some string, which can even be longer than a single character.

Sometimes, it's easier to do things without a complicated regexp. Why not use index to find a starting position of the third appearance of 'c' first?:

my $index2 = -1;
foreach (1 .. 3) {
    $index2 = index($body, "c", $index2 + 1);
    die "No third 'c'" if $index2 < 0;
}
[download]

Then use pos() to find where 'a' finished matching (or use index() as well):

$body =~ /a/g or die "No first 'a'";
my $index1 = pos($body);
# my $index1 = index($body, 'a') + length('a');
[download]

Then grab what's in between using the offsets:

my $inbetween = substr($body, $index1, $index2 - $index1);
[download]

A few things are left as an exercise to the reader:

• What to do if the third occurrence of 'c' appears before the first 'a'.
• Use a match instead of index if 'c' is a pattern instead of a string.
• The above code counts overlapping occurrences. It's easy to adapt if you don't want to count overlaps.
• The code above assumes you want the third occurrence of 'c' from the start of the string. It's easy to adapt to get the third occurrence of 'c' following the 'a'.

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