Roy -

Thanks for shedding some light onto this issue. I am grateful for your functional modification of the code which does indeed produce the desired output.

As for the reasoning why the original code behaves the way it does, however, I am not sure I follow your explanation. Since the regex is missing the //o modifier I would have expected it to get re-compiled on every iteration of the for-loop (please correct me if I am wrong here!). If it doesn't - why not?

On the other hand, your reasoning is supported by the output of the following code modification:

my @strings = qw( aaabbbb ab abb aabb aaabb aabbb );

my $a_counter;
my $b_counter;

for my $string ( @strings )
{
    print "In $string there were $a_counter 'a's and $b_counter 'b's.\
+n" if ( $string =~ /(a(?{$a_counter ++; }  ))+(b(?{$b_counter ++;}))+
+/ );
}
[download]

which produces

In aaabbbb there were 3 'a's and 4 'b's.
In ab there were 4 'a's and 5 'b's.
In abb there were 5 'a's and 7 'b's.
In aabb there were 7 'a's and 9 'b's.
In aaabb there were 10 'a's and 11 'b's.
In aabbb there were 12 'a's and 14 'b's.
[download]

So: Can anybody please explain conclusively what is happening here in the compilation of the regex?