Question

I am looking at Perl script written by someone else, and I found this:

$num2 = '000000';
substr($num2, length($num2)-length($num), length($num)) = $num;
my $id_string = $text."_".$num2

Forgive me ignorance, but for an untrained Perl programmer the second line looks as if the author is assigning the string $num to the result of the function substr. What does this line exactly do?

Answer 1

In Perl, (unlike say, Python, where strings, tuples are not modifiable in-place), strings can be modified in situ. That is what substr is doing here, it is modifying only a part of the string. Instead of this syntax, you can use the more cryptic syntax:

substr($num2, length($num2)-length($num), length($num),$num);

which accomplishes the same thing. You can further stretch it. Imagine you want to replace all instances of foo by bar in a string, but only within the first 50 characters. Perl will let you do it in a one-liner:

substr($target,0,50) =~ s/foo/bar/g;

Great, isn’t it?