In your problem statement, you say that you need everything before the third colon. So your solution should be expressed in a similar form.

You're mentioning substr in the title, but that will require you to find the position of the third colon so you know what length to specify.

Rather than doing that, I'd suggest using split and join: Use split on colons to break the field into chunks, and then keep the first three chunks, gluing them back together with colons to make youre result. Like:

my @chunks = split /:/, $input_string;
my $answer = join(':', $chunks[0], $chunks[1], $chunks[2]);
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You could use regular expressions, too. But if you really want to use substr, then look up perldoc -f index to see how you might find the position of a string. (Of course, since you want the third one, you'll need to call it three times.)

...roboticus

When your only tool is a hammer, all problems look like your thumb.