perl regex matching infohash from magnet link

Question

im trying to extract an info hash from a torrent magnet link using perls regex
the magnet link looks like:

magnet:?xt=urn:btih:8AC3731AD4B039C05393B5404AFA6E7397810B41&dn=ubuntu+11+10+oneiric+ocelot+desktop+cd+i386&tr=http%3A%2F%2Ftracker.openbittorrent.com%2Fannounce

but sometimes it can look like:
magnet:?xt=urn:btih:8AC3731AD4B039C05393B5404AFA6E7397810B41

the part im trying to extract is 8AC3731AD4B039C05393B5404AFA6E7397810B41

im trying to capture everything upto the first '&' or if it only includes the infohash then upto the end of the line, ive tried a couple way but cant get it to work correctly
what i have below only captures the first character

if ($tmpVar =~ m/magnet\:\?xt=urn\:btih\:([[:alnum:]]+?)/i) {
  $mainRes{'hash'} = $1;
}

i also tried adding &|$ after the capture but that just results in an error
Thanks

"just results in an error" -- this statement is next to useless. Instead say what the specific error is. — TLP, Mar 01 '12 at 19:29

score 4 · Accepted Answer · answered Mar 01 '12 at 17:13

4

You could use:

/\burn:btih:([A-F\d]+)\b/i

Or if the hash is always 40 chars:

/\burn:btih:([A-F\d]{40})\b/i

answered Mar 01 '12 at 17:13

Qtax

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ahh thnx, it was the '?' that was screwing me up – Kr0nZ Mar 01 '12 at 17:16
yeh i though about just limiting it to 40 chars, but i think it can sometimes be 32 chars, according to the specs (if i read it correctly) thnx – Kr0nZ Mar 01 '12 at 17:22
@Kr0nZ, you could use `{32,40}` then. – Qtax Mar 01 '12 at 17:52

score 2 · Answer 2 · answered Mar 01 '12 at 18:27

As you've already discovered, you don't want to use the ? in your regular-expressions. Here's why:

The ? in pattern+? makes your regex "non-greedy", meaning it will try to use as few characters as possible while still matching the pattern you specify. So

"8AC3731AD4B039C05393B5404AFA6E7397810B41" =~ /(\w+?)/

just returns "8" while

"8AC3731AD4B039C05393B5404AFA6E7397810B41" =~ /(\w+)/

returns the whole string.

if ($tmpVar =~ m/magnet:\?xt=urn:btih:([[:alnum:]]+)/i) {
    $mainRes{'hash'} = $1;
}

score 0 · Answer 3 · answered Mar 10 '12 at 12:01

This is why the gods of CPAN gave us URI, to parse out parts of URIs, which you can then parse with a regex.

#!/usr/bin/perl
use URI;
use URI::QueryParam;
use Data::Dumper;

my $u = URI->new( shift() );
my $xt = $u->query_form_hash->{xt};

my ($hash) = $xt =~ m{^urn:btih:(.*)$};
print "$hash\n";

Presuming your magnet URI on the command line.

perl regex matching infohash from magnet link

3 Answers3