In the quest for regex-less solutions:

$in='1_abc/2_deg/bla_30_31_blah';

#one-liner with split, nicely complicated (including backdoor regex)
$out=join '/', (split /_|\//, $in, 4)[1,3];

#This works if the second number is a single digit 
$out=substr $in, index( $in, '_') + 1;
substr( $out, index( $out, '_') - 1, 2)='';
[download]

At the moment, I can't think of more.

Cheers,

Jeroen
"We are not alone"(FZ)
Update: Just thought of one substr use that doesn't break on multiple digits...:

$out=substr $in, index( $in, '_') + 1;
substr( $out, $us = index( $out, '/') + 1, index( $out, '_')- $us + 1)
+='';
[download]