- preg_match: documentation ( source)
<?php
$data = <<< 'TEXT'
edit 2
set dst 10.23.255.252 255.255.255.252
set device "abc"
set comment "Static Route site-5552"
TEXT;
$match=[];
preg_match('/set dst ([\d\.]+).*site-5552/s',$data,$match);
echo $match[1];