<?php

$re = '/(?:\G(?!^)\s*|Stream mapping:\s*)\K([a-zA-Z]+\s+#[0-9]+:[0-9]+).*/';
$str = 'Metadata:
      variant_bitrate : 800000
Stream mapping:
  Stream #0:4 -> #0:0 (copy)
  Stream #0:5 -> #0:1 (copy)
Press [q] to stop, [?] for help';

if (preg_match_all($re, $str, $m)) {
 print_r($m[1]);
}