$re = '/(?s).*speed=\s*\K([^\v]*)(?=x.*$)/';
$str = 'frame=13920 fps= 86 q=-1.0 size= 96512kB time=00:07:44.47 bitrate=1702.2kbits/s speed=2.88x
frame=14068 fps= 87 q=-1.0 size= 98048kB time=00:07:49.41 bitrate=1711.1kbits/s speed= 2.9x
frame=14116 fps= 87 q=-1.0 Lsize= 98954kB time=00:07:51.06 bitrate=1720.9kbits/s speed=2.91x
video:86252kB audio:3826kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 9.852874%';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($matches);
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php