$re = '/\bF150\b.*/m';
$str = 'G1 X277.8072 Y212.6482 Z-2.5000
G1 X277.3935 Y212.6617 Z-2.5000
G1 X276.9809 Y212.6737 Z-2.5000
G1 F150 Z-4.0000
G1 X276.9809 Y212.6738 Z-4.0000
G1 X276.5705 Y212.6846 Z-4.0000 ';
$subst = "$0\nG1 F1000";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
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