$re = '/(?:(?:\R|\A).+){3}\K\R.+/m';
$str = '1
00:07:14,120 --> 00:07:16,953
[French line]
[Russian line]
2
00:07:16,960 --> 00:07:19,952
[French line]
[Russian line]
3
00:07:21,240 --> 00:07:24,835
[French line]
[Russian line]';
$subst = "";
$result = preg_replace($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