$re = '/(?sm)^(\d{2}:\d{2}:\d{2}\.\d+) +--> +(\d{2}:\d{2}:\d{2}\.\d+)((?:(?!\r?\n\r?\n).)*)/';
$str = '00:00:02.736 --> 00:00:06.072 line:79.33% position:10.00% align:start
AND YOUR GRACE?
00:00:06.072 --> 00:00:08.875 line:74.00% position:10.00% align:start
WHAT WILL YOU DO
ABOUT THAT?';
$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