$re = '/(?m)(?|^(\d+):()()|\G^()([-+]?\d+)(?![:\d])[ \t]+(.*)|\G^()()()[ \t]*$\R*)\R?/';
$str = '6:
1 my 1st log
-2 my 2nd log
7:
-3 my 3rd log
4 my 4th log
-5 my 5th log
';
$subst = "$1\t$2\t$3\r\n";
$result = preg_replace($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