$re = '~^(.*?sheet/)pub(.*)(&[a-z=]*)$~i';
$str = 'aaa/bbbbb/ccc.d:abcsheet/pub/more/here?aa=0&output=html';
$subst = "$1ccc$2#gid=0";
$result = preg_replace($re, $subst, $str, 1);
echo "The result of the substitution is ".$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