$re = '/(?<!\S)(?<!\bDim\s+)day(s)?(?!\S)/m';
$str = '\'*** This day is the day of all days
\'*** This day is the day of all days (there are 4 spaces at the start)
Dim day as integer
day=day+1';
$subst = "night$1";
$result = preg_replace($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