我不想在一个字符串中允许多个模式，例如，

$string = "php in  stackoverflow"; // multiple spaces not allowed
$string = "php in--stackoverflow"; // multiple hyphens not allowed
$string = "php in__stackoverflow"; // multiple underscores not allowed
etc

字符串
所以，它可以用这些线，

if(preg_match('/\s\s+/', $string)) echo "only a single spaces are allowed";
if(preg_match('/\-\-+/', $string)) echo "only a single hyphens are allowed";
if(preg_match('/\_\_+/', $string)) echo "only a single underscores are allowed";

型
我也不想让下面的组合模式和上面的线条不起作用，

$string = "php in -stackoverflow"; // a space with a hyphen not allowed
$string = "php in-_stackoverflow"; // a hyphens with a underscore not allowed
$string = "php in_ stackoverflow"; // a underscore with a space not allowed
etc

型
有没有办法用一个简单的脚本来实现这一点？