PHP文本框通过preg_match验证

I want to check that a user input in textbox doesnot contains any special character. how can i validate text box without using any javascript/Jquery

Try this, $name is the user input,

if(preg_match('/[^a-z0-9 _]+/i', $name)) {
// preg_match will return true if it finds 
// a character *other than* a-z, 0-9, space and _
// *anywhere* inside the string
}