title: php validate string with preg_match
body: <p>I am trying to verify in PHP with preg_match that an input string contains only "a-z, A-Z, -, _ ,0-9" characters. If it contains just these, then validate.</p>
  
  <p>I tried to search on google but I could not find anything usefull.</p>
  
  <p>Can anybody help?</p>
  
  <p>Thank you !</p>