Jquery check for alphanumeric charactres

In the below code Using jquery how to check for only alaphanumeric charaters.Even space and none of the special symbols should be allowed.I do want o use a external plugin

 var a='a hjdshj%^& ff234 ./.';

  if(var a has special ch开发者_运维问答aracters)
  {
      alert('has special characters');
  } 

EDIT : Even single and double quotes should not be allowed

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There is no need of any plugin to check this

you can make use of Regular exprssion to do it easily..

try this

var str='a hjdshj%^& ff234 ./.';
var re = new RegExp(/\$|\/|\^|-/\s,"g");
specialchars = str.match(re);
alert(specialchars.length);