How to check if a username exits using jquery validation and laravel

so i have a form and i want to check if a username exits before the user submits the form i'm using jquery validation plugin to validate the form and send a post request to check if the user name is vailable but i'm getting a server error of 419 and sometimes 500. here is a snippet of my code i have been told by someone it may have something to do with a csrf token laravel generates when a post request is sent however i don't know what to do ? js code below.

$.validator.addMethod('firstname_regex', function(value,element){
 return this.optional(element) || fnameregex.test(value);
},'This name is not valid.')

$.validator.addMethod('lastname_regex', function (value,element) {
 return this.optional(element) || lnameregex.test(value);
},'This name is not valid')

$.validator.addMethod('username_regex', function (value,element) {
 return this.optional(element) || usernameregex.test(value);
},'This username is not valid')

$.validator.addMethod('password_regex', function (value,element) {
    return this.optional(element) || passregex.test(value);
},'Passwords must be eight characters or more contain one uppercase, one numeric            
    number and one lowercase.')

$(function(){
    $('#signupform').validate({
      rules:{
         firstname:{
               required:true,
             firstname_regex:true,
                nowhitespace:true,
                minlength:3,
         },
         lastname:{
              required:true,
              lastname_regex:true,
              nowhitespace:true,
              minlength:3,
         },
         username:{
              required:true,
             username_regex:true,
             nowhitespace:true,
             remote:{
                    url:"checkusers",
                    type:"post"
               }
          },
            email:{
             required:true,
                email:true,
                 nowhitespace:true,
          },
          password:{
               required:true,
               password_regex:true,
              nowhitespace:true,
           },
          passconfirm:{
              required:true,
              password_regex:true,
             nowhitespace:true,
             equalTo:"#password",
          }

         },
         highlight: function(element) {
            $(element).closest('.form-group').addClass('has-error');
         $(element).closest('.form-control').css("border-color","#a94442");
       },
     unhighlight: function(element) {
           $(element).closest('.form-group').removeClass('has-error');
          $(element).closest('.form-control').css("border-color","#00b8e6");
      },
        messages:{
         passconfirm:{
              equalTo:"Please ensure your passwords match."
              },
             firstname:{
                 minlength:"Please enter a valid name."
          },
         lastname:{
              minlength:"Please enter a valid name."
          },
            username:{
                remote: "This username already exits."
          }
     }
  });
})

code for my controller

Route::post('/checkusers',function(){
$user = users::all()->where('username', Input::get('username'))->first();
   if ($user) {
        return false;
   } else {
          return true;
    }
});

i hope i am being clear enough for you guys to help me thanks very much. someone mention some about a token needs to be sent for laravel active session authentication I don't know what this means or how to go about solving but hopefully that may shed some light on my problem for anyone to help.