<form>
<input type="text" class="form-control" id="name"  placeholder="Enter Name" name="user_name">
<button>Submit</button>
</form>
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validate.js

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function verify_name(name){

    $(".name_error").hide();
    if(name == ""){
    $(".name_error").show();
    $(".name_error").html("plz provide your name");
    }
    else{
    alert('Running AJAX request');
     $.ajax({
        url:'classes/process.php',
        method:'post',
        data:{check_name:1,name:name},
        sucess:function(data){
         $(".name_error").show();
         if(data == "invalid_name"){
            $(".name_error").html("Provide correct name");
          }

        },error: function(req, status, error) {
                alert(req + "\n" + status + "\n" + error);
            }

     })

    }

   }

$("#name").focusout(function(){
    var name=$("#name").val();
    verify_name(name);

    })
_____________________________________________________________________________
Process.php
_____________________________________________________________________________

<?php

include "dbcon.php";

class DataOperation extends database{
public function verify_name($name){

        $regexp = "/^[A-Za-z_]{3,5}$/";
        if(!preg_match($regexp,$name)){
        return "invalid_name";
        }


        }

}

$obj = new DataOperation;
    if(isset($_POST["check_name"])){
     $name=$_POST["name"];
     echo $data=$obj->verify_name($name);
     exit();
 }