Edited Validation

<!DOCTYPE html>

<html>
<head>
    <title>Form validation using Regex</title>

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<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.1.0/js/bootstrap.min.js"></script>

</head>
<body>

    <h1 class="text-center bg-dark text-white">Form validation using REGEX in javascript</h1>

   <div class="container">
    <br>
    <form>

        <div class="form-group">
            <label>Username:</label>
            <input type="text" name="" id="username" class="form-control" >
            <span id="usererror" class="text-danger font-weight-bold"></span>
        </div>

        <div class="form-group">
            <label>Password:</label>
            <input type="text" name="" id="password" class="form-control">
            <span id="passerror" class="text-danger font-wright-bold"></span>
        </div>

        <div class="form-group">
            <label>Confirm password:</label>
            <input type="text" name="" id="cpassword" class="form-control">
            <span id="cpasserror" class="text-danger font-weight-bold"></span>
        </div>

        <div class="form-group">
            <label>Email :</label>
            <input type="text" name="" id="email" class="form-control">
            <span id="emailerror" class="text-danger font-weight-bold"></span>
        </div>

        <div class="form-group">
            <label>Mobile no:</label>
            <input type="text" name="" id="mobile" class="form-control">
            <span id="mobileerror" class="text-danger font-weight-bold"></span>
        </div>

        <input onclick="validation()" type="button" name="button" value="submit" class="btn btn-danger">


    </form>
</div>

<script type="text/javascript">

  function validation(){

    var username = document.getElementById('username').value;
    var password = document.getElementById('password').value;
    var cpassword = document.getElementById('cpassword').value;
    var email = document.getElementById('email').value;
    var mobile = document.getElementById('mobile').value;

    console.log(username);

    var usercheck = /^[A-Za-z. 1-99]{3,85}$/ ;
    var passwordcheck = /^(?=.*[0-9])(?=.*[!@#$%^&*])[A-Za-z0-9!@#$%^&*]{8,18}/;
    var emailcheck = /^[A-za-z0-9_]{3,}@[A-za-z]{3,}[.][A-Za-z.]{2,}$/;
    var mobilecheck = /^[9][0-9]{9}$/;


   if(usercheck.test(username)){

      document.getElementById('usererror').innerHTML =" ";

   }

    else{
        document.getElementById('usererror').innerHTML ="Username is invalid";
        return false;
    }

    if(passwordcheck.test(password)){
       document.getElementById('passerror').innerHTML =" ";

    }

    else{
        document.getElementById('passerror').innerHTML ="Passowrd is invalid";
        return false;
    }


   if(password.match(cpassword)){
      document.getElementById('cpasserror').innerHTML =" ";

   }

   else{
    document.getElementById('cpasserror').innerHTML ="Password donot match";
    return false;
   }

    if(emailcheck.test(email)){
       document.getElementById('emailerror').innerHTML =" ";

    }

    else{
        document.getElementById('emailerror').innerHTML ="Email is invalid";
        return false;
    }

     if(mobilecheck.test(mobile)){
       document.getElementById('mobileerror').innerHTML =" ";

    }

    else{
        document.getElementById('mobileerror').innerHTML ="Passowrd is invalid";
        return false;
    }

  }


</script>

</body>
</html>