js1

function validate() {
 var username = document.getElementById("name");
  var checkName=new RegExp("^[a-zA-Z0-9_]");

 if (username.value.length < 3 ||  username.value.length > 10 || (!checkName.test(username)) ){
    document.getElementById("div1").innerHTML="Потребителското име трябва да е поне 3 символа и не повече от 10";
    document.getElementById("div1").style.color="Red";
    return false;
 }

 }


<!DOCTYPE html>

<html>

<head>
<meta charset="utf-8">
<title>Form to fill</title>
<script src="form.js"> </script>

</head>

<body>

<h2>Fill out the form below and click “order now” to order</h2>

<form id="theForm" action="register.php" method="post" onSubmit="return validate();">


<label for="name">Name:</label>
<input type="text" id="name" name=""> </br> </br>
<div id="div1">
</div>

<div class="row">
<label for="address">Address:</label>
<input type="text" id="address" name=""> </br> </br>
</div>

<div class="row">
<label for="phone">Phone:</label>
<input type="tel" id="phone" name=""> </br> </br>
</div>

<div class="row">
<label for="email">Email:</label>
<input type="email" id="email" name="">
</div>

<br>

<input type="submit" value="Submit">

</form>

</body>

</html>


function validate() {
    var username = document.getElementById("name");
    var checkName=new RegExp("^[a-z0-9]");

     if (username.value.length < 3 ||  username.value.length > 10) {
        document.getElementById("div1").innerHTML="Потребителското име трябва да е поне 3 символа и не повече от 10";
        document.getElementById("div1").style.color="Red";
        return false;
     }


 }

<!DOCTYPE html>

<html>

<head>
<meta charset="utf-8">
<title>Form to fill</title>


</head>

<body>

<script>


function validate() {
    var username = document.getElementById("name").value;
    //var checkName=new RegExp("^[a-z0-9]");
    var test = new RegExp("[a-zA-Z0-9!@#$%^*_|]");
        if(!username.match(test) || username.value.length < 3 ||  username.value.length > 10 )//The test() method executes a search for a match between a regular expression and a specified string. Returns true or false.
        // '\/[ -works '\/[a -doesn't work because match searches the string pass for the characters in test
        //if the input is just 'a' it works why ?
        {
        document.getElementById("div1").innerHTML="Потребителското име трябва да е поне 3 символа и не повече от 10";
        document.getElementById("div1").style.color="Red";
        return false;
     }


 }
</script>


<h2>Fill out the form below and click “order now” to order</h2>

<form id="theForm" action="message.html" method="post" onSubmit="return validate();">


<label for="name">Name:</label>
<input type="text" id="name" name=""> </br> </br>
<div id="div1">
</div>

<div class="row">
<label for="address">Address:</label>
<input type="text" id="address" name=""> </br> </br>
</div>

<div class="row">
<label for="phone">Phone:</label>
<input type="tel" id="phone" name=""> </br> </br>
</div>

<div class="row">
<label for="email">Email:</label>
<input type="email" id="email" name="">
</div>

<br>

<input type="submit" value="Submit" >


</form>

</body>

</html>