In Aspx Code
<script type="text/javascript">
function NotAllow()
{
if(document.form1.TextBox1.value.length<4)
{
alert ("Please enter more than 4 characters")
}
if(document.form1.TextBox1.value.length>16)
{
alert ("Please enter less than 16 characters")
}
var iChars = "!@#$%^&*()+=-[]\\\';,./{}|\":<>? ";
for (var i = 0; i < document.form1.TextBox1.value.length; i++)
{
if (iChars.indexOf(document.form1.TextBox1.value.charAt(i)) != -1)
{
alert ("Your username has special characters. \nThese are not allowed.\n Please remove them and try again.");
return false;
}
}
}
script>
In Aspx.cs Code
Button1.Attributes.Add("onclick", "return NotAllow();");