JavaScript Fucntion.
function ValidateText(source, clientside_arguments)
{
    var txtVal = clientside_arguments.Value;   
    var len = txtVal.length;
    var Search =0;
    if(txtVal.charAt(0) < 'A' || txtVal.charAt(0) > 'Z' && txtVal.charAt(0) <'a' || txtVal.charAt(0)>'z')
    {
               alert("First letter of User Name must be an alphabet");
               clientside_arguments.IsValid=false;
    }
    else
    {
        for(var i = 0;i<len; i++)
        {           
            if(txtVal.charAt(i) < 'A' || txtVal.charAt(i) > 'Z' && txtVal.charAt(i) <'a' || txtVal.charAt(i)>'z')
                   Search = Search + 1;
        }
        if(Search == len)
        {
               alert("Invalid User Name");
               clientside_arguments.IsValid=false;
        }
        else
            clientside_arguments.IsValid=true;
    }
  
}