title: Allow only half width katakana (Japanese) characters in text box
body: <p>In my rails app I have a text field like this </p>
  
  <pre><code>&lt;%= f.text_field :guaranter_furigana_name, autocomplete: "off", class: "form-control", placeholder: "例：ヤマダ タロウ（全角カナ）", maxlength: "50",  :disabled =&gt; @disabled_field %&gt;
  </code></pre>
  
  <p>I am using jQuery to validate this text field accepts only <a href="https://en.wikipedia.org/wiki/Half-width_kana" rel="nofollow">half width katakana</a> characters <strong>(Unicode values from ff60 to ff9f)</strong>. This is what I tried</p>
  
  <pre><code>// Allow only half width kana for guranter フリガナ
  $(function (){
    $("#t_user_name_register_guaranter_furigana_name").on("input", function(e){
  
      var key = e.keyCode || e.charCode;
  
      var inp = String.fromCharCode(key);
  
      var kanaregexp = new RegExp('[\uff00-\uff9f]');
      if (kanaregexp.test(inp) != true){
        if( key != 8 &amp;&amp; key != 46 &amp;&amp; key != 32){
          inp = $(this).val();
          $(this).val(inp.slice(0,-1));
          $("#telno_errmsg").html("半角カナ字のみを入力してください。").show().fadeOut(3000);
          return false;
        }
      }
    });
  });
  </code></pre>
  
  <p>The code is a mess and not working always. Please help</p>