JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://unpkg.com/imask"></script>
<p>ex.1
<input id="foo" placeholder="最初の文字のみ1~6、それ以降は数字、全部で6文字まで">
</p>
<p>ex.2
<input id="bar" placeholder="でんわばんごう">
</p>

CSS

input{width:75%;padding:15px;}

JavaScript

var regExpMask = new IMask(
  document.getElementById('foo'),
  {
    mask: /^[1-6]\d{0,5}$/
  });
  
 var phoneMask = new IMask(
  document.getElementById('bar'), {
  mask: '+{81}(00)0000-0000'
});