title: Regex for b, i and a tags only or none in a string
body: <p>I'd like to know the possible Regex  suitable for checking a string that includes certain HTML tags, namely, <code>&lt;b&gt;</code>, <code>&lt;i&gt;</code> and <code>&lt;a&gt;</code> or it has no any tags. using PHP preg_match.</p>
  
  <p>For example:</p>
  
  <pre><code>"This a text only.." return true
  "This is a &lt;b&gt;bold&lt;/b&gt; text" return true
  "this is &lt;script&gt;alert('hi')&lt;/script&gt;" return false
  "this is &lt;a href="#"&gt;some&lt;/a&gt;and &lt;h1&gt;header&lt;/h1&gt;" return false
  </code></pre>