replacing a particular pattern in a string with an auto incrementing number
<p>I have a string like <code>"Hello how Hello are Hello"</code>
I want to replace the number of times <code>"Hello"</code> occurs in the above string by an autoincrementing number.</p>

<p>Such that the result would be: <code>"1 how 2 are 3"</code></p>

<p><code>"Hello"</code> can occur any number of times</p>
