How to add a specific characters at the end of the words only in the first column?
<p>I need to manipulate the file this way:</p>

<p><strong>File In -</strong></p>

<pre><code>1 34566 34765 

2 45678 45789

3 34567 34799

X 67895 66900

Y 34567 34890
</code></pre>

<p><strong>file out</strong></p>

<pre><code>1_M 34566 34765

2_M 45678 45789

3_M 34567 34799

X_M 67895 66900

Y_M 34567 34890
</code></pre>

<p>Thanks,</p>