title: Bash script : select lines starting with number in specific range
body: <p>I have a file containing lines like:</p>
  
  <pre><code>121&lt;some letters&gt; random text ...
  1234&lt;some letters&gt; random numbers etc...
  </code></pre>
  
  <p>Each line starts with a number followed by some letters.I'm looking for a way to select only the lines which start with a number in a specific interval, for example : [0-9999] . I'm having difficulty in selecting these lines if the number of digits can vary.</p>
  
  <p>Tried using grep but can't seem to find the correct way to write the regex.</p>