title: Extract multiple instances of Text from MySQL column bounded by some characters
body: <p>I'd like to extract certain snippets from a MySQL column which are bounded in some unique way. Here is an example:</p>
  
  <p>I have the following table (TableA):</p>
  
  <pre><code>id | column_a
  ---+---------
  1  | There is a user [u?u=990] and another [u?u=5458855].
  2  | And here is just one user [u?u=9390].
  3  | And here is nothing.
  </code></pre>
  
  <p>And I want to get this:</p>
  
  <pre><code>id | result_a
  ---+---------
  1  | 990
  1  | 5458855
  2  | 9390
  </code></pre>
  
  <p>At the moment I have this query:</p>
  
  <pre><code>SELECT id,SUBSTRING_INDEX(SUBSTRING_INDEX(column_a, '[u?u=', -1), ']', 1) AS result_a FROM TableA
  </code></pre>
  
  <p>But that gives me:</p>
  
  <pre><code>id | result_a
  ---+---------
  1  | 5458855
  2  | 9390
  </code></pre>
  
  <p>How can I improve this query? </p>
  
  <p>Thanks!</p>