title: POSIX regular expression to extract a substring
body: <p>I want to extract the substring in place of <code>md5_object_id</code> from the following string: </p>
  
  <pre><code>{
        "associatedEntity": "60056",
        "associatedObject": "md5_object_id",
        "associatedAttr": "END22206",
        "associatedAttr": ""
  }
  </code></pre>
  
  <p>The md5_object_id is dynamic and it can take any valid md5 string.
  I tried some regex, but I need the most efficient one, since I am using the extracted substring in an SQL Join query. I am using PostgreSQL 9.2, so json functions are not available.</p>