title: How to find data with specific character in specific position of a string in ElasticSearch
body: <h4>dada in ES:</h4>
  <div class="s-table-container">
  <table class="s-table">
  <thead>
  <tr>
  <th>f1 (string)</th>
  <th>f2(string)</th>
  <th>id  (int)</th>
  </tr>
  </thead>
  <tbody>
  <tr>
  <td>apple</td>
  <td>abcdef</td>
  <td>1</td>
  </tr>
  <tr>
  <td>perl</td>
  <td>ed<strong>f</strong>adfe</td>
  <td>2</td>
  </tr>
  <tr>
  <td>perl</td>
  <td>ae<strong>f</strong>asdf</td>
  <td>3</td>
  </tr>
  <tr>
  <td>perl</td>
  <td>fedae</td>
  <td>4</td>
  </tr>
  </tbody>
  </table>
  </div>
  <p>I want to find <code>f2[2]='f'</code>, i.e. the data with id=2 and 3</p>
  <p>how to find it efficiently?</p>
  <p>the length of f2 is fixed, may be up to 336 characters.</p>
  <p>my solution is :</p>
  <pre><code>{
      &quot;query&quot;: {
          &quot;regexp&quot;: {
              &quot;f2&quot;: &quot;.{2}f.*&quot; 
          }
      }
  }
  </code></pre>
  <p>but I don't think using regexp would be efficient, any better choice?</p>