title: A list of the highest x's for each group
body: <pre><code>a_list = [
  'img_229_152_webp_1x', 
  'img_229_152_webp_2x', 
  'img_229_152_webp_3x', 
  'img_229_152_webp_4x', 
  'img_229_152_webp_5x', 
  'img_246_164_webp_1x', 
  'img_246_164_webp_2x', 
  'img_246_164_webp_3x', 
  'img_246_164_webp_4x']
  </code></pre>
  <p>Here we have two groups of images: <code>img_229_152_webp_5x'</code>, and <code>'img_246_164_webp_4x'</code>.</p>
  <p>The list is sorted. And as for the last part (<code>\d+x</code>), each group of images is an   arithmetic progression, no missing element is possible. Example of impossible case: 1x, 2x, 4x.</p>
  <p>Could you help me get a list of the highest x's for each group. In this case: <code>[img_229_152_webp_5x', 'img_246_164_webp_4x']</code>.</p>