title: Mongodb query: search regex in nested array
body: <p>Given the folowing document structure of recorded games:
  <div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>{
  ...
  "rightkingpercenthp":[1,1,1,1,1,1,1,1,1,1,1,0.94,0.95,0],
  "gameDetails":[
   {"playername":"Calvino",
   ...
   "unitsPerWave":[
    ["grarl_unit_id:0.5|9.5"],
    ["hydra_unit_id:3.5|6.5","veteran_unit_id:1.5|6","grarl_unit_id:3.5|5.5","peewee_unit_id:8.5|7"],        ["hydra_unit_id:3.5|6.5","veteran_unit_id:1.5|6","veteran_unit_id:8.5|7","peewee_unit_id:8.5|6",
      "fatalizer_unit_id:2.5|5"],
      ...
      ]
  }</code></pre>
  </div>
  </div>
  
  How can i count the amount of games with at least one e.g. "veteran_unit_id" in gameDetails.unitsPerWave (ignoring coordinates after the name)? </p>