title: jq syntax error extracting key with colon in its name
body: <p>I have the following JSON (which can be retrieved from <a href="https://openlibrary.org/api/books?bibkeys=ISBN:039397281X&amp;format=json" rel="nofollow noreferrer">https://openlibrary.org/api/books?bibkeys=ISBN:039397281X&amp;format=json</a>) as input:</p>
  
  <pre><code>{
    "ISBN:039397281X": {
      "bib_key": "ISBN:039397281X",
      "preview": "borrow",
      "thumbnail_url": "https://covers.openlibrary.org/b/id/7890978-S.jpg",
      "preview_url": "https://archive.org/details/isbn_9780393972818",
      "info_url": "https://openlibrary.org/books/OL348852M/Don_Quijote"
    }
  }
  </code></pre>
  
  <p>My goal is to extract the <code>preview_url</code> -- thus, for the input above, to get the output <code>https://archive.org/details/isbn_9780393972818</code>.</p>
  
  <p>I'm currently using the following:</p>
  
  <pre><code>curl 'https://openlibrary.org/api/books?bibkeys=ISBN:039397281X&amp;format=json' \
    | jq -r '.ISBN:039397281X' | jq -r '.preview_url'
  </code></pre>
  
  <p>However, this emits an error:</p>
  
  <pre><code>jq: error: syntax error, unexpected ':', expecting $end (Unix shell quoting issues?) at &lt;top-level&gt;, line 1:
  .ISBN:039397281X     
  jq: 1 compile error
  </code></pre>
  
  <p>How can it be resolved?</p>