title: Sphinx not search for English word with underscore in Japanese document
body: <p>I created Japanese and English documents in sphinx, which had the same contents. Japanese document includes some English words.
  If you search for an English word with an underline in both documents (for example,
  &quot;OMP_NUM_THREADS&quot;), you will see the search results in English, but not in Japanese.</p>
  <p>Do you know how to fix it ?</p>