\z is a "better" version of $ and should usually be used instead of $:

• The regular expression /^cat$/ matches the input strings cat and cat\n.
• The regular expression /^cat\z/ matches only the input string cat.

This usually doesn't matter, but can occasionally be important, as in D8516.

if you were writing a custom DocumentEngine for say .myformat which is perhaps traditionally mapped to the mime type of text/plain, then I'm guessing adding the following, is a good way to get your engine to be picked as the "default" viewer

protected function getContentScore(PhabricatorDocumentRef $ref) {
    $name = $ref->getName();
    if (preg_match('/\\.myformat\z/i', $name)) {
      return 2000;
    }

    return 500;
  }