patterns are registered does not matter.
</p>

<p>
  This change breaks backwards compatibility in small ways, some obvious&mdash;patterns with "{" and "}" behave differently&mdash;
  and some less so&mdash;treatment of escaped paths has been improved.
  The change is controlled by a <a href="/doc/godebug"><code>GODEBUG</code></a> field named <code>httpmuxgo121</code>.