Use simplified regex for html placeholders

`util` already provides the regex we need for all placeholders, and we're doing extra work parsing for the explicit union of all of them. In documents with ~30000 placeholders, I get a ~5x speedup with this change.

Author: raabrp

docs/change_log/release-3.2.md

@@ -93,4 +93,5 @@ The following bug fixes are included in the 3.2 release:
 * HTML tag placeholders are no longer included in `.toc_tokens` (#899).
 * Unescape backslash-escaped characters in TOC ids (#864).
 * Refactor bold and italic logic in order to solve complex nesting issues (#792).
-* Always wrap CodeHilite code in <code> tags (#862).
+* Always wrap CodeHilite code in <code> tags (#862) </code>
+* Regex for HTML placeholders simplified for speed improvement (#928).

markdown/postprocessors.py

@@ -77,7 +77,7 @@ def run(self, text):
  replacements[self.md.htmlStash.get_placeholder(i)] = html
 
  if replacements:
- pattern = re.compile("|".join(re.escape(k) for k in replacements))
+ pattern = util.HTML_PLACEHOLDER_RE
  processed_text = pattern.sub(lambda m: replacements[m.group(0)], text)
  else:
  return text