Forbid square brackets in reference link IDs (#680)

lib/markdown2.py

@@ -1460,7 +1460,7 @@ def _sanitize_html(self, s: str) -> str:
           [ ]?          # one optional space
           (?:\n[ ]*)?   # one optional newline followed by spaces
           \[
-            (?P<id>.*?)
+            (?P<id>[^\[\]]*?)
           \]
         ''', re.X | re.S)
 
@@ -3179,7 +3179,6 @@ def run(self, text: str):
                 start_idx = text.index('[', curr_pos)
             except ValueError:
                 break
-            text_length = len(text)
 
             # Find the matching closing ']'.
             # Markdown.pl allows *matching* brackets in link text so we
@@ -3190,7 +3189,7 @@ def run(self, text: str):
 
             for p in range(
                 start_idx + 1,
-                min(start_idx + MAX_LINK_TEXT_SENTINEL, text_length)
+                min(start_idx + MAX_LINK_TEXT_SENTINEL, len(text))
             ):
                 ch = text[p]
                 if ch == ']':

test/tm-cases/reference_link_lookalike_issue680.html

@@ -0,0 +1 @@
+<p>[<a href="http://a.com">A</a>] [<a href="http://b.com">B</a>]</p>

test/tm-cases/reference_link_lookalike_issue680.text

@@ -0,0 +1 @@
+[[A](http://a.com)] [[B](http://b.com)]