Don't allow r"$\left\\|\right.$", as in TeX.

anntzer · anntzer · commit 223640485379 · 2022-06-11T15:29:07.000+02:00
AFAICT, (incorrect) support for the double-backslashed version was accidentally introduced in 027dd2c.
diff --git a/lib/matplotlib/_mathtext.py b/lib/matplotlib/_mathtext.py
@@ -1680,7 +1680,7 @@ class _MathStyle(enum.Enum):
 
     _ambi_delim = set(r"""
       | \| / \backslash \uparrow \downarrow \updownarrow \Uparrow
-      \Downarrow \Updownarrow . \vert \Vert \\|""".split())
+      \Downarrow \Updownarrow . \vert \Vert""".split())
 
     _left_delim = set(r"( [ \{ < \lfloor \langle \lceil".split())
 
