matplotlib · oscargus · Aug 20, 2022 · Aug 20, 2022
diff --git a/doc/users/next_whats_new/mathtext_middle.rst b/doc/users/next_whats_new/mathtext_middle.rst
@@ -0,0 +1,17 @@
+``mathtext`` supports ``\middle``
+---------------------------------
+
+The ``\middle`` latex command is now supported by `.mathtext`. It is a
+complement to ``\left`` and ``\right`` and used to add a middle sized
+separator. Currently, only a single ``\middle`` between ``\left`` and
+``\right`` is supported.
+
+.. plot::
+    :include-source: true
+
+    import matplotlib.pyplot as plt
+    plt.figure(figsize=(2, 1))
+    plt.figtext(
+        0.05, 0.55,
+        r"$\left\{\sum_{i=0}^x a_i \middle | x \in \mathbb{N}\right\}$",
+        size=20, math_fontfamily='cm')
diff --git a/lib/matplotlib/_mathtext.py b/lib/matplotlib/_mathtext.py
@@ -1782,7 +1782,7 @@ class _MathStyle(enum.Enum):
 
     _ambi_delims = set(r"""
       | \| / \backslash \uparrow \downarrow \updownarrow \Uparrow
-      \Downarrow \Updownarrow . \vert \Vert""".split())
+      \Downarrow \Updownarrow . \vert \Vert \mid""".split())
     _left_delims = set(r"( [ \{ < \lfloor \langle \lceil".split())
     _right_delims = set(r") ] \} > \rfloor \rangle \rceil".split())
     _delims = _left_delims | _right_delims | _ambi_delims
@@ -1950,7 +1950,10 @@ def csnames(group, names):
 
         p.auto_delim    <<= (
             r"\left" - (p.delim("left") | Error("Expected a delimiter"))
-            + ZeroOrMore(p.simple | p.auto_delim)("mid")
+            + ZeroOrMore(p.simple | p.auto_delim)("mid1")
+            + Optional(r"\middle" - (p.delim("mid") |
+                                     Error("Expected a delimiter")))("middle")
+            + ZeroOrMore(p.simple | p.auto_delim)("mid2")
             + r"\right" - (p.delim("right") | Error("Expected a delimiter"))
         )
 
@@ -2560,11 +2563,14 @@ def overline(self, s, loc, toks):
         hlist = Hlist([rightside])
         return [hlist]
 
-    def _auto_sized_delimiter(self, front, middle, back):
+    def _auto_sized_delimiter(self, front, mid1, back, middle=".", mid2=None):
+        if mid2 is None:
+            mid2 = []
         state = self.get_state()
-        if len(middle):
-            height = max(x.height for x in middle)
-            depth = max(x.depth for x in middle)
+        content = mid1 + mid2
+        if len(content):
+            height = max(x.height for x in content)
+            depth = max(x.depth for x in content)
             factor = None
         else:
             height = 0
@@ -2575,7 +2581,11 @@ def _auto_sized_delimiter(self, front, middle, back):
         if front != '.':
             parts.append(
                 AutoHeightChar(front, height, depth, state, factor=factor))
-        parts.extend(middle)
+        parts.extend(mid1)
+        if middle != ".":
+            parts.append(
+                AutoHeightChar(middle, height, depth, state, factor=factor))
+        parts.extend(mid2)
         if back != '.':
             parts.append(
                 AutoHeightChar(back, height, depth, state, factor=factor))
@@ -2585,6 +2595,9 @@ def _auto_sized_delimiter(self, front, middle, back):
     def auto_delim(self, s, loc, toks):
         return self._auto_sized_delimiter(
             toks["left"],
-            # if "mid" in toks ... can be removed when requiring pyparsing 3.
-            toks["mid"].asList() if "mid" in toks else [],
-            toks["right"])
+            # if "mid*" in toks ... can be removed when requiring pyparsing 3.
+            toks["mid1"].asList() if "mid1" in toks else [],
+            toks["right"],
+            toks["mid"] if "mid" in toks else ".",
+            toks["mid2"].asList() if "mid2" in toks else []
+            )
diff --git a/lib/matplotlib/_mathtext_data.py b/lib/matplotlib/_mathtext_data.py
@@ -526,7 +526,7 @@
     'succnsim'                 : 8937,
     'gimel'                    : 8503,
     'vert'                     : 124,
-    '|'                        : 124,
+    '|'                        : 8214,
     'varrho'                   : 1009,
     'P'                        : 182,
     'approxident'              : 8779,

diff --git a/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext1_dejavusans_04.png b/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext1_dejavusans_04.png
diff --git a/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext_dejavusans_68.pdf b/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext_dejavusans_68.pdf
diff --git a/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext_dejavusans_68.png b/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext_dejavusans_68.png
diff --git a/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext_dejavusans_68.svg b/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext_dejavusans_68.svg
diff --git a/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext_dejavuserif_68.pdf b/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext_dejavuserif_68.pdf
diff --git a/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext_dejavuserif_68.png b/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext_dejavuserif_68.png