matplotlib · ksunden · Jul 21, 2023 · Jul 20, 2023
diff --git a/lib/matplotlib/_mathtext.py b/lib/matplotlib/_mathtext.py
@@ -1965,9 +1965,10 @@ def csnames(group, names):
             | p.boldsymbol
         )
 
+        mdelim = r"\middle" - (p.delim("mdelim") | Error("Expected a delimiter"))
         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 | mdelim)("mid")
             + r"\right" - (p.delim("right") | Error("Expected a delimiter"))
         )
 
@@ -2586,13 +2587,23 @@ def overline(self, s, loc, toks):
     def _auto_sized_delimiter(self, front, middle, back):
         state = self.get_state()
         if len(middle):
-            height = max(x.height for x in middle)
-            depth = max(x.depth for x in middle)
+            height = max([x.height for x in middle if not isinstance(x, str)])
+            depth = max([x.depth for x in middle if not isinstance(x, str)])
             factor = None
+            for idx, el in enumerate(middle):
+                if isinstance(el, str) and el == '\\middle':
+                    c = middle[idx + 1]
+                    if c != '.':
+                        middle[idx + 1] = AutoHeightChar(
+                                c, height, depth, state, factor=factor)
+                    else:
+                        middle.remove(c)
+                    del middle[idx]
         else:
             height = 0
             depth = 0
             factor = 1.0
+
         parts = []
         # \left. and \right. aren't supposed to produce any symbols
         if front != '.':

diff --git a/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext1_dejavusans_07.png b/lib/matplotlib/tests/baseline_images/test_mathtext/mathtext1_dejavusans_07.png
diff --git a/lib/matplotlib/tests/test_mathtext.py b/lib/matplotlib/tests/test_mathtext.py
@@ -136,6 +136,7 @@
     r'$\text{text}_{\text{sub}}^{\text{sup}} + \text{\$foo\$} + \frac{\text{num}}{\mathbf{\text{den}}}\text{with space, curly brackets \{\}, and dash -}$',
     r'$\boldsymbol{abcde} \boldsymbol{+} \boldsymbol{\Gamma + \Omega} \boldsymbol{01234} \boldsymbol{\alpha * \beta}$',
     r'$\left\lbrace\frac{\left\lbrack A^b_c\right\rbrace}{\left\leftbrace D^e_f \right\rbrack}\right\rightbrace\ \left\leftparen\max_{x} \left\lgroup \frac{A}{B}\right\rgroup \right\rightparen$',
+    r'$\left( a\middle. b \right)$ $\left( \frac{a}{b} \middle\vert x_i \in P^S \right)$ $\left[ 1 - \middle| a\middle| + \left( x  - \left\lfloor \dfrac{a}{b}\right\rfloor \right)  \right]$',
 ]
 
 digits = "0123456789"