diff --git a/Tests/test_imagemorph.py b/Tests/test_imagemorph.py
index 515e29cead9..ca192a809c4 100644
--- a/Tests/test_imagemorph.py
+++ b/Tests/test_imagemorph.py
@@ -7,7 +7,7 @@
 
 from PIL import Image, ImageMorph, _imagingmorph
 
-from .helper import assert_image_equal_tofile, hopper
+from .helper import assert_image_equal_tofile, hopper, timeout_unless_slower_valgrind
 
 
 def string_to_img(image_string: str) -> Image.Image:
@@ -266,16 +266,18 @@ def test_unknown_pattern() -> None:
         ImageMorph.LutBuilder(op_name="unknown")
 
 
-def test_pattern_syntax_error() -> None:
+@pytest.mark.parametrize(
+    "pattern", ("a pattern with a syntax error", "4:(" + "X" * 30000)
+)
+@timeout_unless_slower_valgrind(1)
+def test_pattern_syntax_error(pattern: str) -> None:
     # Arrange
     lb = ImageMorph.LutBuilder(op_name="corner")
-    new_patterns = ["a pattern with a syntax error"]
+    new_patterns = [pattern]
     lb.add_patterns(new_patterns)
 
     # Act / Assert
-    with pytest.raises(
-        Exception, match='Syntax error in pattern "a pattern with a syntax error"'
-    ):
+    with pytest.raises(Exception, match='Syntax error in pattern "'):
         lb.build_lut()
 
 
diff --git a/docs/releasenotes/12.0.0.rst b/docs/releasenotes/12.0.0.rst
index e21c243ea55..41edea31844 100644
--- a/docs/releasenotes/12.0.0.rst
+++ b/docs/releasenotes/12.0.0.rst
@@ -150,3 +150,10 @@ others prepare for 3.14, and to ensure Pillow could be used immediately at the r
 of 3.14.0 final (2025-10-07, :pep:`745`).
 
 Pillow 12.0.0 now officially supports Python 3.14.
+
+ImageMorph operations must have length 1
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Valid ImageMorph operations are 4, N, 1 and M. By limiting the length to 1 character
+within Pillow, long execution times can be avoided if a user provided long pattern
+strings. Reported by Jang Choi.
diff --git a/src/PIL/ImageMorph.py b/src/PIL/ImageMorph.py
index f0a066b5bd8..bd70aff7b48 100644
--- a/src/PIL/ImageMorph.py
+++ b/src/PIL/ImageMorph.py
@@ -150,7 +150,7 @@ def build_lut(self) -> bytearray:
 
         # Parse and create symmetries of the patterns strings
         for p in self.patterns:
-            m = re.search(r"(\w*):?\s*\((.+?)\)\s*->\s*(\d)", p.replace("\n", ""))
+            m = re.search(r"(\w):?\s*\((.+?)\)\s*->\s*(\d)", p.replace("\n", ""))
             if not m:
                 msg = 'Syntax error in pattern "' + p + '"'
                 raise Exception(msg)