From 2dfdcb644d42e17f2a3e571dfb69250a1662ac1f Mon Sep 17 00:00:00 2001
From: Henry Jones <henryivesjones@gmail.com>
Date: Tue, 7 Apr 2026 14:17:13 +1200
Subject: [PATCH 1/2] Fix Generator._make_boundary behavior with CRLF line
 endings.

The Generator._make_boundary regex does not match on boundary phrases correctly when using CRLF line endings due to re.MULTILINE not considering \r\n as a line ending.
---
 Lib/email/generator.py                        |  2 +-
 Lib/test/test_email/test_generator.py         | 35 +++++++++++++++++++
 ...-04-07-14-13-40.gh-issue-148192.34AUYQ.rst |  2 ++
 3 files changed, 38 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Library/2026-04-07-14-13-40.gh-issue-148192.34AUYQ.rst

diff --git a/Lib/email/generator.py b/Lib/email/generator.py
index cebbc416087fee..ba11d63fba600a 100644
--- a/Lib/email/generator.py
+++ b/Lib/email/generator.py
@@ -392,7 +392,7 @@ def _make_boundary(cls, text=None):
         b = boundary
         counter = 0
         while True:
-            cre = cls._compile_re('^--' + re.escape(b) + '(--)?$', re.MULTILINE)
+            cre = cls._compile_re('^--' + re.escape(b) + '(--)?\r?$', re.MULTILINE)
             if not cre.search(text):
                 break
             b = boundary + '.' + str(counter)
diff --git a/Lib/test/test_email/test_generator.py b/Lib/test/test_email/test_generator.py
index c2d7d09d591e86..febafe181b9c9e 100644
--- a/Lib/test/test_email/test_generator.py
+++ b/Lib/test/test_email/test_generator.py
@@ -1,13 +1,20 @@
 import io
 import textwrap
 import unittest
+import random
+import sys
 from email import message_from_string, message_from_bytes
 from email.message import EmailMessage
+from email.mime.multipart import MIMEMultipart
+from email.mime.text import MIMEText
 from email.generator import Generator, BytesGenerator
+import email.generator
 from email.headerregistry import Address
 from email import policy
 import email.errors
 from test.test_email import TestEmailBase, parameterize
+import test.support
+
 
 
 @parameterize
@@ -288,6 +295,34 @@ def test_keep_long_encoded_newlines(self):
         g.flatten(msg)
         self.assertEqual(s.getvalue(), self.typ(expected))
 
+    def _test_boundary_detection(self, linesep):
+        # Generate a boundary token in the same way as _make_boundary
+        token = random.randrange(sys.maxsize)
+
+        def _patch_random_randrange(*args, **kwargs):
+            return token
+
+        with test.support.swap_attr(
+            random, "randrange", _patch_random_randrange
+        ):
+            boundary = self.genclass._make_boundary(text=None)
+            boundary_in_part = (
+                "this goes before the boundary\n--"
+                + boundary
+                + "\nthis goes after\n"
+            )
+            msg = MIMEMultipart()
+            msg.attach(MIMEText(boundary_in_part))
+            self.genclass(self.ioclass()).flatten(msg, linesep=linesep)
+            # .0 is appended if the boundary was found.
+            self.assertEqual(msg.get_boundary(), boundary + ".0")
+
+    def test_lf_boundary_detection(self):
+        self._test_boundary_detection("\n")
+
+    def test_crlf_boundary_detection(self):
+        self._test_boundary_detection("\r\n")
+
 
 class TestGenerator(TestGeneratorBase, TestEmailBase):
 
diff --git a/Misc/NEWS.d/next/Library/2026-04-07-14-13-40.gh-issue-148192.34AUYQ.rst b/Misc/NEWS.d/next/Library/2026-04-07-14-13-40.gh-issue-148192.34AUYQ.rst
new file mode 100644
index 00000000000000..c736dfd311b2fd
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-04-07-14-13-40.gh-issue-148192.34AUYQ.rst
@@ -0,0 +1,2 @@
+:func:`email.generator.Generator._make_boundary` now correctly finds the
+boundary when using CRLF linesep.

From 540b8d91fc2ec5c54dc9af9f8aab3ed1c2532ae5 Mon Sep 17 00:00:00 2001
From: Henry Jones <henryivesjones@gmail.com>
Date: Tue, 7 Apr 2026 14:26:53 +1200
Subject: [PATCH 2/2] remove :func: on blurb

---
 .../next/Library/2026-04-07-14-13-40.gh-issue-148192.34AUYQ.rst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Misc/NEWS.d/next/Library/2026-04-07-14-13-40.gh-issue-148192.34AUYQ.rst b/Misc/NEWS.d/next/Library/2026-04-07-14-13-40.gh-issue-148192.34AUYQ.rst
index c736dfd311b2fd..7c5f4ad37ad6a2 100644
--- a/Misc/NEWS.d/next/Library/2026-04-07-14-13-40.gh-issue-148192.34AUYQ.rst
+++ b/Misc/NEWS.d/next/Library/2026-04-07-14-13-40.gh-issue-148192.34AUYQ.rst
@@ -1,2 +1,2 @@
-:func:`email.generator.Generator._make_boundary` now correctly finds the
+``email.generator.Generator._make_boundary`` now correctly finds the
 boundary when using CRLF linesep.