diff --git a/sphinx/ext/doctest.py b/sphinx/ext/doctest.py
index 6c0327fab02..7e6ce75a6ac 100644
--- a/sphinx/ext/doctest.py
+++ b/sphinx/ext/doctest.py
@@ -38,7 +38,7 @@
 logger = logging.getLogger(__name__)
 
 blankline_re = re.compile(r'^\s*<BLANKLINE>', re.MULTILINE)
-doctestopt_re = re.compile(r'#\s*doctest:.+$', re.MULTILINE)
+doctestopt_re = re.compile(r'\s*#\s*doctest:.+$', re.MULTILINE)
 
 
 def is_allowed_version(spec: str, version: str) -> bool:
diff --git a/tests/test_transforms/test_transforms_post_transforms_code.py b/tests/test_transforms/test_transforms_post_transforms_code.py
index a4243fb85bc..381f4ac5980 100644
--- a/tests/test_transforms/test_transforms_post_transforms_code.py
+++ b/tests/test_transforms/test_transforms_post_transforms_code.py
@@ -15,6 +15,7 @@ def test_trim_doctest_flags_html(app):
     assert 'QUUX' not in result
     assert 'CORGE' not in result
     assert 'GRAULT' in result
+    assert '<span class="n">now</span><span class="p">()</span>   \n' not in result
 
 
 @pytest.mark.sphinx(