diff --git a/sphinx/builders/epub3.py b/sphinx/builders/epub3.py
index 44b2cb10e16..d60c9dd132c 100644
--- a/sphinx/builders/epub3.py
+++ b/sphinx/builders/epub3.py
@@ -65,7 +65,7 @@ class NavPoint(NamedTuple):
     '|[\ufdf0-\ufffd]|[\U00010000-\U000effff]'
 )
 _xml_name_char = (
-    _xml_name_start_char + r'\-|\.' '|[0-9]|\u00b7|[\u0300-\u036f]|[\u203f-\u2040]'
+    _xml_name_start_char + r'\-|\.|[0-9]|\u00b7|[\u0300-\u036f]|[\u203f-\u2040]'
 )
 _XML_NAME_PATTERN = re.compile(f'({_xml_name_start_char})({_xml_name_char})*')