diff --git a/src/calibre/gui2/markdown_syntax_highlighter.py b/src/calibre/gui2/markdown_syntax_highlighter.py
index 1eae02e57e..118365550e 100644
--- a/src/calibre/gui2/markdown_syntax_highlighter.py
+++ b/src/calibre/gui2/markdown_syntax_highlighter.py
@@ -14,7 +14,7 @@ from calibre.gui2.palette import dark_link_color, light_link_color
 class MarkdownHighlighter(QSyntaxHighlighter):
 
     MARKDOWN_KEYS_REGEX = {
-        'Bold' : re.compile('(?P<delim>\*\*)(?P<text>.+)(?P=delim)'),
+        'Bold' : re.compile(r'(?P<delim>\*\*)(?P<text>.+)(?P=delim)'),
         'uBold': re.compile('(?P<delim>__)(?P<text>[^_]{2,})(?P=delim)'),
         'Italic': re.compile('(?P<delim>\*)(?P<text>[^*]{2,})(?P=delim)'),
         'uItalic': re.compile('(?P<delim>_)(?P<text>[^_]+)(?P=delim)'),