diff --git a/src/calibre/ebooks/markdown/util.py b/src/calibre/ebooks/markdown/util.py
index 1036197c47..f07da1e111 100644
--- a/src/calibre/ebooks/markdown/util.py
+++ b/src/calibre/ebooks/markdown/util.py
@@ -36,7 +36,7 @@ STX = '\u0002'  # Use STX ("Start of text") for start-of-placeholder
 ETX = '\u0003'  # Use ETX ("End of text") for end-of-placeholder
 INLINE_PLACEHOLDER_PREFIX = STX+"klzzwxh:"
 INLINE_PLACEHOLDER = INLINE_PLACEHOLDER_PREFIX + "%s" + ETX
-INLINE_PLACEHOLDER_RE = re.compile(INLINE_PLACEHOLDER % r'([0-9]{4})')
+INLINE_PLACEHOLDER_RE = re.compile(INLINE_PLACEHOLDER % r'([0-9]+)')
 AMP_SUBSTITUTE = STX+"amp"+ETX
 
 """