diff --git a/markdown/extensions/attr_list.py b/markdown/extensions/attr_list.py
index 683bdf831..f42b99585 100644
--- a/markdown/extensions/attr_list.py
+++ b/markdown/extensions/attr_list.py
@@ -72,7 +72,7 @@ def isheader(elem):
 
 class AttrListTreeprocessor(Treeprocessor):
 
-    BASE_RE = r'\{\:?([^\}]*)\}'
+    BASE_RE = r'\{\:([^\}]*)\}'
     HEADER_RE = re.compile(r'[ ]+%s[ ]*$' % BASE_RE)
     BLOCK_RE = re.compile(r'\n[ ]*%s[ ]*$' % BASE_RE)
     INLINE_RE = re.compile(r'^%s' % BASE_RE)