allow whitespace before code attributes

fixes #29

notedown/notedown.py

@@ -72,9 +72,10 @@ class MarkdownReader(NotebookReader):
     fenced_regex = r"""
     ^(?P<raw>
     (?P<fence>`{3,}|~{3,})  # a line starting with a fence of 3 or more ` or ~
-    (?P<attributes>.*)      # followed by the group 'attributes'
-    \n                      # followed by a newline
-    (?P<content>            # start a group 'content'
+    [ \t]*                  # followed by any amount of whitespace,
+    (?P<attributes>.*)      # the group 'attributes',
+    \n                      # a newline,
+    (?P<content>            # the 'content' group,
     [\s\S]*?)               # that includes anything
     \n(?P=fence)$\n)        # up until the same fence that we started with
     """

tests.py

@@ -173,6 +173,23 @@
 ```
 """
 
+attribute_markdown = u"""Attribute test
+
+```lang
+code1
+```
+
+```{.attr}
+code2
+```
+
+```  {.attr}
+code3
+```
+"""
+
+ref_attributes = ['lang', r'{.attr}', r'{.attr}']
+
 
 def create_json_notebook(markdown):
     reader = notedown.MarkdownReader()
@@ -280,6 +297,14 @@ def test_format_agnostic():
     assert(fenced_markdown_cells == indented_markdown_cells)
 
 
+def test_attributes():
+    """Are code block attributes correctly parsed?"""
+    cells = parse_cells(attribute_markdown)
+    attributes = [cell['attributes'] for cell in cells if cell['type'] == 'code']
+    for attr, ref in zip(attributes, ref_attributes):
+        assert attr == ref
+
+
 def test_pre_process_text():
     """test the stripping of blank lines"""
     block = {}