Only allow '>>>' prompt without space afterwards if line is blank

Closes gh-6025

IPython/core/inputtransformer.py

@@ -453,8 +453,8 b' def _strip_prompts(prompt_re, initial_re=None):'
 def classic_prompt():
     """Strip the >>>/... prompts of the Python interactive shell."""
     # FIXME: non-capturing version (?:...) usable?
-    prompt_re = re.compile(r'^(>>> ?|\.\.\. ?)')
-    initial_re = re.compile(r'^(>>> ?)')
+    prompt_re = re.compile(r'^(>>>|\.\.\.)( |$)')
+    initial_re = re.compile(r'^>>>( |$)')
     return _strip_prompts(prompt_re, initial_re)
 
 @CoroutineInputTransformer.wrap

IPython/core/tests/test_inputsplitter.py

@@ -455,9 +455,9 b' class IPythonInputTestCase(InputSplitterTestCase):'
         isp = self.isp
         for raw, name, line, cell in [
             ("%%cellm a\nIn[1]:", u'cellm', u'a', u'In[1]:'),
-            ("%%cellm \nline\n>>>hi", u'cellm', u'', u'line\n>>>hi'),
-            (">>>%%cellm \nline\n>>>hi", u'cellm', u'', u'line\nhi'),
-            ("%%cellm \n>>>hi", u'cellm', u'', u'hi'),
+            ("%%cellm \nline\n>>> hi", u'cellm', u'', u'line\n>>> hi'),
+            (">>> %%cellm \nline\n>>> hi", u'cellm', u'', u'line\nhi'),
+            ("%%cellm \n>>> hi", u'cellm', u'', u'hi'),
             ("%%cellm \nline1\nline2", u'cellm', u'', u'line1\nline2'),
             ("%%cellm \nline1\\\\\nline2", u'cellm', u'', u'line1\\\\\nline2'),
         ]: