Add Markdown block lexer/parser for LaTeX blocks

The block lexer/parser was splitting equations like this

$$
x
=
2
$$

So the inline lexer/parser was never seeing the whole equation, and
it wasn't getting properly rendered. This fixes such breaking by
adding a block-level lexer/parser to the LaTeX equations written as
either $$...$$ or \\[...\\]

The inline "block math" parsing code was kept as is, since the above
equation could have been part of a paragraph like "$$x = 2$$" to keep
the compatibility with Jupyter Notebook rendering engine (and because
there's a test enforcing that behavior)

nbconvert/filters/markdown_mistune.py

@@ -21,6 +21,12 @@
 from nbconvert.filters.strings import add_anchor
 
 
+class MathBlockGrammar(mistune.BlockGrammar):
+    block_math = re.compile(r"^\$\$(.*?)\$\$|^\\\\\[(.*?)\\\\\]", re.DOTALL)
+    latex_environment = re.compile(r"^\\begin\{([a-z]*\*?)\}(.*?)\\end\{\1\}",
+                                   re.DOTALL)
+
+
 class MathInlineGrammar(mistune.InlineGrammar):
     inline_math = re.compile(r"^\$(.+?)\$|^\\\\\((.+?)\\\\\)", re.DOTALL)
     block_math = re.compile(r"^\$\$(.*?)\$\$|^\\\\\[(.*?)\\\\\]", re.DOTALL)
@@ -29,6 +35,31 @@ class MathInlineGrammar(mistune.InlineGrammar):
     text = re.compile(r'^[\s\S]+?(?=[\\<!\[_*`~$]|https?://| {2,}\n|$)')
 
 
+class MathBlockLexer(mistune.BlockLexer):
+    default_rules = (['block_math', 'latex_environment']
+                     + mistune.BlockLexer.default_rules)
+
+    def __init__(self, rules=None, **kwargs):
+        if rules is None:
+            rules = MathBlockGrammar()
+        super(MathBlockLexer, self).__init__(rules, **kwargs)
+
+    def parse_block_math(self, m):
+        """Add token for a $$math$$ block"""
+        self.tokens.append({
+            'type': 'block_math',
+            'text': m.group(1) or m.group(2)
+        })
+
+    def parse_latex_environment(self, m):
+        """Add token for a \begin{.} ...LaTeX stuff... \end{.} block"""
+        self.tokens.append({
+            'type': 'latex_environment',
+            'name': m.group(1),
+            'text': m.group(2)
+        })
+
+
 class MathInlineLexer(mistune.InlineLexer):
     default_rules = (['block_math', 'inline_math', 'latex_environment']
                      + mistune.InlineLexer.default_rules)
@@ -53,8 +84,17 @@ class MarkdownWithMath(mistune.Markdown):
     def __init__(self, renderer, **kwargs):
         if 'inline' not in kwargs:
             kwargs['inline'] = MathInlineLexer
+        if 'block' not in kwargs:
+            kwargs['block'] = MathBlockLexer
         super(MarkdownWithMath, self).__init__(renderer, **kwargs)
 
+    def output_block_math(self):
+        return self.renderer.block_math(self.token["text"])
+
+    def output_latex_environment(self):
+        return self.renderer.latex_environment(self.token["name"],
+                                               self.token["text"])
+
 
 class IPythonRenderer(mistune.Renderer):
     def block_code(self, code, lang):

nbconvert/filters/tests/test_markdown.py

@@ -137,6 +137,18 @@ def test_markdown2html_math(self):
             "$$a<b&b<lt$$",
             "$$a<b&lt;b>a;a-b<0$$",
             "$$<k'>$$",
+            """$$x
+=
+2$$""",
+            r"""$$
+ b_{l_1,l_2,l_3}^{\phi\phi\omega} = -8\, l_1\times l_2 \,l_1\cdot l_2 \int_0^{\chi_*} d\chi \frac{W(\chi,\chi_*)^2}{\chi^2} \int_0^{\chi}d\chi' \frac{W(\chi',\chi)W(\chi',\chi_*)}{{\chi'}^2} \left[
+ P_\psi\left(\frac{l_1}{\chi},z(\chi)\right) P_\Psi\left(\frac{l_2}{\chi'},z(\chi')\right)
+- (l_1\leftrightarrow l_2)
+\right]
+$$""",
+            r"""\begin{equation*}
+x = 2 *55* 7
+\end{equation*}""",
             """$
 \\begin{tabular}{ l c r }
   1 & 2 & 3 \\