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In part 1, we created a Python-Markdown extension which implements a new syntax for defining <ins> and <del> tags. Now we need to alter Markdown's existing syntax for bold and italics. As a reminder, your new syntax should look like this:

Two hyphens for --delete--.
Two underscores for __insert__.
Two asterisks for **strong**.
Two slashes for //emphasis//.

First, we need to define our new regular expressions. We can just use the same expressions from last time with a few modifications.

STRONG_RE = r'(\*\*)(.*?)\*\*'
EMPH_RE = r'(\/\/)(.*?)\/\/'

Now we need to insert these into the markdown parser. However, unlike with ins and del, we need to override the existing inline patterns. A quick look at the source indicates that strong and emphasis are currently implemented with four inline patterns; "strong", "emphasis", "emphasis2" and "strong_em".

Let's override "strong" first.

class MyExtension(markdown.Extension):
    def extendMarkdown(self, md, md_globals):
        ...
        # Create new strong pattern
        strong_tag = markdown.inlinepatterns.SimpleTagPattern(STRONG_RE, 'strong')
        # Override existing strong pattern
        md.inlinepatterns['strong'] = strong_tag

Notice that rather than "add"ing a new pattern before or after an existing pattern, we simple reassigned the value of a pattern named "strong". This is because the old pattern named "strong" already existed and we don't need to change its location in the parser. So we simply assign a new pattern instance to it.

We can do the same for emphasis:

class MyExtension(markdown.Extension):
    def extendMarkdown(self, md, md_globals):
        ...
        emph_tag = markdown.inlinepatterns.SimpleTagPattern(EMPH_RE, 'emphasis')
        md.inlinepatterns['emphasis'] = emph_tag

Now we have two old patterns left, "strong_em" and "emphasis2". "Emphasis2" was just a special case so that underscored_words were not mistaken for emphasis, but as our new syntax required double underscores, it's not needed any more. Therefore, we can delete it. The same applies for strong_em. With the old syntax, due to both strong and emphasis using the same characters, a special case was needed to match the two nested together (i.e.: ***like this***). Again this isn't needed. We can delete the two in the same way we would delete dict items:

class MyExtension(markdown.Extension):
    def extendMarkdown(self, md, md_globals):
        ...
        del md.inlinepatterns['strong_em']
        del md.inlinepatterns['emphasis2']

That should do it. For completeness, the entire extension should look like this:

import markdown

DEL_RE = r'(--)(.*?)--'
INS_RE = r'(__)(.*?)__'
STRONG_RE = r'(\*\*)(.*?)\*\*'
EMPH_RE = r'(\/\/)(.*?)\/\/'

class MyExtension(markdown.Extension):
    def extendMarkdown(self, md, md_globals):
        del_tag = markdown.inlinepatterns.SimpleTagPattern(DEL_RE, 'del')
        md.inlinepatterns.add('del', del_tag, '>not_strong')
        ins_tag = markdown.inlinepatterns.SimpleTagPattern(INS_RE, 'ins')
        md.inlinepatterns.add('ins', ins_tag, '>del')
        strong_tag = markdown.inlinepatterns.SimpleTagPattern(STRONG_RE, 'strong')
        md.inlinepatterns['strong'] = strong_tag
        emph_tag = markdown.inlinepatterns.SimpleTagPattern(EMPH_RE, 'emphasis')
        md.inlinepatterns['emphasis'] = emph_tag
        del md.inlinepatterns['strong_em']
        del md.inlinepatterns['emphasis2']

def makeExtension(configs=None):
    return MyExtension(configs=configs)

In part 3 (coming soon) we'll combine all four of those patterns into one new pattern. Yes, that means we'll be writing our own InlinePattern class.