Escape colons in relative links e.g. MediaWiki category links

[peterjc]

Referring to https://www.mediawiki.org/wiki/Help:Categories in mediawiki,

• [[Category:Name]] --> part of category "Name" (used in some outputs from pandoc)
• [[Category:Name|Sort]] --> part of category "Name" using sort key "Sort" in place of page title
• [[:Category:Help]] --> link with text "Category:Help"
• [[:Category:Help|Help category]] link with text "Help category"

Try converting this to RST using pandoc v1.17.0.2 (Mac OS X),

$ echo "This page is in [[Category:Example]] and here is a link to the [[:Category:Help|Help category]], and an [http://example.org external site] too." | pandoc -f mediawiki -t rst
This page is in and here is a link to the `Help
category <:Category:Help>`__, and an `external
site <http://example.org>`__ too.

Or, to HTML,

$ echo "This page is in [[Category:Example]] and here is a link to the [[:Category:Help|Help category]], and an [http://example.org external site] too." | pandoc -f mediawiki -t html
<p>This page is in and here is a link to the <a href=":Category:Help" title="wikilink">Help category</a>, and an <a href="http://example.org">external site</a> too.</p>
<p><a href="Category:Example" class="uri" title="wikilink">Category:Example</a></p>

The unsupported tag [[Category:Example]] is dropped here (fine - see comment below using markdown output). The external link to http://example.org is perfect.

The bug is the URL used for the "Help Category". This is Category:Example in HTML which breaks (see below), but :Category:Example in the RST which is worse - notice Mediawiki's special syntax leading colon is still there.

Due to the special URL scheme notation, this URL should be presented as Category%3AExample where the colon is %3A, as otherwise the browser seems to interpret this as a (undefined) URL scheme (like http://example.org or https://example.org or here category:...).

[peterjc]

Pandoc does handle the category tag in markdown output, e.g.

$ echo "This page is in [[Category:Example]] and here is a link to the [[:Category:Help|Help category]], and an [http://example.org external site] too." | pandoc -f mediawiki -t markdown_github-hard_line_breaks
This page is in and here is a link to the [Help
category](:Category:Help "wikilink"), and an [external
site](http://example.org) too.

<Category:Example>

As an aside, clearly my example should not really have put the MediaWiki [[Category:Example]] mid sentence.

Note that the same URL problem is shown here too.

[peterjc]

There is an overlapping/related issue with linking to user pages, and any other MediaWiki special pages with a colon in their name, e.g.

$ echo "This was written by [[User:pjc|Peter]]." | pandoc -f mediawiki -t markdown_github-hard_line_breaks
This was written by [Peter](User:pjc "wikilink").

In order to prevent the browser trying to interpret this as a URL scheme user:, it seems simplest to escape the colon in the URL:

This was written by [Peter](User%3Apjc "wikilink").