[snowman-20]

[1]And now it’s all this

I just said what I said and it was wrong
Or was taken wrong

[2]Next post [3]Previous post

[4]Tidying Markdown reference links

September 17, 2012 at 9:15 PM by Dr. Drang

Oscar Wilde—who would have been great on Twitter—[5]said “I couldn’t help it. I
can resist everything except temptation.” That’s my excuse for this post.

Several days ago I got an email from a reader, asking if I knew of a script
that would tidy up [6]Markdown reference links in a document. She wanted them
reordered and renumbered at the end of the document to match the order in which
they appear in the body of the text. I didn’t know of one^[7]1 and suggested
she write it herself and let me know when it’s done. I’ve been getting progress
reports, but her script isn’t finished yet.

There’s certainly no need to tidy the links up that way. Markdown doesn’t care
what order the reference links appear in or the labels that are assigned to
them. I’ve written dozens of posts in which the order of the references at the
end of the Markdown source were way off from the order of the links in body.
But…

But there is an attraction to putting everything in apple pie order, even when
no one but me will ever see it. Last night I succumbed and wrote a script to
tidy up the links. Sorry, Phaedra.

Here’s an example of a short Markdown document with out-of-order reference
links:

Species and their hybrids, How simply are these facts! How
strange that the pollen of each But we may thus have
[succeeded][2] in selecting so many exceptions to this rule.
but the species would not all the same species living on the
White Mountains, in the arctic regions of that large island.
The exceptions which are now large, and triumphant, and
which are known to every naturalist: scarcely a single
[character][4] in the descendants of the Glacial period,
would have been of use to the plants, have been accumulated
and if, in both regions.

Supposed to be extinct and unknown, form. We have seen that
it yields readily, when subjected as [under confinement][3],
to new and improved varieties will have been much
compressed, we may assume that the species, which are
already present in the ordinary spines serve as a prehensile
or snapping apparatus. Thus every gradation, from animals
with true lungs are descended from a marsupial form), "and
if so, there can be followed by which viscid matter, such as
that of making [slaves][1]. Let it be remembered that
selection may be extended--to the stigma of.

[1]: http://daringfireball.net/markdown/
[2]: http://www.google.com/
[3]: http://docs.python.org/library/index.html
[4]: http://www.kungfugrippe.com/

Note that the references are numbered 1, 2, 3, 4 at the bottom of the document,
but that they appear in the body in the order 2, 4, 3, 1. The purpose of the
script is to change the document to

Species and their hybrids, How simply are these facts! How
strange that the pollen of each But we may thus have
[succeeded][1] in selecting so many exceptions to this rule.
but the species would not all the same species living on the
White Mountains, in the arctic regions of that large island.
The exceptions which are now large, and triumphant, and
which are known to every naturalist: scarcely a single
[character][2] in the descendants of the Glacial period,
would have been of use to the plants, have been accumulated
and if, in both regions.

Supposed to be extinct and unknown, form. We have seen that
it yields readily, when subjected as [under confinement][3],
to new and improved varieties will have been much
compressed, we may assume that the species, which are
already present in the ordinary spines serve as a prehensile
or snapping apparatus. Thus every gradation, from animals
with true lungs are descended from a marsupial form), "and
if so, there can be followed by which viscid matter, such as
that of making [slaves][4]. Let it be remembered that
selection may be extended--to the stigma of.


[1]: http://www.google.com/
[2]: http://docs.python.org/library/index.html
[3]: http://www.kungfugrippe.com/
[4]: http://daringfireball.net/markdown/

Now the links are numbered 1, 2, 3, 4 in both the text and the end references.
The HTML produced when this document is run through a Markdown processor will
be the same as the previous one—the links will still go to the right places—but
the Markdown source looks better.

Here’s the script that does it:

python:
 1:  #!/usr/bin/python
 2:
 3:  import sys
 4:  import re
 5:
 6:  '''Read a Markdown file via standard input and tidy its
 7:  reference links. The reference links will be numbered in
 8:  the order they appear in the text and placed at the bottom
 9:  of the file.'''
10:
11:  # The regex for finding reference links in the text. Don't find
12:  # footnotes by mistake.
13:  link = re.compile(r'\[([^\]]+)\]\[([^^\]]+)\]')
14:
15:  # The regex for finding the label. Again, don't find footnotes
16:  # by mistake.
17:  label = re.compile(r'^\[([^^\]]+)\]:\s+(.+)$', re.MULTILINE)
18:
19:  def refrepl(m):
20:    'Rewrite reference links with the reordered link numbers.'
21:    return '[%s][%d]' % (m.group(1), order.index(m.group(2)) + 1)
22:
23:  # Read in the file and find all the links and references.
24:  text = sys.stdin.read()
25:  links = link.findall(text)
26:  labels = dict(label.findall(text))
27:
28:  # Determine the order of the links in the text. If a link is used
29:  # more than once, its order is its first position.
30:  order = []
31:  for i in links:
32:    if order.count(i[1]) == 0:
33:      order.append(i[1])
34:
35:  # Make a list of the references in order of appearance.
36:  newlabels = [ '[%d]: %s' % (i + 1, labels[j]) for (i, j) in enumerate(order) ]
37:
38:  # Remove the old references and put the new ones at the end of the text.
39:  text = label.sub('', text).rstrip() + '\n'*3 + '\n'.join(newlabels)
40:
41:  # Rewrite the links with the new reference numbers.
42:  text = link.sub(refrepl, text)
43:
44:  print text

The regular expressions in Lines 13 and 17 are fairly easy to understand. The
first one looks for the links in the body of the text and the second looks for
the labels.

The key to the script are the four data structures: links, labels, order, and
newlabels. For our example document, links is the list of tuples

[('succeeded', '2'),
 ('single character', '4'),
 ('under confinement', '3'),
 ('slaves', '1')]

labels is the dictionary

{'1': 'http://daringfireball.net/markdown/',
 '3': 'http://docs.python.org/library/index.html',
 '2': 'http://www.google.com/',
 '4': 'http://www.kungfugrippe.com/'}

order is the list

['2', '4', '3', '1']

and newlabels is the list of strings

['[1]: http://www.google.com/',
 '[2]: http://docs.python.org/library/index.html',
 '[3]: http://www.kungfugrippe.com/',
 '[4]: http://daringfireball.net/markdown/']

links and labels are built via the regex findall method in Lines 25-26. links
is the direct output of the method and maintains the order in which the links
appear in the text. labels is that same output, but converted to a dictionary.
Its order, which we don’t care about, is lost in the conversion, but it can be
used to easily access the URL from the link label.

order is the order in which the link labels first appear in the text. The if
statement in Line 32 ensures that repeated links don’t overwrite each other.

newlabels is built from labels and order in Line 36. It’s the list of labels
after the renumbering. Line 39 deletes the original label lines and puts the
new ones at the end of the document.

Finally, Line 42 replaces all the link labels in the body of the text with the
new values. Rather than a replacement string, it uses a simple replacement
function defined in Lines 19-21 to do so.

Barring any bugs I haven’t found yet, this script (or filter) will work on any
Markdown document and can be used either directly from the command line or
through whatever system your text editor uses to call external scripts. I have
it stored in BBEdit’s Text Filters folder under the name “Tidy Markdown
Reference Links.py,” so I can call it from the Text ‣ Apply Text Filter
submenu.

I should mention that although this script is fairly compact and simple, it
didn’t spring from my head fully formed. There were starts and stops as I
figured out which data structures were needed and how they could be built. Each
little subsection of the script was tested as I went along. The order list was
originally a list of tuples; it wasn’t until I had a working version of the
entire script that I realized that it could be simplified down to a list of
link labels. That change shortened the script by five lines or so and, more
importantly, clarified its logic.

Despite these improvements, the script is hardly foolproof. The Markdown source
of this very post confuses the hell out it. Not only does it think there are
links in the sample document (which you’d probably guess), it also thinks the
[%s][%d] in Line 21 of the script is a link (and the one in this sentence,
too). And why wouldn’t it? To distinguish between real links and things that
look like links in embedded source code, the script would have to be able to
parse Markdown, not just match a couple of short regular expressions. This is a
variant on what Hamish Sanderson said in the comments on [8]an earlier post.

At the moment, I’m not willing to sacrifice the simplicity of the Tidy script
to get it to handle weird posts like this one. But if I find that it fails
often with the kind of input I commonly give it, I’ll have to revisit that
decision.

As Wilde also said, “Experience is the name everyone gives to their mistakes.”

━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

 1. I didn’t think [9]Seth Brown’s formd did that, but [10]this tweet from
    Brett Terpsta says I was wrong about that. [11]↩

[12]Next post [13]Previous post

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References:

[1] https://leancrew.com/all-this/
[2] https://leancrew.com/all-this/2012/09/some-kind-of-druid-dudes-lifting-the-veil/
[3] https://leancrew.com/all-this/2012/09/implementing-pubsubhubbub/
[4] https://leancrew.com/all-this/2012/09/tidying-markdown-reference-links/
[5] http://www.gutenberg.org/dirs/etext97/lwfan10h.htm
[6] http://daringfireball.net/projects/markdown/syntax#link
[7] https://leancrew.com/all-this/2012/09/tidying-markdown-reference-links/#fn:formd
[8] http://www.leancrew.com/all-this/2012/09/applescript-syntax-highlighting-finally/
[9] http://www.drbunsen.org/formd-a-markdown-formatting-tool.html
[10] https://twitter.com/ttscoff/status/247398632377184256
[11] https://leancrew.com/all-this/2012/09/tidying-markdown-reference-links/#fnref:formd
[12] https://leancrew.com/all-this/2012/09/some-kind-of-druid-dudes-lifting-the-veil/
[13] https://leancrew.com/all-this/2012/09/implementing-pubsubhubbub/
[23] https://leancrew.com/all-this/archive/
[24] https://leancrew.com/all-this/feed/
[25] https://leancrew.com/all-this/feed.json
[26] https://fosstodon.org/@drdrang
[27] http://github.com/drdrang
[28] http://www.mathjax.org/
[29] http://creativecommons.org/licenses/by-sa/3.0/
