Convert Inline Footnotes to Regular Footnotes in Markdown

Yihui Xie 2023-02-23

After using Hugo 0.25.1 for my personal website for nearly six years, I have finally decided to upgrade it. I’ll explain the reasons later. One problem I encountered was that I had to convert inline footnotes in Markdown to regular footnotes (because Hugo 0.25.1 supported the former but Hugo no longer supported it since a certain later version). That is, I need to convert

text^[this is a footnote]

to

text[^1]

[^1]: this is a footnote

I could use Pandoc to do it, but since I have a few thousand posts, I don’t want to let them all go through a Pandoc conversion, to avoid accidental breakage. Instead, I just wrote a little R script to help:

files = list.files('.', '[.]R?md$', recursive = TRUE, full.names = TRUE)
for (f in files) {
  x = xfun::read_utf8(f)
  r = '\\^\\[([^]]+)\\]'  # eyes bleeding...
  if (length(grep(r, x)) == 0) next
  fn = NULL
  m = gregexpr(r, x)
  regmatches(x, m) = lapply(regmatches(x, m), function(z) {
    if ((n <- length(z)) == 0) return(z)
    z2 = sprintf('[^%d]', length(fn) + seq_len(n))
    fn <<- c(fn, gsub(r, '\\1', z))
    z2
  })
  fn = sprintf('[^%d]: %s', seq_along(fn), fn)
  x = c(x, rbind('', fn))
  xfun::write_utf8(x, f)
}

Basically, it reads a .md (or .Rmd) file, detects all footnotes of the form ^[text] in the text, replaces them with [^n] (where n is the footnote number), appends new footnotes [^n]: text to the text, and writes out to the original file. Since my files are under version control, I can manually inspect the GIT changes to confirm the script did what I expected.

It did help me a lot, and I only had one special case—footnotes that contain ], e.g., ^[this is a [footnote](url)]. Fortunately, I didn’t have too many posts that had this type of footnote, so I manually corrected them.

Why upgrade Hugo? I didn’t really have strong reasons. One reason is that I’m tired of having to use .Scratch due to the old version of Hugo. Another reason is that as the maintainer of the blogdown package, I want to keep an eye on newer versions of Hugo. For normal users, usually I don’t recommend upgrading Hugo, as long as a certain version works fine for you.