A script I’d batch-run on my Markdown files had inserted a UTF-8 non-breaking-space between Markdown heading indicator and the text, which meant that # My title actually got rendered as that, instead of an H3 title.

Looking at the file contents, I could see it wasn’t just a space between the # and the text, but a non-breaking space.

Both od and hexdump can be used :

$ grep -e "Kit$" content/post/tools-i-use-ipad-pro.md | od -aox
0000000    #   #   #   �   �   K   i   t  nl
           021443  141043  045640  072151  000012
             2323    c223    4ba0    7469    000a
0000011

$ grep -e "Kit$" content/post/tools-i-use-ipad-pro.md | hexdump -C
00000000  23 23 23 c2 a0 4b 69 74  0a                       |###..Kit.|

So here the hash (#) character is hex 23, and the UTF8 non-breaking space c2 a0 in hex.

You can also see this with printf:

$ printf '\xC2\xA0\x23' | od -aox
0000000    �   �   #
           120302  000043
             a0c2    0023

$ printf '\xC2\xA0\x23' | hexdump -C
00000000  c2 a0 23                                          |..#|
00000003

On Linux it’s easy enough to use sed to replace the UTF-8 non-breaking-space with a plain space:

$ printf '\xC2\xA0\x23' | sed 's/\xC2\xA0/ /g' | hexdump -C
00000000  20 23                                             | #|
00000002

On the Mac though, no such luck - it just doesn’t work:

$ printf '\xC2\xA0\x23' | sed 's/\xC2\xA0/ /g' | hexdump -C
00000000  c2 a0 23 0a                                       |..#.|
00000004

Thanks to StackOverflow I found that a magic dollar sign before the sed string is all it takes:

printf '\xC2\xA0\x23' | sed $'s/\xC2\xA0/ /g' | hexdump -C
00000000  20 23 0a                                          | #.|
00000003

So now prototype the batch conversion, targeting a single file:

$ sed -i'.bak' $'s/\x23\xC2\xA0/# /g' content/post/tools-i-use-ipad-pro.md
$ grep -e "Kit$" content/post/tools-i-use-ipad-pro.md | hexdump -C
00000000  23 23 23 20 4b 69 74 0a                           |### Kit.|
00000008

Looks good; now to convert all the Markdown files:

$ sed -i'.bak' $'s/\x23\xC2\xA0/# /g' content/post/*.md

Useful reference: https://superuser.com/a/517852/66380