Goal: replace ![alttext](url.png) with <img src="url.png" alt="alttext">.

The ! and (...) parts work fine, it's the [...] part that I can't get to work.

As a simple example, I want to match the input a [test] b. This is the command I use:

echo 'a [test] b' | sed 's/\[[^\]]*\]/x/'

It doesn't work. When I replace the square brackets with angle brackets, it works just fine:

echo 'a <test> b' | sed 's/<[^>]*>/x/'

Why does sed not understand the same regex but with escaped square brackets? It works fine in other tools such as Python.

  • It's a complex task to parse markdown. Here's the commonmark spec for reference. Commented Feb 20, 2019 at 15:15
  • @glennjackman Thanks for the pointer, but I'm not trying to create a complete markdown parser. I just wanted a small shell script which replaces h1-h3, links, images, bold, italics, and paragraphs. That's just a few sed rules and makes a webpage readable so someone can semi-preview it without having to deploy it to the markdown-based CMS system.
    – Luc
    Commented Feb 20, 2019 at 16:40
  • IMO, it would be easier to just use a markdown tool to compile the markdown text into html, instead of rolling your own semi-solution. Commented Feb 20, 2019 at 18:06

1 Answer 1


Within the character class definition ] is taken literally when directly following the ^. So don't escape this. Also you are missing a quantifier like +, to say "one or more characters" or * to say "zero or more characters". Furthermore there is one sed too many in your command line.

$ echo 'a [test] b' | sed 's/\[[^]]\+\]/x/'
a x b

(\+ being a GNU extension (now also supported by a few other implementations), the standard equivalent is \{1,\})

or more readable:

$ echo 'a [test] b' | sed -E 's:\[[^]]+\]:x:'
a x b

(-E being a BSD extension, now also supported by GNU sed and some other implementations; use -r in older versions of GNU sed).

  • Ugh, the issue with hastily written questions when others are waiting for me to go to lunch: I only have one sed in the real command and the missing * is a typo as well. I'll edit the question to not confuse future readers, you might want to edit the answer. Not escaping the ^] indeed works, thanks finswimmer!
    – Luc
    Commented Feb 20, 2019 at 12:29

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