How to remove markdown links from headers with sed?

Question

I'm trying to use sed to remove links like and leave just the title:

## [Some title](#some-title)

This is my command:

sed 's/^\(\#*\) *\[\([^\]]*\)\].*/\1 \2/'

I expect to have just the text without the link:

## Some title

But it doesn't work. What I do wrong?

I'm using Linux with GNU sed.

Answer 1

Based on what you've told us so far (I'm trying to ... leave just the title:) and the sample input you provided (## [Some title](#some-title)) this might be what you're trying to do, using any awk:

$ awk -F'[][]' '{print $2}' file
Some title

or any sed:

$ sed 's/.*\[\([^]]*\)].*/\1/' file
Some title

but without more truly representative sample input and expected output that's just a guess.

As for what's wrong with your sed script:

sed -i 's/^\(\#*\) *\[\([^\]]*\)\].*/\1 \2/'

Using -i like that will do "inplace" editing in GNU sed but in other sed versions, even BSD sed which also supports inplace editing but requires a backup file name, it'll do different things so you don't tell us what problem you're experiencing when running your script but maybe that's it?

Beyond that, in the first regexp segment \(\#*\):

1. You're escaping the literal char # as \# which is undefined behavior per POSIX when you wanted just #.
2. You're using #* which matches zero or more #s when you wanted 1 or more which is ##* or #\{1,\} in a BRE as sed uses by default (or #+ if you were using an ERE).

In the separating spaces part <blank>*:

1. You're using <blank>* which matches zero or more <blank>s when you wanted 1 or more which is <blank><blank>* or <blank>\{1,\} in a BRE (or <blank>+ if you were using an ERE).

In the last regexp segment \[\([^\]]*\)\].*:

1. You're using [^\]] and so escaping ] which is undefined behavior per POSIX when you wanted just [^]].
2. You're using \] at the end which is undefined behavior per POSIX since there's no unescaped [ before it when you wanted just ].

If you fixed all of those issues you'd get:

$ sed 's/^\(##*\)  *\[\([^]]*\)].*/\1 \2/' file
## Some title

or

$ sed 's/^\(#\{1,\}\) \{1,\}\[\([^]]*\)].*/\1 \2/' file
## Some title

and since you're using GNU sed which supports EREs you could write that as:

$ sed -E 's/^(#+) +\[([^]]*)].*/\1 \2/' file
## Some title

And then to leave just the title as you said you wanted just means removing the first capture group:

$ sed 's/^##*  *\[\([^]]*\)].*/\1/' file
Some title

$ sed 's/^#\{1,\} \{1,\}\[\([^]]*\)].*/\1/' file
Some title

$ sed -E 's/^#+ +\[([^]]*)].*/\1/' file
Some title

Answer 2

It looks like this pattern [^\]] doesn't work in sed.

This seems to work:

sed 's/^\(#*\) \[\(.*\)\].*/\1 \2/'

Answer 3

Writing a pandoc filter can handle the most general version of this problem:

Remove any link within any level of heading.

Headers can differ by depth and style, their contents can be formatted, and header-like strings can appear in comments and code blocks. So, for example, your markdown file could be like this:

## [Some title](#some-title)
Some text here

Another *header [with a `link`](https://www.konami.com/yugioh/)*
------------
wow!

# What if [a header link](#like-this) appears outside of code?

    # What if [a header link](#like-this) appears in code?

<!--
# a [header link](#keep-me) that should not be altered
because it's commented out -->

Pandoc knows all about these cases. I don’t want to have to think about them. I just want to say “if you find a link somewhere in a heading, get rid of it.” That’s a filter.

Here’s a pandoc filter (Haskell version)

Based on the behead.hs example in the documentation:

#!/usr/bin/env runhaskell
-- removeheaderlinks.hs
import Text.Pandoc.JSON
import Text.Pandoc.Walk

main :: IO ()
main = toJSONFilter removeheaderlinks

-- if this Inline is a link, remove the link but keep the attributes
removelink :: Inline -> Inline
removelink (Link at xs _) = Span at xs
removelink x = x

-- remove all links if the block is a header
removeheaderlinks :: Block -> Block
removeheaderlinks (Header n attr content) = Header n attr $ walk removelink content
removeheaderlinks x = x

You need to have haskell installed, as well as pandoc-types, so run cabal v2-update && cabal v2-install --lib pandoc-types --package-env . first.

Then run this to convert:

pandoc -f markdown -t markdown --filter removeheaderlinks.hs ./example.md

Result:

## Some title

Some text here

## Another *header with a `link`*

wow!

# What if a header link appears outside of code?

    # What if [a header link](#like-this) appears in code?

<!--
# a [header link](#keep-me) that should not be altered
because it's commented out -->