This took me a while to get right, so let me share it. Below unix command can be used to replace the markdown links to HTML links in a text file:
sed -i -e 's/\[\([^\]*\)\](\([^)]*\))/<a href="\2">\1<\/a>/g' RELEASE-NOTES.md Tricky points were that sed does not recognize the non-greedy qualifier, so you need to do [^x]* to match _up to_ the next `x`. Also round brackets are taken literally and capturing group brackets need to be escaped (the opposite from what I expected).
