When I try to use sed for find edit in multiple files, always I remember that perl -pe is better suited for this task. Today this happened again. I tried to find and replace lines starting with # Bla bla with title: Bla bla and it was easier to use perl -pe 's|^#+ (.*)|title: $1|g than identifying what kind of regular expressions does sed use.

For Hugo front matter at the beginning of files, it's possible to determine type but not possible to set the section. The latter is the top level directory after content that contains the file.