How to delete remote refs from git matching a filter?

While moving from GitLab to GitHub, after I executed git push --mirror <URL>, I noticed that git ls-remote --refs (in the target repository on GitHub) displayed some refs starting with the refs/merge-requests/ prefix.

Since they are irrelevant to GitHub (they are only used by GitLab),
I want to delete them; how can I do it?

In other words, how can I delete refs from a remote matching a search/filter criterion?

Solution

You could use this one-liner shell script to do so (you can use this on Windows, too, e.g. using Git Bash):

git ls-remote --refs             \
    | grep refs/merge-requests/  \
    | cut -f 2                   \
    | xargs -I {} git push --delete origin {}

Summary:

List remote refs
Include only the `merge-requests refs from the list
Extract only ref names
Execute git push --delete origin on each matching ref to delete them from remote.

Notes:

A drawback of this solution is that this deletes refs one by one, so it can take some time to delete hundreds of remote refs
Modify the script accordingly if your remote's name is not origin (though it probably is)