How to make a Ruby string safe for a filesystem?

From http://web.archive.org/web/20110529023841/http://devblog.muziboo.com/2008/06/17/attachment-fu-sanitize-filename-regex-and-unicode-gotcha/:

def sanitize_filename(filename)
  returning filename.strip do |name|
   # NOTE: File.basename doesn't work right with Windows paths on Unix
   # get only the filename, not the whole path
   name.gsub!(/^.*(\\|\/)/, '')

   # Strip out the non-ascii character
   name.gsub!(/[^0-9A-Za-z.\-]/, '_')
  end
end

In Rails you might also be able to use ActiveStorage::Filename#sanitized:

ActiveStorage::Filename.new("foo:bar.jpg").sanitized # => "foo-bar.jpg"
ActiveStorage::Filename.new("foo/bar.jpg").sanitized # => "foo-bar.jpg"

For Rails I found myself wanting to keep any file extensions but using parameterize for the remainder of the characters:

filename = "my§doc$is°° very&itng___thsIs%nie445.doc.pdf"
cleaned = filename.split(".").map(&:parameterize).join(".")

Implementation details and ideas see source: https://github.com/rails/rails/blob/master/activesupport/lib/active_support/inflector/transliterate.rb

def parameterize(string, separator: "-", preserve_case: false)
  # Turn unwanted chars into the separator.
  parameterized_string.gsub!(/[^a-z0-9\-_]+/i, separator)
  #... some more stuff
end

If your goal is just to generate a filename that is "safe" to use on all operating systems (and not to remove any and all non-ASCII characters), then I would recommend the zaru gem. It doesn't do everything the original question specifies, but the filename produced should be safe to use (and still keep any filename-safe unicode characters untouched):

Zaru.sanitize! "  what\ēver//wëird:user:înput:"
# => "whatēverwëirduserînput"
Zaru.sanitize! "my§docu*ment$is°°   very&interes:ting___thisIs%nice445.doc.pdf" 
# => "my§document$is°° very&interesting___thisIs%nice445.doc.pdf"