Filename sanitizer. Comments welcome.

filename.rb

# clean filenames for OS X, Windows and Linux
# this is for Ruby 1.8 and Rails 2.3
# requires activesupport 2.3.x

class Filename
  CHARACTER_FILTER = /[\x00-\x1F\/\\:\*\?\"<>\|]/u
  UNICODE_WHITESPACE = /[[:space:]]+/u
  
  def initialize(filename)
    @raw = filename.to_s.freeze
  end
  
  # strip whitespace on beginning and end
  # collapse intra-string whitespace into single spaces
  def normalize
    @normalized ||= @raw.mb_chars.strip.to_s.gsub(UNICODE_WHITESPACE,' ').mb_chars
  end
  
  # remove characters that aren't allowed cross-OS
  def sanitize
    @sanitized ||= normalize.to_s.gsub(CHARACTER_FILTER,'')
  end
  
  # normalize unicode string and cut off at 255 characters
  def truncate
    @truncated ||= sanitize.mb_chars.normalize.mb_chars.slice(0..254)
  end
  
  # convert back from multibyte string
  def to_s
    truncate.to_s
  end
end

Filename.new("   ühere\\ x\tis a   totally:unsafeöø.name!!ø ").to_s
# => "ühere x is a totallyunsafeöø.name!!ø"

For a laugh I thought I'd strip out almost everything and with your basic example, it still worked.. and across Ruby 1.8, 1.9, and 2.0 and without the insidious ActiveSupport too! ;-) https://gist.github.com/41cc5c231fe280120034

I think I cheated :-D However, I have to shoot off now and there are surely trivial ways to break this (Ruby 1.8's handling of unicode regexes is not my forté), but might be worth a fiddle anyway if you ever want to run on production releases of MRI.

@peterc without the mb_chars the slicing won't work on Ruby 1.8

This is now a gem! https://github.com/madrobby/zaru