Jeroen Heijmans asked:
> Hi, I'm looking for a function that can do the same thing as the strtr
> function in PHP (and possibly other languages, I don't know). That is, I
> would like to replace multiple parts of a strings simultaneously, so
> that you can (among others) swap strings:
>
> strtr("AA BB", array("AA" => "BB", "BB" => "AA")); // "BB AA"
>
> I've been trying to find a Ruby equivalent, but haven't so far. The tr
> functions in String only deal with single characters, and replace or
> gsub only deal with one replacement at a time.
>
> Any functions that I missed, or perhaps a library with this
> functionality available?

tr comes from Perl, where it has the meaning of PHP's three-argument version 
of strtr(), which is the same as Ruby's tr.

The two argument version, with the arrays, isn't in standard Ruby, but to 
get equivalent functionality, you'd have to do something like this:

class String
  # PHP's two argument version of strtr
  def strtr(replace_pairs)
    keys = replace_pairs.map {|a, b| a }
    values = replace_pairs.map {|a, b| b }
    self.gsub(
      /(#{keys.map{|a| Regexp.quote(a) }.join( ')|(' )})/
      ) { |match| values[keys.index(match)] }
  end
end

# Call using a hash for the replacement pairs:
"AA BB".strtr("AA" => "BB", "BB" => "AA") #=> "BB AA"

# Or use an array of pairs if order matters
"AA BB".strtr([["AA", "BB"], ["BB", "AA"]]) #=> "BB AA"

Cheers,
Dave