gistfile1.rb

query = "param1=1&param2=2&offset=3"

unless query.empty?
  # build an Hash based on the query string
  # we want to build {"param1"=>"1", "param2"=>"2", "offset"=>"3"}
  temp_params = {}
  query.split("&").each {|v| temp_params.merge!(Hash[*v.split("=")]) unless (v.split("=").count < 2)}
  # remove the offset parameter 
  temp_params.delete("offset")
  # convert the Hash back to a String
  new_query = temp_params.map{|k,v| "#{k}=#{v}"}.join("&")
end

Hum, tens razão, parti do principio que estarias a usar o Rails.