encoding_test.rb

a = "\x91" # Windows-1252 hex code for curly quote
puts a
puts a.valid_encoding?
puts a.force_encoding('Windows-1252')
puts a.force_encoding('Windows-1252').encode('UTF-8')

Output

irb(main):034:0> a = "\x91" # Windows-1252 hex code for curly quote
=> "\x91"
irb(main):035:0> puts a
�
=> nil
irb(main):036:0> puts a.valid_encoding?
false
=> nil
irb(main):037:0> puts a.force_encoding('Windows-1252')
�
=> nil
irb(main):038:0> puts a.force_encoding('Windows-1252').encode('UTF-8')
‘
=> nil
irb(main):039:0>