fmfm
(ruby 2.1.0p0 (2013-12-25 revision 44422) [x86_64-darwin12.0]使ってます。)
irb(main):054:0> /^[0-9A-Za-z]*$/.match("\naaa\nあああ")
=> #<MatchData "">
irb(main):055:0> /^[0-9A-Za-z]*$/.match("aaa\nあああ")
=> #<MatchData "aaa">
irb(main):057:0> /\A[0-9A-Za-z]*$/.match("\naaa\nあああ")
=> #<MatchData "">
irb(main):058:0> /\A[0-9A-Za-z]*$/.match("aaa\nあああ")
=> #<MatchData "aaa">
irb(main):059:0> /\A[0-9A-Za-z]*z/.match("aaa\nあああ")
=> nil
