問題

irbでEncoding.default_externalの設定変更や、日本語文字を含む正規表現などを試していたあと、irbがエラーで起動できなくなった。

C:\WINDOWS\system32>irb
C:/Ruby24-x64/lib/ruby/site_ruby/rbreadline.rb:6135:in `delete': invalid byte sequence in UTF-8 (ArgumentError)
        from C:/Ruby24-x64/lib/ruby/site_ruby/rbreadline.rb:6135:in `alloc_history_entry'
        from C:/Ruby24-x64/lib/ruby/site_ruby/rbreadline.rb:6168:in `add_history'
        from C:/Ruby24-x64/lib/ruby/site_ruby/readline.rb:368:in `<<'
        from C:/Ruby24-x64/lib/ruby/2.4.0/irb/ext/save-history.rb:76:in `block (2 levels) in load_history'
        from C:/Ruby24-x64/lib/ruby/2.4.0/irb/ext/save-history.rb:76:in `each'
        from C:/Ruby24-x64/lib/ruby/2.4.0/irb/ext/save-history.rb:76:in `block in load_history'
        from C:/Ruby24-x64/lib/ruby/2.4.0/irb/ext/save-history.rb:75:in `open'
        from C:/Ruby24-x64/lib/ruby/2.4.0/irb/ext/save-history.rb:75:in `load_history'
        from C:/Ruby24-x64/lib/ruby/2.4.0/irb/ext/save-history.rb:65:in `extended'
        from C:/Ruby24-x64/lib/ruby/2.4.0/irb/ext/save-history.rb:21:in `extend'
        from C:/Ruby24-x64/lib/ruby/2.4.0/irb/ext/save-history.rb:21:in `init_save_history'
        from C:/Ruby24-x64/lib/ruby/2.4.0/irb/ext/save-history.rb:45:in `save_history='
        from C:/Ruby24-x64/lib/ruby/2.4.0/irb/context.rb:92:in `initialize'
        from C:/Ruby24-x64/lib/ruby/2.4.0/irb.rb:412:in `new'
        from C:/Ruby24-x64/lib/ruby/2.4.0/irb.rb:412:in `initialize'
        from C:/Ruby24-x64/lib/ruby/2.4.0/irb.rb:383:in `new'
        from C:/Ruby24-x64/lib/ruby/2.4.0/irb.rb:383:in `start'
        from C:/Ruby24-x64/bin/irb.cmd:19:in `<main>'

解決

~/.irb_history から、日本語文字が含まれる行を削除したら起動できるようになった。