# File ext/nkf/lib/kconv.rb, line 173
  def isjis(str)
    /\A [\t\n\r\x20-\x7E]*
      (?:
        (?:\x1b \x28 I      [\x21-\x7E]*
          |\x1b \x28 J      [\x21-\x7E]*
          |\x1b \x24 @      (?:[\x21-\x7E]{2})*
          |\x1b \x24 B      (?:[\x21-\x7E]{2})*
          |\x1b \x24 \x28 D (?:[\x21-\x7E]{2})*
        )*
        \x1b \x28 B [\t\n\r\x20-\x7E]*
      )*
     \z/nox =~ str.dup.force_encoding('BINARY') ? true : false
  end