Kconv#isjis

# File tmp/rubies/ruby-3.3.0/ext/nkf/lib/kconv.rb, line 174
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