--- sub(pattern[, replace])
--- sub!(pattern[, replace])
--- sub(pattern) {|matched| ... }
--- sub!(pattern) {|matched| ... }

    ƥѿ $_ λؤʸ pattern 
    ޥåǽʬ replace ֤ʸ֤ޤ
     replace ά줿ˤϥƥ졼Ȥư֥
    ɾִ̤Ԥޤ֥åˤϰȤƥޥå
    ʸϤޤ

    sub!  $_ λؤƤʸ󤽤ΤΤ񤭴ޤ
    ܺ٤ϡString#sub [String/sub] 򻲾ȤƤ
    String#sub ȴؿ sub Ǥϰʲǰ㤤ޤ

    * sub᥽åɤ $_ ͤ򥳥ԡơԡ򹹿
      $_ ((**))ޤ

