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

    Ȥ߹ѿ $_ λؤʸ pattern 
    ޥåʬ replace ֤ʸ֤ޤ
     replace ά줿ˤϥƥ졼Ȥư֥
    ɾִ̤Ԥޤ֥åˤϰȤƥޥå
    ʸϤޤ

       $_ = "foobar"
       p gsub(/o+/) {|m|
         m.upcase
       }
       # => "fOObar"

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

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

