ruby gsub使用

str.gsub(/(\d{4})\/(\d{2})\/(\d{2})(\s\d{2}:\d{2}:\d{2})/, '"\1-\2-\3\4"')
这句话是什么意思，前面的/(\d{4})\/(\d{2})\/(\d{2})(\s\d{2}:\d{2}:\d{2})/我知道是匹配一个正则表达式串，但是后面的"\1-\2-\3\4"我不是很清楚,有知道的告诉一下