如何构建一个正则表达式，匹配一个包含任何字符但必须包含21个逗号的任何长度的字符串？
/^([^,]*,){21}[^,]*$/
那是：
^ Start of string
( Start of group
[^,]* Any character except comma,zero or more times,A comma
){21} End and repeat the group 21 times
[^,zero or more times again
$ End of string
