正则环视

环视结构不匹配任何字符，只匹配文本中的特定位置。

分类

顺序环视：顺序（从左到右）查看文本
逆序环视：逆序（从右到左）查看文本
肯定环视：匹配成功则表示命中
否定环视：匹配失败则表示命中

• **(?=)**肯定顺序环视：子表达式能够匹配右侧文本
• **(?!)**否定顺序环视：子表达式不能匹配右侧文本
• **(?<=)**肯定逆序环视：子表达式能够匹配左侧文本
• **(?<!)**否定逆序环视：子表达式不能匹配左侧文本

示例

1）肯定顺序环视

"Total dollors: 123".replace(/(?=123)/g, "$");
// 'Total dollors: $123'

2)肯定逆序环视

"Total dollors: 123".replace(/(?<=123)/g, "$");
("Total dollors: 123$");

3)否定顺序环视

"Total dollors: 123".replace(/(?!123)/g, "$");
// '$T$o$t$a$l$ $d$o$l$l$o$r$s$:$ 1$2$3$''

4)否定逆序环视

"Total dollors: 123".replace(/(?<!123)/g, "$");
// '$T$o$t$a$l$ $d$o$l$l$o$r$s$:$ $1$2$3'

1. 数字格式化’,’表达

let num = "12345678";

num.replace(/(?=(\d\d\d))/g, ",");
// ',1,2,3,4,5,678'

num.replace(/(?=(\d\d\d)(?!\d))/g, ",");
// '12345,678'

num.replace(/(?=(\d\d\d)+$)/g, ",");
// '12,345,678'

num = "The population of 12345678 is growing";

num.replace(/(?=(\d\d\d)+$)/g, ",");
// 'The population of 12345678 is growing'

num.replace(/(?=(\d\d\d)+(?!\d))/g, ",");
// 'The population of 12,345,678 is growing'

num = "This is 2020s";
num.replace(/(?=(\d\d\d)+(?!\d))/g, ",");
// 'This is 2,020s'