正则表达式入门（八）环视

环视是一种非捕获分组，它根据某个模式之前或之后的内容匹配其他模式。环视也称为零宽度断言。

环视包括：

• 正前瞻
• 反前瞻
• 正后顾
• 反后顾

THE RIME OF THE ANCYENT MARINERE, IN SEVEN PARTS.

ARGUMENT.

How a Ship having passed the Line was driven by Storms to the cold
Country towards the South Pole; and how from thence she made her course
to the tropical Latitude of the Great Pacific Ocean; and of the strange
things that befell; and in what manner the Ancyent Marinere came back to
his own Country.

I.

1      It is an ancyent Marinere,
2        And he stoppeth one of three:
3      "By thy long grey beard and thy glittering eye
4 "Now wherefore stoppest me?

正前瞻

假设要匹配单词ancyent，且要求紧跟的单词是marinere，可以使用正前瞻。

(?i)ancyent(?=marinere)

(?i)不区分大小写，所以现在就是在每一行中寻找后跟marinere的单词ancyent。

反前瞻

反前瞻是正前瞻的取反操作。说明要在需要匹配的模式后找不到给定的前瞻内容。

(?i)ancyent(?!marinere)

这里，正前瞻的=变成了反前瞻的!。

正后顾

正后顾会查看前面的内容，和正前瞻的方向相反。

(?i)(?<=ancyent)marinere

反后顾

(?i)(?<!ancyent)marinere