javascript regexp negative lookbehind doesn't match in some case

Question

i use node 10.15

i want the expression to match only line2 but not line 1 and line 3

why this negative lookbehind failed?

(?<!月)\d+日(?!后)

Answer 1

在负数后面添加一个数字：

(?<!月|\d)\d+日(?!后)

Answer 2

It's fairly easy. You only have to guarantee, that the match is not preceded by a digit.

(?<!月|\d)\d+日(?!后)
------^^^

This adds contrast to your RegEx. See RexEgg for details

See here for online example

Answer 3

As you are using Negative look behind, you should be on EcmaScript2018.

You should put a word boundary \\b before \\d in your regex to avoid it matching partially and then it will stop matching your first line. Try this regex,

(?<!月)\b\d+日(?!后)

Demo