Why the positive lookahead not working for this regex [closed]

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Closed 7 days ago.

This is my regex

^(settings|issues(?=a))/$

I want to match settings/ and issuesa/. The problem is I am only able to match settings/ and not issuesa/.

回答1:

The “a” in the lookahead is not part of the match.

It would match if the regex was

^(settings|issues(?=a))a/$

(though it would not match settings/ then)

or

^(settings|issues(?=a)a)/$

(but then I don't see a point for the lookahead).

回答2:

Try it

regex = '^(settings|issues(a*))/$'

You can also look it up here how regex works: https://regex101.com/r/vQr5cP/1

回答3:

With (?=a), you require a to appear at the same location in the string as /. Since / can't be a, issues(?=a) will always fail to match.

You need to put all the right-hand boundaries into lookahead constructs:

^(settings(?=/$)|issues(?=a/$))

See the regex demo

Details

• ^ - start of string
• (settings(?=/$)|issues(?=a/$)) - Capturing group 1:
  • settings(?=/$) - settings substring that is immediately followed with / and end of string position
  • | - or
  • issues(?=a/$) - issues substring that is immediately followed with a/ and end of string position.

来源：https://stackoverflow.com/questions/59299644/why-the-positive-lookahead-not-working-for-this-regex