JS regexp positive lookbehind no match

Question

Here's the string and the sentence I want to match (extract).

let pigs = "\X The animal 0000 I really dig \X Above all others is the pig. 222 \X Pigs are noble. Pigs are clever 3333, \X Pigs are 5555 courteous.\X However, Now and then, to break this 6666 rule, \X One meets 7777 a pig who is a fool. \X"

" Pigs are 5555 courteous."

Here's two versions of my code. When I check in on various regexp-check websites, it gives the desired match. But when I run in the browser console, it gives null. I have the latest Chrome version. Why doesn't the browser console output the match here?

pigs.match(/(?<=\\X)[^\\X]*5555[^\\X]*(?=\\X)/g);
pigs.match(/(?:\\X)[^\\X]*5555[^\\X]*(?:\\X)/g);

You need to escape the backslashes in the `pigs` string if you want them to be literal. — Barmar, Sep 16 '21 at 05:03
Try `console.log(pigs)` and you'll see that `\X` has turned into `X`. — Barmar, Sep 16 '21 at 05:04
Why does the browser eliminate backslashes automatically in every string? — Lukasz Pospiech, Sep 16 '21 at 05:26
Backslash is the escape prefix. It either starts an escape sequence (like `\n` for newline) or makes the next character literal. — Barmar, Sep 16 '21 at 05:44

score 0 · Answer 1 · answered Sep 16 '21 at 06:19

You could use following pattern:

(?<=\X)(?:.(?!\X))+5555.+?(?=\X)

What I changed is: (?:.(?!\X))+ - which is non-capturing group and expression matches one or more characters, which are not followed by \X.

Also, you wrongly used [^\\X] - it's just negated class, it will match any character other than \ or X.

Below JS example:

let pigs = "\X The animal 0000 I really dig \X Above all others is the pig. 222 \X Pigs are noble. Pigs are clever 3333, \X Pigs are 5555 courteous.\X However, Now and then, to break this 6666 rule, \X One meets 7777 a pig who is a fool. \X"

let match = pigs.match(/(?<=\X)(?:.(?!\X))+5555.+?(?=\X)/g)

console.log(match)

JS regexp positive lookbehind no match

1 Answers1