JS regexp (?

Question

Does anyone know how translate the POSIX regexp (?<!X)A in JS?

Find A only if not preceded by X.

You might find this interesting: http://blog.stevenlevithan.com/archives/mimic-lookbehind-javascript. — pimvdb, Aug 11 '11 at 18:21
I don't think that negative lookbehinds are in POSIX, lookarounds are not supported in BRE nor ERE. ;-) — Qtax, Aug 11 '11 at 19:13

Platinum Azure · Answer 1 · 2011-08-11T18:12:47.913

5

Simply check for either the beginning (ergo there is no X) or that there is a non-X character.

(^|[^X])A

For more than one character, you could check for A and then check the matched text for X followed by A, and discard the match if it matches the second pattern.

edited Aug 11 '11 at 18:12

answered Aug 11 '11 at 18:06

Platinum Azure

43,544
11
104
132

+1 This will behave the same unless the global modifier is on. – Paul Aug 11 '11 at 18:08

score 3 · Answer 2 · edited May 23 '17 at 12:19

3

Short answer: you can't.

JavaScript's RegExp Object does not support negative lookbehind.

edited May 23 '17 at 12:19

Community

1
1

answered Aug 11 '11 at 18:03

FK82

4,747
4
26
40

@dotweb, -1, it does not, and your answer does not prove otherwise. – Qtax Aug 11 '11 at 19:12

score 0 · Answer 3 · answered Aug 11 '11 at 18:26

0

Try this:

var str = "ab"; 
console.log(/a(?!x)/i.exec(str)); //a

var str = "ax"; 
console.log(/a(?!x)/i.exec(str)); //null

if you need of part after "a", try:

   /a(?!x).*/i

answered Aug 11 '11 at 18:26

The Mask

16,429
36
107
177

JS regexp (?

3 Answers3