Negative lookbehind regex in Javascript

Question

I want to match all strings that start with an @ unless they have other characters in front of the @ as well. Simple example: in @one @two and bla@three I want to match @one and @two but not @three. It's for highlighting usernames in a chatroom.

These strings can be anywhere in a sentence (right at the start or in the middle).

I actually thought that (?![a-zA-Z])@[a-zA-Z]+ should work but it still matches @three too.

You want negative lookbehind, not negative lookahead. But JS doesn't have that. See http://stackoverflow.com/questions/641407/javascript-negative-lookbehind-equivalent for a workaround. — Barmar, Apr 10 '14 at 22:13
So, essentially you're looking for a pattern that matches an "@something" which has a space in front? Or in other words: what characters _other_ than a space should be allowed in front of the "@"? — S.B., Apr 10 '14 at 22:13
It should start with a space or be the beginning of the sentence. — Kevin Renskers, Apr 10 '14 at 22:15
`/(?:^| )(@\S+)/` should do just fine. Or `@[a-z]+`, if there can be something else used as separator than ` `. — raina77ow, Apr 10 '14 at 22:15
The only problem is that this also matches the space before @two — Kevin Renskers, Apr 10 '14 at 22:21
@raina77ow Looks like you nailed it :) http://regex101.com/r/gE5cE4 — Jason Sperske, Apr 10 '14 at 22:23
@mixedCase Of course, you'll have to use capturing groups - there's no look-behind in JS. — raina77ow, Apr 10 '14 at 22:26
One of my favorite things to do with regex questions is show people that they don't need to use lookaround. Give `\B@\w+` a try. — CAustin, Apr 10 '14 at 22:37
Ha amazingly simple. Please turn it into an answer so I can accept it! — Kevin Renskers, Apr 11 '14 at 10:36

score 5 · Accepted Answer · answered Apr 08 '15 at 21:12

5

You don't need a regex look around, you can use a simple regex like this:

\B@\w+

Working demo

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answered Apr 08 '15 at 21:12

Federico Piazza

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As an additional example: http://regexr.com/3apif on the RegExr site if you go to reference on the left, then anchors it'll explain \b RegExr is my go to for creating RegEx as the reference guides are right there without being in the way. – zfrisch Apr 08 '15 at 21:21
@MicTech you could use `(?<=^| )@\w+` – Federico Piazza Mar 01 '16 at 15:54
@FedericoPiazza JavaScript doesn't support that. – MicTech Mar 01 '16 at 18:08
@MicTech true, forgot about that. Anyway, this question was posted 1 year ago and also marked as resolved, so guess it solved OP problem. – Federico Piazza Mar 01 '16 at 19:59

Negative lookbehind regex in Javascript

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