Javascript regex with negative lookbehind (review)

Question

I was hoping someone could review my regular expression to make sure that it's doing what I want it to do.

So here's what I'm after:

Search for a word within a word boundary - so it can be a word on its own, or a word within another word
Grab the preceding 30 characters (if preceding characters exist), but only if they do not contain the word I am searching for.
Grab the next 30 characters (if they exist)

So if I were searching for "car" for 1, I have:

(\b\w*car\w*\b)

For 2, I have:

((?!\b\w*car\w*\b).{30}|.{0,30})

For 3, I have:

.{0,30}

All together:

((?!\b\w*car\w*\b).{30}|.{0,30})(\b\w*car\w*\b).{0,30}

Have I got it right, will this do what I'm after?

this http://codereview.stackexchange.com/ might be a better place for this question — depperm, Dec 12 '15 at 04:54
Can you add some examples of valid and invalid strings for this expression? — Musa Haidari, Dec 12 '15 at 04:57
Run this in a the console and check if the output is what you want/expect: `"I'm an psychodelic purple and green electric smartcar that goes insanely really fast and can lap the Earth on a single charge".match(/((?!\b\w*car\w*\b).{30}|.{0,30})(\b\w*car\w*\b)(.{0,30})/)` — br3nt, Dec 12 '15 at 05:05
*Have I got it right, will this do what I'm after?* I don't know, does it? — , Dec 12 '15 at 05:12
This appears to be an attempt at a workaround to JavaScript's lack of lookbehind support. In that light, perhaps you'd like to read more about [how to properly implement lookbehind in JavaScript](https://stackoverflow.com/questions/35142364/regex-negative-lookbehind-not-valid-in-javascript/35143111#35143111). — Adam Katz, Feb 09 '16 at 01:36

score 1 · Accepted Answer · answered Dec 12 '15 at 05:13

There is a slight change that needed to be made to your regex.

The last .{0, 30} needs to be put into a group. The full regex is then becomes this:

/((?!\b\w*car\w*\b).{30}|.{0,30})(\b\w*car\w*\b)(.{0,30})/

If you run this in a javascript console you can see the output:

var str = "I'm an psychodelic purple and green electric smartcar that goes insanely really fast and can lap the Earth on a single charge"
var match = str.match(/((?!\b\w*car\w*\b).{30}|.{0,30})(\b\w*car\w*\b)(.{0,30})/);

alert(
  "Word containing car: " + match[2] +
  "\nFirst 30: " + match[1] +
  "\nLast 30: " + match[3]
);

You should get the following alert message:

Word containing car: smartcar
First 30: lic purple and green electric 
Last 30:  that goes insanely really fas

Javascript regex with negative lookbehind (review)

1 Answers1