Positive look behind in JavaScript regular expression

Question

I've a document from which I need to extract some data. Document contain strings like these

Text:"How secure is my information?"

I need to extract text which is in double quotes after the literal Text:

How secure is my information?

How do I do this with regex in Javascript

@MattBall, how do you figure? One could match on one or more characters with a lookbehind for `Text:"` and a lookahead for `"`. — henrebotha, Jun 24 '16 at 08:52
I suppose I can understand thinking about lookbehind as a solution for this, but it's much better suited for grouping parentheses. i.e. `/Text: "([^"]*)"/` — Vince, Mar 15 '17 at 00:15

score 73 · Accepted Answer · edited Mar 13 '22 at 14:28

73

Lookbehind assertions were recently finalised for JavaScript and will be in the next publication of the ECMA-262 specification. They are supported in Chrome 66 (Opera 53), but no other major browsers at the time of writing (caniuse).

var str = 'Text:"How secure is my information?"',
    reg = /(?<=Text:")[^"]+(?=")/;

str.match(reg)[0];
// -> How secure is my information?

Older browsers do not support lookbehind in JavaScript regular expression. You have to use capturing parenthesis for expressions like this one instead:

var str = 'Text:"How secure is my information?"',
    reg = /Text:"([^"]+)"/;

str.match(reg)[1];
// -> How secure is my information?

This will not cover all the lookbehind assertion use cases, however.

edited Mar 13 '22 at 14:28

x-yuri

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answered Aug 25 '10 at 18:35

Andy E

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But how do I extract all such data from a large document? Say into an array or something? – Raj Aug 25 '10 at 19:00
You use the /g modifier like what I had in my answer. – Crayon Violent Aug 25 '10 at 22:57
@Raj: you can use the `/g` modifier, as Crayon Violent said. This is the global modifier, without it the regular expression will stop execution when it finds the first match, with it the regular expression continues until it finds all matches, returning an array with all the matches in it. – Andy E Aug 26 '10 at 08:21
1

Hm. I get an undefined when I test the above code. When I console the `str.match(reg)` without index 1, it outputs `["Text:"How secure is my information?""]` thoughts? – jmk2142 Jun 19 '13 at 14:42
1

@orangewarp: the `g` modifier needed removing for the example in my answer. – Andy E Jun 19 '13 at 14:50
5

Cool. Works. Why does the **g** modifier kill the capturing parentheses? I was wondering because I can imagine a scenario where you might have multiple targets in a string. `str = 'Something more things " ...'` If I wanted all id values in an array, it seems you would want to use **g** but then the capturing parentheses would be gone. What would the best way be here? `reg = /id="([^"]+)"/g;` -> ["id="12345"","id="qwerty""] -> then run a foreach with `/id="([^"]+)"/` ? Can it be done in one regex step? – jmk2142 Jun 19 '13 at 15:55
2

@orangewarp: that's the standard behaviour for match with a global flag. It basically repeats a call to `exec` behind the scenes, returning only the first element from the result array in each iteration. The best solution is probably to run your own loop, calling `RegExp.prototype.exec` and parsing the result yourself, or use the [search and don't replace](http://ejohn.org/blog/search-and-dont-replace/) method (both of which are more or less the same, but the latter offers a little more convenience in some cases). – Andy E Jun 20 '13 at 07:45
@AndyE Good to know the behind the scenes. Follow-Up: Great link. Helped simplify my code. I've included a sample for other people to learn. Thanks once again! **[JS Fiddle Example](http://jsfiddle.net/TvNA3/)** – jmk2142 Jun 20 '13 at 17:17
Only thing different to a lookbehind is, that `Text:"` is consumed by the match. If I want to replace with `"…"` p.e. I have to do `str.replace(/(Text:")([^"]+)/, "$1…")`instead of `str.replace(/Text:"([^"]+)/, "…")`. – v01pe Jun 24 '13 at 00:09
How about the case when the lookahead string is a multichar string i.e. instead of ", it is ":Text e.g. - i/p - Text:"How secure is my information?":Text o/p - How secure is my information? – user1412066 Oct 23 '16 at 14:29
ufff! wasted quite some time fiddling with _firefox 68.0.1_ :-( thanks – Anand Rockzz Aug 05 '19 at 23:22
In 2020, [lookbehind expressions are still not supported in all browsers](https://caniuse.com/#search=lookbehind), which means that they should be avoided unless you can guarantee the use of a browser that supports them. – Dave F Mar 26 '20 at 12:52

Eddy · Answer 2 · 2016-07-30T12:43:33.373

26

I just want to add something: JavaScript doesn't support lookbehinds like (?<= ) or (?<! ).

But it does support lookaheads like (?= ) or (?! ).

edited Jul 30 '16 at 12:43

answered Jan 11 '14 at 12:15

Eddy

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score 12 · Answer 3 · answered Aug 25 '10 at 18:36

12

You can just do:

/Text:"(.*?)"/

Explanation:

Text:" : To be matched literally
.*? : To match anything in non-greedy way
() : To capture the match
" : To match a literal "
/ / : delimiters

answered Aug 25 '10 at 18:36

codaddict

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How do you use this? Do you have some context? – Sjoerd Aug 25 '10 at 18:39
that is the regex, you are supposed to use it with for instance .match() – Crayon Violent Aug 25 '10 at 22:56

Bill Criswell · Answer 4 · 2015-11-25T20:41:39.797

2

If you want to avoid the regular expression all together you can do:

var texts = file.split('Text:"').slice(1).map(function (text) {
  return text.slice(0, text.lastIndexOf('"')); 
});

edited Nov 25 '15 at 20:41

answered Nov 25 '15 at 15:07

Bill Criswell

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score 2 · Answer 5 · answered Aug 25 '10 at 18:35

2

string.match(/Text:"([^"]*)"/g)

answered Aug 25 '10 at 18:35

Crayon Violent

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How to avoid matching Text: in the result? – Raj Aug 25 '10 at 19:30
1

string[0] will always have the full regex match. string[1] will have the captured text. If there was a 2nd capture (parenthesis) in the regex, it would be put in string[2], etc... – Crayon Violent Aug 25 '10 at 19:59
1

I think with /g flag you will only get full regex match for all matches. – Raj Aug 25 '10 at 21:16
1

The g modifier will look for everything on the page that matches, not just stop at the first match. You said you have strings (plural), so that's why I put the /g modifier there. – Crayon Violent Aug 25 '10 at 22:54

score 2 · Answer 6 · answered Aug 25 '10 at 18:38

2

<script type="text/javascript">
var str = 'Text:"How secure is my information?"';
var obj = eval('({'+str+'})')
console.log(obj.Text);
</script>

answered Aug 25 '10 at 18:38

Sjoerd

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Modern browsers also have [JSON.parse](https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/JSON/parse), which may be preferred over `eval`. – Sjoerd Jun 20 '12 at 12:39

score 1 · Answer 7 · answered Oct 20 '16 at 12:06

Here is an example showing how you can approach this.

1) Given this input string:

const inputText = 
`Text:"How secure is my information?"someRandomTextHere
Voice:"Not very much"
Text:"How to improve this?"
Voice:"Don't use '123456' for your password"
Text:"OK just like in the "Hackers" movie."`;

2) Extract data in double quotes after the literal Text: so that the results is an array with all matches like so:

["How secure is my information?",
 "How to improve this?",
 "OK just like in the \"Hackers\" movie."]

SOLUTION

function getText(text) {
  return text
    .match(/Text:".*"/g)
    .map(item => item.match(/^Text:"(.*)"/)[1]);
}

console.log(JSON.stringify(    getText(inputText)    ));

RUN SNIPPET TO SEE A WORKING DEMO

const inputText = 
`Text:"How secure is my information?"someRandomTextHere
Voice:"Not very much"
Text:"How to improve this?"
Voice:"Don't use '123456' for your password"
Text:"OK just like in the "Hackers" movie."`;



function getText(text) {
  return text
    .match(/Text:".*"/g)
    .map(item => item.match(/^Text:"(.*)"/)[1]);
}

console.log(JSON.stringify(    getText(inputText)    ));

score 0 · Answer 8 · answered Jul 02 '20 at 01:24

If you, like me, get here while researching a bug related to the Cloudinary gem, you may find this useful:

Cloudinary recently released version 1.16.0 of their gem. In Safari, this crashes with the error 'Invalid regular expression: invalid group specifier name'.

A bug report has been filed. In the meantime I reverted to 1.15.0 and the error went away.

Hope this saves someone some lifetime.

Positive look behind in JavaScript regular expression

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