Regular expression works on regex101.com, but not on prod

(?:(?<=\s)|^)@(\S+) <-- the problem in positive lookbehind

Working like this on prod: (?:\s|^)@(\S+), but I need a correct start index (without space).

Here is in JS:

var regex = new RegExp(/(?:(?<=\s)|^)@(\S+)/g); 

Error parsing regular expression: Invalid regular expression: /(?:(?<=\s)|^)@(\S+)/

What am I doing wrong?

UPDATE

Ok, no lookbehind in JS :(

But anyways, I need a regex to get the proper start and end index of my match. Without leading space.

1 Answer

Make sure you always select the right regex engine at regex101.com. See an issue that occurred due to using a JS-only compatible regex with [^] construct in Python.

JS regex - at the time of answering this question - did not support lookbehinds. Now, it becomes more and more adopted after its introduction in ECMAScript 2018. You do not really need it here since you can use capturing groups:

var re = /(?:\s|^)@(\S+)/g; var str = 's @vln1\n@vln2\n'; var res = []; while ((m = re.exec(str)) !== null) { res.push(m[1]); } console.log(res);

The (?:\s|^)@(\S+) matches a whitespace or the start of string with (?:\s|^), then matches @, and then matches and captures into Group 1 one or more non-whitespace chars with (\S+).

To get the start/end indices, use

var re = /(\s|^)@\S+/g; var str = 's @vln1\n@vln2\n'; var pos = []; while ((m = re.exec(str)) !== null) { pos.push([m.index+m[1].length, m.index+m[0].length]); } console.log(pos);

BONUS

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• line-breaks - Line endings at regex101.com are LF only, you can't test strings with CRLF endings, see regex101.com VS myserver - different results. Solutions can be different for each regex library: either use \R (PCRE, Java, Ruby) or some kind of \v (Boost, PCRE), \r?\n, (?:\r\n?|\n)/(?>\r\n?|\n) (good for .NET) or [\r\n]+ in other libraries (see answers for C#, PHP).
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