Regex negative lookahead is not working properly - regex

Question

I have the following javascript code
var d = '<some string 2>';
var a = ...;
var b = '<some string 1>';
.
.
.
var c = b.map.....
To find the closest string declaration to c, I was using something like this:
var \w+ = '(.*?)';(?:(?!var \w+ = '(.*?)';.*?var c = \w+\.map)).*?var c = \w+\.map
I am using negative lookahead to not match a declaration if there is another declaration ahead that matches. However, the result I am getting is still <some string 2>. Could someone please explain me what causes that to happen?

Answer 1

The are more ways to match a string in JavaScript, and even more ways to break the pattern as this is very very brittle.
For the example data in the question, using a negative lookahead to assert that the next line does not start with the .map part or a string:
^var\s+\w\s*=\s*(["'`])(.*?)\1;\n(?:(?!var\s+\w+\s*=\s*(?:\w+\.map\b|(["'`]).*?\3);\n).*\n)*var\s+\w+\s*=\s*\w\.map\b
^ Start of string
var\s+\w\s*=\s* Match a possible var format and the equals sign
(["'`]) Capture group 1, match one of " ' or `
(.*?) Capture group 2 capture the data that you want to find
\1;\n A back reference to match up the quote in group 1
(?: Non capture group, to repeat as a whole
(?! Negative lookahead, assert what is directly to the right is not
var\s+\w+\s*=\s* Match a var part with =
(?: Non capture group for the alternation |
\w+\.map\b Match the .map part
| Or
(["'`]).*?\3 Capture group 3 to match one of the quotes with a back reference to match it up
) Close non capture group
;\n Match ; and a newline
) Close lookahead
.*\n Match the whole line and a newline
)* Close non capture group and optionally repeat to match all lines
var\s+\w+\s*=\s*\w\.map\b Match the line with .map
Regex demo
const regex = /^var\s+\w\s*=\s*(["'`])(.*?)\1;\n(?:(?!var\s+\w+\s*=\s*(?:\w+\.map\b|(["'`]).*?\3);\n).*\n)*var\s+\w+\s*=\s*\w\.map\b/m;
const str = `var d = '<some string 2>';
var a = ...;
var b = '<some string 1>';
.
.
.
var c = b.map.....`;
const m = str.match(regex);
if (m) {
console.log(m[2]);
}