Lookahead https://javascript.info/regexp-lookahead-lookbehind

0_lookahead.txt

Lookahead

The syntax is: X(?=Y), it means "look for X, but match only if followed by Y". There may be any pattern instead of X and Y.

For an integer number followed by %, the regexp will be \d+(?=%):

1_lookahead.js

let str = "1 turkey has a discount of 30%";

alert(str.match(/\d+(?=%)/)); // 30, the number 1 is ignored, as it's not followed by %

2_negative_lookahead.txt

Negative lookahead

Let’s say that we want a quantity instead, not a price from the same string. That’s a number \d+, NOT followed by $.

For that, a negative lookahead can be applied.

The syntax is: X(?!Y), it means "search X, but only if not followed by Y".

3_negative_lookahead.js

let str = "2 turkeys have a discount of 60%";

alert(str.match(/\d+\b(?!%)/g)); // 2 (the discount is ignored)