const regex = /\A(?:.*\R){15}\K\X+/gm;
// Alternative syntax using RegExp constructor
// const regex = new RegExp('\\A(?:.*\\R){15}\\K\\X+', 'gm')
const str = `1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20`;
const subst = ``;
// The substituted value will be contained in the result variable
const result = str.replace(regex, subst);
console.log('Substitution result: ', result);
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for JavaScript, please visit: https://developer.mozilla.org/en/docs/Web/JavaScript/Guide/Regular_Expressions