const regex = /(^---[\s\S]+?date: )\d{4}\/\d{2}\/\d{2}([\s\S]+?^---)/gm;
// Alternative syntax using RegExp constructor
// const regex = new RegExp('(^---[\\s\\S]+?date: )\\d{4}\\\/\\d{2}\\\/\\d{2}([\\s\\S]+?^---)', 'gm')
const str = `---
title: test
date: 2018/10/17
description: some thing
---
lorem ipsum ---
date: 2018/10/18
lorem ipsum
---
title: test
date: 2018/10/17
description: some thing
---
---
title: test
nodatehere:
description: some thing
---`;
const subst = `$1replacement$2`;
// 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