const regex = new RegExp('-----Message d\\'origine-----.*(?:\\r?\\n(?!\\r?\\n).*)*\\s*', 'gm')
const str = `Something above
-----Message d'origine-----
De : Myself <myself@himself.com>
Envoyé : vendredi 8 mars 2019 14:30
À : Someone <someone@himself.com>
Cc : AnotherGuy <another@himself.com>
Objet : My bad I forgot how to do it
Hi,
blabla`;
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