const regex = new RegExp('(?s)^((?:https?://)?(?:www\\.)?\\S+\\.\\S+)\\n(?=.*^\\1$)', 'gm')
const str = `operator.livrareflori.md
operator.livrareflori.md
operator.livrareflori.md
operator.livrareflori.md
operator.livrareflori.md
operator.livrareflori.md
operator.livrareflori.md
operator.livrareflori.md
operator.livrareflori.md
operator.livrareflori.md
operator.livrareflori.md
operator.livrareflori.md
amazon.de
fonts.gstatic.com
fonts.gstatic.com
fonts.gstatic.com
erovoyeurism.net
tugtechnologyandbusiness.com`;
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