const regex = /#[0-9a-fA-F]{6,6};/;
// Alternative syntax using RegExp constructor
// const regex = new RegExp('#[0-9a-fA-F]{6,6};', '')
const str = `<a href="//10.1.1.175/Intranetv2/content_kostenstellen.php" style="margin-top:10px; background-color:#800000; border-color:#800000;" class="btn btn-warning">Kostenstellen</a>`;
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