const regex = /<a href="entry:\/\/([^_]+)_([^_]+)_([^_]+)" title="/gmsi;
// Alternative syntax using RegExp constructor
// const regex = new RegExp('<a href="entry:\\\/\\\/([^_]+)_([^_]+)_([^_]+)" title="', 'gmsi')
const str = `<ul><li><span class="Latn" lang="en"><a href="entry://Thousand_Island_dressing" title="Thousand Island dressing">Thousand Island dressing</a></span></li></ul>`;
const subst = `<a href="entry://$1 $2 $3" title="`;
// 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