const regex = /(?<!\[|F|\[F)\w+(?!W|\]|W\]\])/g;
// Alternative syntax using RegExp constructor
// const regex = new RegExp('(?<!\\[|F|\\[F)\\w+(?!W|\\]|W\\]\\])', 'g')
const str = `Fixing this [HFW] takes alot of [HFW] time [HFW] two`;
const subst = `[CW]`;
// 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