discourse/app/assets/javascripts/discourse/lib/highlight-html.js

function highlight(node, pattern, nodeName, className) {
  if (
    ![Node.ELEMENT_NODE, Node.TEXT_NODE].includes(node.nodeType) ||
    ["SCRIPT", "STYLE"].includes(node.tagName) ||
    (node.tagName === nodeName && node.className === className)
  ) {
    return 0;
  }

  if (node.nodeType === Node.ELEMENT_NODE && node.childNodes) {
    for (let i = 0; i < node.childNodes.length; i++) {
      i += highlight(node.childNodes[i], pattern, nodeName, className);
    }
    return 0;
  }

  if (node.nodeType === Node.TEXT_NODE) {
    const match = node.data.match(pattern);

    if (!match) {
      return 0;
    }

    const element = document.createElement(nodeName);
    element.className = className;
    element.innerText = match[0];
    const matchNode = node.splitText(match.index);
    matchNode.splitText(match[0].length);
    matchNode.parentNode.replaceChild(element, matchNode);
    return 1;
  }

  return 0;
}

export default function(node, words, opts = {}) {
  let settings = {
    nodeName: "span",
    className: "highlighted",
    wholeWord: false,
    matchCase: false
  };

  Object.assign(settings, opts);
  words = typeof words === "string" ? [words] : words;
  words = words
    .filter(Boolean)
    .map(word => word.replace(/[-\/\\^$*+?.()|[\]{}]/g, "\\$&"));

  if (!words.length) return node;

  let pattern = `(${words.join("|")})`;
  let flag;

  if (settings.wholeWord) {
    const hasUnicode = words.some(word => {
      return !word.match(new RegExp(`\b${word}\b`));
    });
    pattern = hasUnicode
      ? `(?<=[\\s,.:;"']|^)${pattern}(?=[\\s,.:;"']|$)`
      : `\b${pattern}\b`;
  }

  if (settings.matchCase) {
    flag = "i";
  }

  highlight(
    node,
    new RegExp(pattern, flag),
    settings.nodeName.toUpperCase(),
    settings.className
  );

  return node;
}

export function unhighlightHTML(opts = {}) {
  let settings = {
    nodeName: "span",
    className: "highlighted"
  };

  Object.assign(settings, opts);

  document
    .querySelectorAll(`${settings.nodeName}.${settings.className}`)
    .forEach(e => {
      const parentNode = e.parentNode;
      parentNode.replaceChild(e.firstChild, e);
      parentNode.normalize();
    });
}