This script will spy the page for what you type and when will match what you find will inform

spy-typing.js

function spy(regexp, callback) {
  let state = "";
  document.addEventListener("keydown", function(e) {
    state += e.key;
    console.log(e.key);
    const match = state.match(regexp);
    if (match) {
      callback(match[0]);
      state = "";
    }
  });
}

spy(/typing this should catch$/, function(code) {
  console.log(code);
  alert(code);
});