A very simple regular expression engine

regex.js

//A walkthrough of this solution exists here: https://nickdrane.com/build-your-own-regex/

function matchOne(pattern, text) {
  if (!pattern) return true;
  if (!text) return false;
  return pattern === "." || text === pattern;
}

function search(pattern, text) {
  if (pattern[0] === "^") {
    return match(pattern.slice(1), text);
  } else {
    return match(".*" + pattern, text);
  }
}

function match(pattern, text) {
  if (!pattern) return true;
  else if (!text && pattern === "$") return true;
  else if (pattern[1] === "?") {
    return matchQuestion(pattern, text);
  } else if (pattern[1] === "*") {
    return matchStar(pattern, text);
  } else if (pattern[0] === "(") {
    return matchGroup(pattern, text);
  } else {
    return matchOne(pattern[0], text[0]) && match(pattern.slice(1), text.slice(1));
  }
}

function matchQuestion(pattern, text) {
  return (
    (matchOne(pattern[0], text[0]) && match(pattern.slice(2), text.slice(1))) ||
    match(pattern.slice(2), text)
  );
}

function matchStar(pattern, text) {
  return (
    (matchOne(pattern[0], text[0]) && match(pattern, text.slice(1))) ||
    match(pattern.slice(2), text)
  );
}

// Does not support nested groups
function matchGroup(pattern, text) {
  const groupEnd = pattern.indexOf(")");
  const groupPattern = pattern.slice(1, groupEnd);
  if (pattern[groupEnd + 1] === "?") {
    const remainderPattern = pattern.slice(groupEnd + 2); // +2 needed to slice off the ')?'
    return (
      (match(groupPattern, text.slice(0, groupPattern.length)) &&
        match(remainderPattern, text.slice(groupPattern.length))) ||
      match(remainderPattern, text)
    );
  } else if (pattern[groupEnd + 1] === "*") {
    const remainderPattern = pattern.slice(groupEnd + 2); // +2 needed to slice off the ')*'
    return (
      (match(groupPattern, text.slice(0, groupPattern.length)) &&
        match(pattern, text.slice(groupPattern.length))) ||
      match(remainderPattern, text)
    );
  } else {
    const remainderPattern = pattern.slice(groupEnd + 1); // +1 needed to slice off the ')'
    return (
      match(groupPattern, text.slice(0, groupPattern.length)) &&
      match(remainderPattern, text.slice(groupPattern.length))
    );
  }
}