module.exports = function (glob, opts) {
  if (glob == null) {
    return null;
  }

  var str = String(glob);

  // The regexp we are building, as a string.
  var reStr = "";

  // Whether we are matching so called "extended" globs (like bash) and should
  // support single character matching, matching ranges of characters, group
  // matching, etc.
  var extended = opts ? !!opts.extended : false;

  // If we are doing extended matching, this boolean is true when we are inside
  // a group (eg {*.html,*.js}), and false otherwise.
  var inGroup = false;
  // RegExp flags (eg "i" ) to pass in to RegExp constructor.
  var flags = opts && typeof( opts.flags ) === "string" ? opts.flags : "";
  var c;
  for (var i = 0, len = str.length; i < len; i++) {
    c = str[i];

    switch (c) {
    case "\\":
    case "/":
    case "$":
    case "^":
    case "+":
    case ".":
    case "(":
    case ")":
    case "=":
    case "!":
    case "|":
      reStr += "\\" + c;
      break;

    case "?":
      if (extended) {
        reStr += ".";
	    break;
      }

    case "[":
    case "]":
      if (extended) {
        reStr += c;
	    break;
      }

    case "{":
      if (extended) {
        inGroup = true;
	    reStr += "(";
	    break;
      }

    case "}":
      if (extended) {
        inGroup = false;
	    reStr += ")";
	    break;
      }

    case ",":
      if (inGroup) {
        reStr += "|";
	    break;
      }
      reStr += "\\" + c;
      break;

    case "*":
      reStr += ".*";
      break;

    default:
      reStr += c;
    }
  }

  // When regexp 'g' flag is specified don't
  // constrain the regular expression with ^ & $
  if (!flags || !~flags.indexOf('g')) {
    reStr = "^" + reStr + "$";
  }

  return new RegExp(reStr, flags);
};
