parseRegexGroup(e, t) {
    for (var r = this.fetch(), a = r, i = "", l; (l = this.fetch()).text !== "EOF" && e.test(i + l.text); )
      a = l, i += a.text, this.consume();
    if (i === "")
      throw new G("Invalid " + t + ": '" + r.text + "'", r);
    return r.range(a, i);
  }
Parses an ordinary group, which is either a single nucleus (like "x")
or an expression in braces (like "{x+y}") or an implicit group, a group
that starts at the current position, and ends right before a higher explicit
group ends, or at EOF.