Attach (recommended) or Link to PDF file here:
PDF from #4106, https://mail.gnome.org/archives/evince-list/2013-November/pdfNC0hkVW67a.pdf.

Steps to reproduce the problem:
1. Open the PDF

What is the expected behavior?
![immagine](https://user-images.githubusercontent.com/1616846/121865567-67ca4200-ccfe-11eb-8673-1d6e863fc8d5.png)

What went wrong?
The PDF doesn't load, `TypeError: can't access property Symbol(), match is null` error is logged in the console.
let pos = name.length;
  while (pos < expr.length) {
    const spos = pos;
    const char = expr.charAt(pos++);
    if (char === "[") {
      match = expr.slice(pos).match(indexPattern);
      if (!match) {
        warn("XFA - Invalid index in SOM expression");
        return null;
      }
      parsed[parsed.length - 1].index = parseIndex(match[0]);
      pos += match[0].length + 1;
      continue;
    }
    let operator;
    switch (expr.charAt(pos)) {
      case ".":
        if (!dotDotAllowed) {
          return null;
        }
        pos++;
        operator = operators.dotDot;
        break;
      case "#":
        pos++;
        operator = operators.dotHash;
        break;
      case "[":
        
        operator = operators.dotBracket;