message: Explicitly check undefined
diff: @@ -251,7 +251,7 @@ export const createRegExpParser = createParser((ParserCore) =>
  index,
  nextChar,
  );
  - if (octalValue && isOct(octalValue.toString())) {
  + if (octalValue !== undefined && isOct(octalValue.toString())) {
  const octal = parseInt(octalValue.toString(), 8);
  return this.finishComplexToken('Character', {
  value: String.fromCharCode(octal),
  @@ -316,7 +316,7 @@ export const createRegExpParser = createParser((ParserCore) =>
  index,
  nextChar,
  );
  - if (referenceValue) {
  + if (referenceValue !== undefined) {
  let backReference = referenceValue.toString();
  // \8 \9 are treated as escape char
  if (referenceValue === 8 || referenceValue === 9) {
  