Method push

java/com/google/re2j/Parser.java:106–142

(Regexp re)

  // push pushes the regexp re onto the parse stack and returns the regexp.
  // Returns null for a CHAR_CLASS that can be merged with the top-of-stack.
  private Regexp push(Regexp re) {
    if (re.op == Regexp.Op.CHAR_CLASS && re.runes.length == 2 && re.runes[0] == re.runes[1]) {
      // Collapse range [x-x] -> single rune x.
      if (maybeConcat(re.runes[0], flags & ~RE2.FOLD_CASE)) {
        return null;
      }
      re.op = Regexp.Op.LITERAL;
      re.runes = new int[] {re.runes[0]};
      re.flags = flags & ~RE2.FOLD_CASE;
    } else if ((re.op == Regexp.Op.CHAR_CLASS
            && re.runes.length == 4
            && re.runes[0] == re.runes[1]
            && re.runes[2] == re.runes[3]
            && Unicode.simpleFold(re.runes[0]) == re.runes[2]
            && Unicode.simpleFold(re.runes[2]) == re.runes[0])
        || (re.op == Regexp.Op.CHAR_CLASS
            && re.runes.length == 2
            && re.runes[0] + 1 == re.runes[1]
            && Unicode.simpleFold(re.runes[0]) == re.runes[1]
            && Unicode.simpleFold(re.runes[1]) == re.runes[0])) {
      // Case-insensitive rune like [Aa] or [Δδ].
      if (maybeConcat(re.runes[0], flags | RE2.FOLD_CASE)) {
        return null;
      }

      // Rewrite as (case-insensitive) literal.
      re.op = Regexp.Op.LITERAL;
      re.runes = new int[] {re.runes[0]};
      re.flags = flags | RE2.FOLD_CASE;
    } else {
      // Incremental concatenation.
      maybeConcat(-1, 0);
    }

    stack.add(re);
    return re;
  }

  // maybeConcat implements incremental concatenation
  // of literal runes into string nodes.  The parser calls this