docs: private void checkHexDigit(int c) throws IOException {
          if (c >= '0' && c <= '9') {
              return;
          }
          if (c >= 'a' && c <= 'f') {
              return;
          }
          if (c >= 'A' && c <= 'F') {
              return;
          }
          // Causes the invalid escape to be skipped
          hasNextChar = true;
          nextChar = c;
          throw new IOException("Did not find four digit hex character code."
                  + " line: " + lexer.getLine() + " col:" + lexer.getColumn());
      }