code: public RegexMatcher addExpression(String expr, T attach, Option... options)
      {
          if (nfa == null)
          {
              nfa = parser.createNFA(nfaScope, expr, attach, options);
          }
          else
          {
              NFA<T> nfa2 = parser.createNFA(nfaScope, expr, attach, options);
              nfa = new NFA<>(nfaScope, nfa, nfa2);
          }
          return this;
      }
docs: Add expression.
  @param expr
  @param attach
  @param options
  @return