code: public static Pattern get(String regex, int flags) {
  		final RegexWithFlag regexWithFlag = new RegexWithFlag(regex, flags);
  
  		Pattern pattern = POOL.get(regexWithFlag);
  		if (null == pattern) {
  			pattern = Pattern.compile(regex, flags);
  			POOL.put(regexWithFlag, pattern);
  		}
  		return pattern;
  	}
docs: 先从Pattern池中查找正则对应的{@link Pattern}，找不到则编译正则表达式并入池。
  
  @param regex 正则表达式
  @param flags 正则标识位集合 {@link Pattern}
  @return {@link Pattern}