Java 8 LocalDate：确定本月的哪个星期一（第一，第二，第三等）(Java 8 LocalDate: Determine What Monday of the Month its (1st, 2nd, 3rd, etc))

 就像已经在这里回答的，最简单的方法就是删除+ ： 
 
^123\\d{9}$
 
 要么 
 
^123\\d{6}$
 
 取决于你确切需要什么。 
 
 你也可以使用另一种更复杂一般的方法，一种负面的看法： 
 
(?!.{10,})^123\\d+$
 
 说明： 
 
 这个： (?!.{10,})是一个负向预读 （ ?=会是一个正向预读），这意味着如果预读之后的表达式匹配这个模式，那么整个字符串不会' t匹配。 粗略地说，这意味着： 只有在负面预测中的模式不匹配时，才能满足此正则表达式的标准 。 
 
 在这种情况下， 只有在.{10}不匹配的情况下字符串才匹配，这意味着10个或更多字符，因此只有当前面的模式匹配最多9个字符时才匹配。 
 
 积极的预见反其道而行之，只有在先行标准也匹配的情况下才能进行匹配。 
 
 只是为了好奇而把它放在这里，它比你需要的更复杂。 

Like already answered here, the simplest way is just removing the +:
 
^123\\d{9}$
 
or 
 
^123\\d{6}$
 
Depending on what you need exactly.
 
You can also use another, a bit more complicated and generic approach, a negative lookahead:
 
(?!.{10,})^123\\d+$
 
Explanation:
 
This: (?!.{10,}) is a negative look-ahead (?= would be a positive look-ahead), it means that if the expression after the look-ahead matches this pattern, then the overall string doesn't match. Roughly it means: The criteria for this regular expression is only met if the pattern in the negative look-ahead doesn't match.
 
In this case, the string matches only if .{10} doesn't match, which means 10 or more characters, so it only matches if the pattern in front matches up to 9 characters.
 
A positive look-ahead does the opposite, only matching if the criteria in the look-ahead also matches.
 
Just putting this here for curiosity sake, it's more complex than what you need for this.