java 字符串的Java正则表达式以数字和固定长度开头
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Java Regex of String start with number and fixed length
提问by waseem
I made a regular expression for checking the length of String , all characters are numbers and start with number e.g 123 Following is my expression
我做了一个正则表达式来检查 String 的长度，所有字符都是数字并以数字开头，例如 123 以下是我的表达式
REGEX =^123\d+{9}$";
But it was unable to check the length of String. It validates those strings only their length is 9 and start with 123. But if I pass the String 1234567891 it also validates it. But how should I do it which thing is wrong on my side.
但它无法检查字符串的长度。它仅验证这些字符串的长度为 9 并以 123 开头。但如果我传递字符串 1234567891，它也会验证它。但是我应该怎么做，哪件事情在我这边是错误的。
回答by pcalcao
Like already answered here, the simplest way is just removing the +:
就像这里已经回答的一样，最简单的方法就是删除+：
^123\d{9}$
or
或者
^123\d{6}$
Depending on what you need exactly.
完全取决于您的需求。
You can also use another, a bit more complicated and generic approach, a negative lookahead:
您还可以使用另一种更复杂和通用的方法，即否定前瞻：
(?!.{10,})^123\d+$
Explanation:
解释：
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.
这：(?!.{10,})是一个否定的前瞻（?=将是一个肯定的前瞻），这意味着如果前瞻之后的表达式匹配这个模式，那么整个字符串不匹配。粗略地说，它的意思是：仅当否定前瞻中的模式不匹配时，才满足此正则表达式的条件。
In this case, the string matches onlyif .{10}doesn't match, which means 10 or more characters, so it only matches if the pattern in front matches up to 9 characters.
在这种情况下，字符串仅在.{10}不匹配时才匹配，这意味着 10 个或更多字符，因此仅当前面的模式匹配最多 9 个字符时才匹配。
A positive look-ahead does the opposite, only matching if the criteria in the look-ahead alsomatches.
积极的前瞻做相反的事情，只有在前瞻中的标准也匹配时才匹配。
Just putting this here for curiosity sake, it's more complex than what you need for this.
只是为了好奇而把它放在这里，它比你需要的更复杂。
回答by Doorknob
Try using this one:
尝试使用这个：
^123\d{6}$
I changed it to 6because 1, 2, and 3 should probably still count as digits.
我将其更改为6因为 1、2 和 3 可能仍应计为数字。
Also, I removed the +. With it, it would match 1 or more \ds (therefore an infinite amount of digits).
另外，我删除了+. 有了它，它将匹配 1 个或多个\ds（因此是无限数量的数字）。
回答by jlordo
Based on your comment below Doorknobs's answeryou can do this:
根据您在Doorknobs 回答下的评论，您可以执行以下操作：
int length = 9;
String prefix = "123"; // or whatever
String regex = "^" + prefix + "\d{ " + (length - prefix.length()) + "}$";
if (input.matches(regex)) {
// good
} else {
// bad
}
