当提供正则表达式时,Java 中的 String.split() 方法究竟是如何工作的? [英] How exactly does String.split() method in Java work when regex is provided?
问题描述
我正在准备 OCPJP 考试,我遇到了以下示例:
I'm preparing for OCPJP exam and I ran into the following example:
class Test {
public static void main(String args[]) {
String test = "I am preparing for OCPJP";
String[] tokens = test.split("\S");
System.out.println(tokens.length);
}
}
此代码打印 16.我期待类似 no_of_characters + 1 的结果.有人可以解释一下,在这种情况下 split() 方法实际上做了什么?我就是不明白...
This code prints 16. I was expecting something like no_of_characters + 1. Can someone explain me, what does the split() method actually do in this case? I just don't get it...
推荐答案
它在每个 "\S"
上拆分,在正则表达式引擎中表示 S
非空白特点.
It splits on every "\S"
which in regex engine represents S
non-whitespace character.
所以让我们尝试在非空格 (S
) 上拆分 "x x"
.由于此正则表达式可以与一个字符匹配,因此我们可以遍历它们以标记拆分的位置(为此我们将使用管道 |
).
So lets try to split "x x"
on non-whitespace (S
). Since this regex can be matched by one character lets iterate over them to mark places of split (we will use pipe |
for that).
- 是
'x'
非空格吗?是的,所以让我们标记它|x
- 是
' '
非空格吗?不,所以我们保持原样 - 最后一个
'x'
是非空格吗?是的,所以让我们标记它||
- is
'x'
non-whitespace? YES, so lets mark it| x
- is
' '
non-whitespace? NO, so we leave it as is - is last
'x'
non-whitespace? YES, so lets mark it| |
因此,我们需要在开始和结束时拆分字符串,这最初为我们提供结果数组
So as result we need to split our string at start and at end which initially gives us result array
["", " ", ""]
^ ^ - here we split
但由于尾随 空字符串 被删除,结果将是
But since trailing empty strings are removed, result would be
[""," "] <- result
,""] <- removed trailing empty string
so split 返回只包含两个元素的数组 ["", " "]
.
so split returns array ["", " "]
which contains only two elements.
顺便说一句.要关闭删除最后一个空字符串,您需要使用 split(regex,limit)
和负值限制,如 split("\S",-1)
.
BTW. To turn off removing last empty strings you need to use split(regex,limit)
with negative value of limit like split("\S",-1)
.
现在让我们回到你的例子.如果您的数据正在拆分每个
Now lets get back to your example. In case of your data you are splitting on each of
I am preparing for OCPJP
| || ||||||||| ||| |||||
这意味着
""|" "|""|" "|""|""|""|""|""|""|""|""|" "|""|""|" "|""|""|""|""|""
所以这代表这个数组
[""," ",""," ","","","","","","","",""," ","",""," ","","","","",""]
但由于尾随空字符串 ""
被删除(如果它们的存在是由拆分引起的 - 更多信息请访问:来自 String.split 的混淆输出)
but since trailing empty strings ""
are removed (if their existence was caused by split - more info at: Confusing output from String.split)
[""," ",""," ","","","","","","","",""," ","",""," ","","","","",""]
^^ ^^ ^^ ^^ ^^
你得到的结果数组只包含这部分:
you are getting as result array which contains only this part:
[""," ",""," ","","","","","","","",""," ","",""," "]
正好是 16 个元素.
which are exactly 16 elements.
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