当提供正则表达式时，Java 中的 String.split() 方法究竟是如何工作的？
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How exactly does String.split() method in Java work when regex is provided?
提问by peterremec
I'm preparing for OCPJP exam and I ran into the following example:
我正在准备 OCPJP 考试，我遇到了以下示例：
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);
}
}
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...
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...
采纳答案by Pshemo
It splits on every "\\S"which in regex engine represents \Snon-whitespace character.
它"\\S"在正则表达式引擎中代表\S非空白字符的每个字符上进行拆分。
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).
因此，让我们尝试"x x"在非空格 ( \S)上进行拆分。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）。
- 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| |
- 是
'x'一个非空白？是的，所以让我们标记它| x - 是
' '一个非空白？不，所以我们保持原样 - 最后一个
'x'非空格？是的，所以让我们标记它| |
So as result we need to split our string at start and at end which initially gives us result array
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
["", " ", ""]
^ ^ - here we split
But since trailing empty stringsare removed, result would be
但由于尾随空字符串被删除，结果将是
[""," "] <- result
,""] <- removed trailing empty string
so split returns array ["", " "]which contains only two elements.
所以 split 返回["", " "]只包含两个元素的数组。
BTW. To turn off removing last empty strings you need to use split(regex,limit)with negative value of limit like split("\\S",-1).
顺便提一句。要关闭删除最后一个空字符串，您需要使用split(regex,limit)limit 的负值，例如split("\\S",-1).
Now lets get back to your example. In case of your data you are splitting on each of
现在让我们回到你的例子。如果您的数据正在拆分每个
I am preparing for OCPJP
| || ||||||||| ||| |||||
which means
意思是
""|" "|""|" "|""|""|""|""|""|""|""|""|" "|""|""|" "|""|""|""|""|""
So this represents this array
所以这代表这个数组
[""," ",""," ","","","","","","","",""," ","",""," ","","","","",""]
but since trailing empty strings ""are removed (if their existence was caused by split - more info at: Confusing output from String.split)
但由于尾随空字符串""被删除（如果它们的存在是由 split 引起的 - 更多信息：来自 String.split 的混淆输出）
[""," ",""," ","","","","","","","",""," ","",""," ","","","","",""]
^^ ^^ ^^ ^^ ^^
you are getting as result array which contains only this part:
你得到的结果数组只包含这部分：
[""," ",""," ","","","","","","","",""," ","",""," "]
which are exactly 16 elements.
正好是 16 个元素。
