Java 用于指定空字符串的正则表达式
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Regex for specifying an empty string
提问by prabha
I use a validator that requires a regex to be specified. In the case of validating against an empty string, I don't know how to generate such a regex. What regex can I use to match the empty string?
我使用需要指定正则表达式的验证器。在针对空字符串进行验证的情况下，我不知道如何生成这样的正则表达式。我可以使用什么正则表达式来匹配空字符串？
回答by Chowlett
I don't know about Java specifically, but ^$usually works (^matches only at the start of the string, $only at the end).
我不特别了解 Java，但^$通常有效（^仅在字符串的开头匹配，$仅在结尾匹配）。
回答by polygenelubricants
The regex ^$matches only empty strings (i.e. strings of length 0). Here ^and $are the beginning and end of the string anchors, respectively.
正则表达式^$只匹配空字符串（即长度为 0 的字符串）。这里^和$分别是字符串锚点的开始和结束。
If you need to check if a string contains only whitespaces, you can use ^\s*$. Note that \sis the shorthand for the whitespace character class.
如果您需要检查字符串是否仅包含空格，您可以使用^\s*$. 请注意，这\s是空白字符类的简写。
Finally, in Java, matchesattempts to match against the entirestring, so you can omit the anchors should you choose to.
最后，在 Java 中，matches尝试匹配整个字符串，因此您可以选择省略锚点。
References
参考
API references
API 参考
Non-regex solution
非正则解决方案
You can also use String.isEmpty()to check if a string has length 0. If you want to see if a string contains only whitespace characters, then you can trim()it first and thencheck if it's isEmpty().
您还可以使用String.isEmpty()检查一个字符串的长度为0。如果你想看看字符串只包含空格字符，那么你可以trim()首先和然后检查它是否是isEmpty()。
回答by neo_r23
For checking empty string i guess there is no need of regex itself... u Can check length of the string directly ..
为了检查空字符串，我想不需要正则表达式本身......你可以直接检查字符串的长度..
in many cases empty string and null checked together for extra precision.
在许多情况下，空字符串和空值一起检查以获得额外的精度。
like String.length >0 && String != null
像 String.length >0 && String != null
回答by David Abragimov
If you have to use regexp in Java for checking empty string you can use
如果您必须在 Java 中使用正则表达式来检查空字符串，您可以使用
String testString = "";
System.out.println(testString.matches(""));
or for checking if only white-spaces:
或检查是否只有空格：
String testString = " ";
testString.trim().matches("");
but anyway using
但无论如何使用
testString.isEmpty();
testString.trim().isEmpty();
should be match better from performance perspective.
从性能的角度来看，应该更好地匹配。
public class Main {
public static void main(String[] args) {
String testString = "";
long startTime = System.currentTimeMillis();
for (int i =1; i <100000000; i++) {
testString.matches("");
}
long endTime = System.currentTimeMillis();
System.out.println("Total testString.matches execution time: " + (endTime-startTime) + "ms");
startTime = System.currentTimeMillis();
for (int i =1; i <100000000; i++) {
testString.isEmpty();
}
endTime = System.currentTimeMillis();
System.out.println("Total testString.isEmpty execution time: " + (endTime-startTime) + "ms");
}
}
}
Output:
输出：
C:\Java\java8\bin\java.exe
Total testString.matches execution time: 18776ms
Total testString.isEmpty execution time: 12ms
