blocks|key|350268|text|第一个匹配一个空格，第二个匹配一个或多个空格。它们是所谓的正则表达式量词，它们执行如下匹配(取自documentation)：|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|350269|Greedy+quantifiers
X?++X,+once+or+not+at+all
X*++X,+zero+or+more+times
X%2B++X,+one+or+more+times
X{n}++++X,+exactly+n+times
X{n,}+++X,+at+least+n+times
X{n,m}++X,+at+least+n+but+not+more+than+m+times

Reluctant+quantifiers
X??+X,+once+or+not+at+all
X*?+X,+zero+or+more+times
X%2B?+X,+one+or+more+times
X{n}?+++X,+exactly+n+times
X{n,}?++X,+at+least+n+times
X{n,m}?+X,+at+least+n+but+not+more+than+m+times

Possessive+quantifiers
X?%2B+X,+once+or+not+at+all
X*%2B+X,+zero+or+more+times
X%2B%2B+X,+one+or+more+times
X{n}%2B+++X,+exactly+n+times
X{n,}%2B++X,+at+least+n+times
X{n,m}%2B+X,+at+least+n+but+not+more+than+m+times|code-block|syntax|javascript|350270|entityMap|0|LINK|mutability|MUTABLE|url|http://docs.oracle.com/javase/7/docs/api/java/util/regex/Pattern.html^0|1C|D|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@$A|R|B|S|1|T]]|C|$]]|$1|D|3|E|5|F|7|U|8|@]|9|@]|C|$G|H]]|$1|I|3|-4|5|6|7|V|8|@]|9|@]|C|$]]]|J|$K|$5|L|M|N|C|$O|P]]]]

The first one matches a single whitespace, whereas the second one matches one or many whitespaces. They're the so-called regular expression quantifiers, and they perform matches like this (taken from the <a href="http://docs.oracle.com/javase/7/docs/api/java/util/regex/Pattern.html" rel="noreferrer">documentation</a>):

<pre><code>Greedy quantifiers
X? X, once or not at all
X* X, zero or more times
X+ X, one or more times
X{n} X, exactly n times
X{n,} X, at least n times
X{n,m} X, at least n but not more than m times

Reluctant quantifiers
X?? X, once or not at all
X*? X, zero or more times
X+? X, one or more times
X{n}? X, exactly n times
X{n,}? X, at least n times
X{n,m}? X, at least n but not more than m times

Possessive quantifiers
X?+ X, once or not at all
X*+ X, zero or more times
X++ X, one or more times
X{n}+ X, exactly n times
X{n,}+ X, at least n times
X{n,m}+ X, at least n but not more than m times
</code></pre>

blocks|key|350323|text|第一个正则表达式将匹配一个空格字符。第二个正则表达式将不情愿地匹配一个或多个空白字符。对于大多数目的，这两个正则表达式非常相似，除了在第二种情况下，正则表达式可以匹配字符串的更多部分，前提是它可以防止正则表达式匹配失败。来自http://www.coderanch.com/t/570917/java/java/regex-difference的|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|350324|entityMap|0|LINK|mutability|MUTABLE|url|http://www.coderanch.com/t/570917/java/java/regex-difference^0|34|1O|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@$A|M|B|N|1|O]]|C|$]]|$1|D|3|-4|5|6|7|P|8|@]|9|@]|C|$]]]|E|$F|$5|G|H|I|C|$J|K]]]]

The first regex will match one whitespace character. The second regex will reluctantly match one or more whitespace characters. For most purposes, these two regexes are very similar, except in the second case, the regex can match more of the string, if it prevents the regex match from failing.  from <a href="http://www.coderanch.com/t/570917/java/java/regex-difference" rel="nofollow">http://www.coderanch.com/t/570917/java/java/regex-difference</a>

blocks|key|350356|text|首先，您需要了解两个语句的最终输出将是相同的，即删除给定字符串中的所有空格。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|350357|然而，x.replaceAll("\\s%2B",+"");将是更有效的修剪空格的方法(如果字符串可以有多个连续的空格)，因为由于正则表达式\\s%2B一次匹配一个或多个空格并将它们替换为空字符串的事实，没有替换的可能性更小。|offset|length|style|CODE|350358|因此，即使您从这两个命令中获得相同的输出，最好使用：|350359|x.replaceAll("\\s%2B",+"");|code-block|syntax|javascript|350360|entityMap^0|0|3|P|1W|4|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|R|8|@$D|S|E|T|F|G]|$D|U|E|V|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|W|8|@]|9|@]|A|$]]|$1|J|3|K|5|L|7|X|8|@]|9|@]|A|$M|N]]|$1|O|3|-4|5|6|7|Y|8|@]|9|@]|A|$]]]|P|$]]

First of all you need to understand that final output of both the statements will be same i.e. to remove all the spaces from given string.

However <code>x.replaceAll("\\s+", "");</code> will be more efficient way of trimming spaces (if string can have multiple contiguous spaces) because of potentially less no of replacements due the to fact that regex <code>\\s+</code> matches 1 or more spaces at once and replaces them with empty string.

So even though you get the same output from both it is better to use:

<pre><code>x.replaceAll("\\s+", "");
</code></pre>

What's the difference between the following two expressions?

<pre><code>x = x.replaceAll("\\s", "");
x = x.replaceAll("\\s+", "");
</code></pre>

Regex expressions in Java, \\s vs. \\s+

下面两个表达式之间的区别是什么？x = x.replaceAll("\\s", "");x = x.replaceAll("\\s+", "");

问Java中的正则表达式与\\s+
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