Java正则表达式转义字符 [英] Java regex escaped characters

问题描述

匹配某些字符（例如换行符）时，可以使用正则表达式\\ n或实际上只使用\ n。例如，以下内容将字符串拆分为一个行数组：

When matching certain characters (such as line feed), you can use the regex "\\n" or indeed just "\n". For example, the following splits a string into an array of lines:

String[] lines = allContent.split("\\r?\\n");

但以下情况也同样如此：

But the following works just as well:

String[] lines = allContent.split("\r?\n");

我的问题：

执行以上操作两个完全以相同的方式工作，还是有任何微妙的区别？如果是后者，你能举例说明你会得到不同的结果吗？

Do the above two work in exactly the same way, or is there any subtle difference? If the latter, can you give an example case where you get different results?

或者只有[可能/理论]表现存在差异？

Or is there a difference only in [possible/theoretical] performance?

推荐答案

目前的情况没有区别。通常的字符串转义序列是在单个反斜杠的帮助下形成的，然后是有效的转义字符（\ n，\ r 等）和正则表达式转义序列是在文字反斜杠的帮助下形成的（即Java字符串中的双反斜杠） literal）和有效的正则表达式转义字符（\\ n，\\d ，等等。）。

There is no difference in the current scenario. The usual string escape sequences are formed with the help of a single backslash and then a valid escape char ("\n", "\r", etc.) and regex escape sequences are formed with the help of a literal backslash (that is, a double backslash in the Java string literal) and a valid regex escape char ("\\n", "\\d", etc.).

\ n（一个转义序列）是文字LF（换行符）和\\ n是与LF符号匹配的正则表达式转义序列。

"\n" (an escape sequence) is a literal LF (newline) and "\\n" is a regex escape sequence that matches an LF symbol.

\ r（一个转义序列）是一个文字CR（回车）和\\\\是一个与CR符号匹配的正则表达式转义序列。

"\r" (an escape sequence) is a literal CR (carriage return) and "\\r" is a regex escape sequence that matches an CR symbol.

\t（转义序列）是文字标签符号，\\t是一个匹配标签符号的正则表达式转义序列。

"\t" (an escape sequence) is a literal tab symbol and "\\t" is a regex escape sequence that matches a tab symbol.

请参阅正则表达式转义列表的.html＃sumrel =nofollow noreferrer> Java正则表达式文档。

See the list in the Java regex docs for the supported list of regex escapes.

但是，如果您使用 Pattern.COMMENTS 标志（用于引入注释并很好地格式化模式，使正则表达式引擎忽略模式中所有未转义的空格），您将需要使用\\ n或\\\ n定义换行符（LF） Java字符串文字和<$​​ c $ c>\\\\或\\\\\来定义回车（CR）。

However, if you use a Pattern.COMMENTS flag (used to introduce comments and format a pattern nicely, making the regex engine ignore all unescaped whitespace in the pattern), you will need to either use "\\n" or "\\\n" to define a newline (LF) in the Java string literal and "\\r" or "\\\r" to define a carriage return (CR).

查看 Java测试：

String s = "\n";
System.out.println(s.replaceAll("\n", "LF")); // => LF
System.out.println(s.replaceAll("\\n", "LF")); // => LF
System.out.println(s.replaceAll("(?x)\\n", "LF")); // => LF
System.out.println(s.replaceAll("(?x)\\\n", "LF")); // => LF
System.out.println(s.replaceAll("(?x)\n", "<LF>")); 
// => <LF>
//<LF>

为什么最后一个产生< LF> +换行符+ < LF> ？因为（？x）\ n等于，一个空模式，它匹配一个换行前和后面的空格。

Why is the last one producing <LF>+newline+<LF>? Because "(?x)\n" is equal to "", an empty pattern, and it matches an empty space before the newline and after it.

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