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i write a piece of program to fetch content from a string between ":"(may not have) and "@" and order guaranteed，for example a string like "url:123@my.com",the I fetch "123",or "123@my.com" then i fetch "123" ,too; so I write a regular expression to implement it ,but i can not work,behind is first version:

Pattern pattern = Pattern.compile("(?<=:?).*?(?=@)");
Matcher matcher = pattern.matcher("sip:+8610086@dmcw.com");
if (matcher.find()) {
     Log.d("regex", matcher.group());
} else {
     Log.d("regex", "not match");
}

it can not work because in the first case:"url:123@my.com" it will get the result:"url:123" obviously not what i want:

so i write the second version:

Pattern pattern = Pattern.compile("(?<=:??).*?(?=@)");

but it get the error,somebody said java not support variable length in look behind;

so I try the third version:

Pattern pattern = Pattern.compile("(?<=:).*?(?=@)|.*?(?=@)");

and its result is same as the first version ,BUT SHOULD NOT THE FIRST CONDITION BE CONSIDERED FIRST?

it same as 

Pattern pattern = Pattern.compile(".*?(?=@)|(?<=:).*?(?=@)");

not left to right! I consider I understood regular expression before ,but confused again.thanks in advance anyway.

4

3 回答 3

1

试试这个（稍微编辑，见评论）：

String test = "sip:+8610086@dmcw.com";
String test2 = "8610086@dmcw.com";
Pattern pattern = Pattern.compile("(.+?:)?(.+?)(?=@)");
Matcher matcher = pattern.matcher(test);
if (matcher.find()) {
    System.out.println(matcher.group(2));
}
matcher = pattern.matcher(test2);
if (matcher.find()) {
    System.out.println(matcher.group(2));
}

输出：

+8610086
8610086

如果您需要有关模式的解释，请告诉我。

于 2013-05-29T08:45:36.690 回答
0

正如你所说，你不能在java中做一个变量lookbehind。

然后，你可以做这样的事情，你不需要lookbehind或lookaround。

正则表达式：:?([^@:]*)@

示例在这个示例中（忘记 \n，它是因为 regex101），您将在第一组中获得您需要的内容，并且您不必做任何特别的事情。有时最简单的解决方案是最好的。

于 2013-05-29T08:46:50.460 回答
0

你真的不需要任何前瞻或后视。你需要的可以通过使用贪婪的量词和一些替代来完成：

    .*(?:^|:)([^@]+)

默认情况下，java 正则表达式量词 ( *+{n}?) 都是贪婪的（将匹配尽可能多的字符，直到找不到匹配项。可以通过在量词后使用问号使它们变得懒惰，如下所示：.*?

您将要为此表达式输出捕获组 1，输出捕获组 0 将返回整个匹配项。

于 2013-05-29T08:47:37.383 回答