Java正则表达式为什么这两个正则表达式不同

Question

I have a java string demonstrating a div element:

String source = "<div class = \"ads\">\n" +
                "\t<dl style = \"font-size:14px; color:blue;\">\n" +
                "\t\t<li>\n" +
                "\t\t\t<a href = \"http://ggicci.blog.163.com\" target = \"_blank\">Ggicci's Blog</a>\n" +
                "\t\t</li>\n" +
                "\t</dl>\n" +
                "</div>\n";

which in html form is:

<div class = "ads">
    <dl style = "font-size:14px; color:blue;">
        <li>
            <a href = "http://ggicci.blog.163.com" target = "_blank">Ggicci's Blog</a>
        </li>
    </dl>
</div>

And I write such a regex to extract dl element:

<dl[.\\s]*?>[.\\s]*?</div>

But it finds nothing and I modified it to be:

<dl(.|\\s)*?>(.|\\s)*?</div>

then it works. So I tested like this:

System.out.println(Pattern.matches("[.\\s]", "a")); --> false
System.out.println(Pattern.matches("[abc\\s]", "a")); --> true

so why the '.' cant match 'a' ?

Answer 1

Inside the square brackets, the characters are treated literaly. [.\\\\s] means "Match a dot, or a backslash or as".

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(.|\\\\s) is equivalent to . .

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I think you really want the following regex:

<dl[^>]*>.*?</div>

Answer 2

+1 for above.

I would do:

<dl[^>]*>(.*?)</dl>

To match the content of dl

Answer 3

the syntax [.\\\\s] makes no sense, because, and Daniel said, the . just means "a dot" in this context.

Why can't you replace your [.\\\\s] with a much simpler . ?

Answer 4

When you include regexes in a post, it's a good idea to post them as you're actually using them--in this case, as Java string literals.

"[.\\\\s]" is a Java string literal representing the regex [.\\s] ; it matches a literal dot or a whitespace character. Your regex is not trying to match a backslash or an 's', as others have said, but the crucial factor is that . loses its special meaning inside a character class.

"(.|\\\\s)" is a Java string literal representing the regex (.|\\s) ; it matches ( anything but a line separator character OR any whitespace character ). It works as you intended, but don't use it! It leaves you extremely vulnerable to catastrophic backtracking , as explained in this answer .

But no worries, all you really need to do is use DOTALL mode (also known as single-line mode), which enables . to match anything including line separator characters.

(?s)<dl\b[^>]*>.*?</dl>