Could java regex match a string which exists or not?

I want to match the following strings:

<a href="http://xyz">xyz</a>
<a href="https://xyz">xyz</a>
<a href="xyz">xyz</a>

and I tried to use the regex but failed:

<a href="((http://|https://|).+)">\1</a>

What is th开发者_运维知识库e correct solution to achieve this? Thanks!

The following regex does that:

<a\s+href="(?:https?://)?([^"]+)">\1</a>

An explanation:

<a\s+href="       # match `<a href="`
(?:https?://)?    # optionally match `http://` or `https://`
([^"]+)           # match one or more chars other than `"`, and store it in group 1
">                # match `">`
\1                # match the same as group 1
</a>              # match `</a>`

A Java demo:

public class Main {
    public static void main(String[] args) {
        String[] tests = {
                "<a href=\"http://xyz\">xyz</a>",
                "<a href=\"https://xyz\">xyz</a>",
                "<a href=\"xyz\">xyz</a>",
                "<a href=\"xyz\">xyzzz</a>"
        };
        String regex = "<a\\s+href=\"(?:https?://)?([^\"]+)\">\\1</a>";
        for(String test : tests) {
            System.out.println(test.matches(regex));
        }
    }
}

prints:

true
true
true
false

I think that is what you need:

<a href="(https?://)?(.+)">\2</a>