a strange regular on look behind

Question

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.

Answer 1

Try this (slightly edited, see comments):

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));
}

Output:

+8610086
8610086

Let me know if you need explanations on the pattern.