Using zero-width assertions negative lookahead to match a string that does's contains string "abc"

Hi all: I'm trying using zero-开发者_C百科width assertions negative lookahead to match a string that does's contains string "abc",and this is what I got:

    Pattern pattern = new Perl5Compiler().compile("((?!abc).)+");
    Perl5Matcher matcher = new Perl5Matcher();
    System.out.println(matcher.matches("abc", pattern));
    System.out.println(matcher.matches("abdas dfas", pattern));
    System.out.println(matcher.matches("d abc ", pattern));
    System.out.println(matcher.matches("fafabcdef", pattern));

and the result is:

    false
    true
    false
    false

What I can't understand is why the string "abc" doesn't matches, it doesn't contains any character after the assertion "abc". Can anyone please figure out how this works? tks~

---

Look-arounds start doing their thing on positions, not on characters. So, with the string "abc", this part of your regex: (?!abc). starts looking ahead on the position before the "a" in your string. The position is the empty string before "a". That's why it fails to match.

---

Hum, that differs from how Perl5 actually works.

$ perl -E'
    for ("abc", "abdas dfas", "d abc ", "fafabcdef") {
        say "$_: ", /((?!abc).)+/ ? "true ($&)" : "false";
    }
'
abc: true (bc)
vabdas dfas: true (abdas dfas)
d abc : true (d )
fafabcdef: true (faf)

It must have an implicit ^ and \z.

$ perl -E'
    for ("abc", "abdas dfas", "d abc ", "fafabcdef") {
        say "$_: ", /^((?!abc).)+\z/ ? "true ($&)" : "false";
    }
'
abc: false
abdas dfas: true (abdas dfas)
d abc : false
fafabcdef: false

The ones that don't match because there some position that matches /abc/.