Question

Does anybody know where in Pattern API the behaviour of this line of code is described

System.out.println("000".matches("(0)\\10“));

I think few people can say what it prints until they run it. API says

\n Whatever the n-th capturing group matched

It does not say that n must be 1 digit. Is it 10-th or 1-th group in my test?

Answer 1

You attempt to match the character 0 between parenthesis, and then you want the previous matched character \1 to be there also, followed by a 0 character. 000 does verify that pattern and thus the match() method returns true, so it prints true.

Since the matcher did not found 10 capturing groups, it interprets it as the first one \1 then the character 0.

A more complex example shows that if the matcher find N capturing group > 9 and that the available number of capturing groups is enough, it works also:

System.out.println(
    "01234567891011 01120".matches(
        "(0)(1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11) \\1\\12\\30"
    )
);

Is true because 0 is in the first capturing group \1 and 11 is in the capturing group \12, finally there is no captured group number \30 so it is interpreted as back reference \3 (which is character 2) then the character 0.