使用命名模式子例程的PCRE正则表达式

Question

I am experimenting with the named subpattern/'subroutine' regex features in PHP's PCRE and I'm hoping someone can explain the following strange output:

$re = "/
(?(DEFINE)
    (?<a> a )
)

^(?&a)$

/x";

var_dump(preg_match($re, 'a', $match)); // (int) 1 as expected
var_dump($match); // Array( [0] => 'a' ) <-- Why?

I can't understand why the named group "a" is not in the result (with the contents "a"). Changing preg_match to preg_match_all puts "a" and "1" in the match data but both contain only an empty string.

I really like the idea of writing regular expressions this way, as you can make them incredibly powerful whilst keeping them very maintainable (see this answer for a good example of this), however if the subpatterns are not available in the match data then it's not much use really.

Am I missing something here or should I just mourn what could have been and move on?

Answer 1

It makes perfect sense these subpatterns would not capture a group - their main purpose it to be used more than once, so you can't really capture them all. In addition, if the default was to capture all subpatterns it wouldn't give you an option not to capture a group where you don't want it - not the best default behavior. The opposite is trivial - you can capture by adding another group around the (?&a) statement.
I couldn't find a reference to this on PCRE.org. The closest is this, which is relevant because you don't match (?<a>...) directly (though you might expect an empty group):

Any capturing parentheses that are set during the subroutine call revert to their previous values afterwards.

It is clearer on the Perl manual (relevant part highlighted):

An example of how this might be used is as follows:

/(?<NAME>(?&NAME_PAT))(?<ADDR>(?&ADDRESS_PAT))
(?(DEFINE)
(?<NAME_PAT>....)
(?<ADRESS_PAT>....)
)/x

Note that capture buffers matched inside of recursion are not accessible after the recursion returns, so the extra layer of capturing buffers is necessary.