preg_match is inconsistent in how it handles unmatched capturing groups

Description

The following code:

<?php
$pattern = '/A(?<a>a)|B(?<b>b)/';
preg_match($pattern, 'Aa', $matches);
echo json_encode($matches)."\n";
preg_match($pattern, 'Bb', $matches);
echo json_encode($matches)."\n";

Resulted in this output:

{"0":"Aa","a":"a","1":"a"}
{"0":"Bb","a":"","1":"","b":"b","2":"b"}

But I expected this output instead:

{"0":"Aa","a":"a","1":"a"}
{"0":"Bb","b":"b","2":"b"}

I would expect that a capturing group only appears in the match if the pattern captured that group. However, it seems like in some cases groups on the left side of an alternation will appear with an empty string as the value while groups on the right side are omitted when they aren't captured.

This makes it difficult to easily ask "Did group N get captured?" because, depending on the structure of the regex, sometimes "not captured" will report as empty string and sometimes it will report as an omitted key. The problem is even more confusing if empty string was a possible capture for the group; in that case there's no way to tell what happened without using PREG_OFFSET_CAPTURE which gives -1 for the extraneous matches.

PHP Version

PHP 8.2.12

Operating System

Windows 11

まず PHP 8.2.12 で preg_match の例を再現し、両方の交替分岐について matches 配列を比較します。次に、preg_match の実装における未一致のキャプチャグループの処理を追跡し、未一致のグループの出力が一貫したものになり、示されている期待結果が生成されるように、回帰テストを追加または更新します。