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

Inizia riproducendo l’esempio di preg_match su PHP 8.2.12 e confronta gli array delle corrispondenze per entrambi i rami dell’alternanza. Traccia la gestione dei gruppi di cattura senza corrispondenza nell’implementazione di preg_match, quindi aggiungi o aggiorna un test di regressione in modo che i gruppi senza corrispondenza abbiano un output coerente e venga prodotto il risultato atteso mostrato.