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

Comienza reproduciendo el ejemplo de preg_match en PHP 8.2.12 y compara los arrays de coincidencias para ambas ramas de alternancia. Rastrea el manejo de los grupos de captura sin coincidencia en la implementación de preg_match y, después, añade o actualiza una prueba de regresión para que los grupos sin coincidencia tengan una salida coherente y se produzca el resultado esperado mostrado.