0xD34F

Не нужны тут регулярные выражения, для разбора html есть специальные инструменты:

$doc = new DOMDocument();
$doc->loadHTML($html);
$a = $doc->getElementsByTagName('a');
$result = array_map([ $doc, 'saveHTML' ], iterator_to_array($a));

Но, конечно, дурацкое дело нехитрое:

$result = preg_split('/(?<=\/a>)(?=<a)/', $html);

// или

preg_match_all('/<a .*?>.*?<\/a>/', $html, $matches);
$result = $matches[0];

$newStr = preg_replace('~\b(\d)(\d)\b~', '$2$1', $str);

$grouped = array_values(array_reduce($arr, function($acc, $n) {
  $acc[$n['api_id']][] = $n;
  return $acc;
}, []));

$grouped = array_values(array_reduce($arr, function($acc, $n) {
  $acc[$n[0]] ??= [ $n[0] ];
  array_push($acc[$n[0]], ...array_slice($n, 1));
  return $acc;
}, []));

preg_match_all('~https:\/\/\S+~', $str, $matches);
$arr = array_unique($matches[0]);

А может не стоит забивать гвозди лбом? - count(array_unique($arr)).

str_pad($a + 1, strlen($a), 0, STR_PAD_LEFT)

$result = array_sum(array_filter(
  array_count_values($all_items),
  fn($n) => in_array($n, $filters),
  ARRAY_FILTER_USE_KEY
));

или

$result = count(array_intersect($all_items, $filters));

$sum = array_sum(array_column($arr, 'price'));
foreach ($arr as &$n) {
  $n['weight'] = number_format($n['price'] / $sum * 100, 2).'%';
}

$grouped = [];

foreach ($items as $n) {
  $grouped[$n['type']] ??= [
    'type' => $n['type'],
    'typeAmount' => 0,
    'amount' => 0,
  ];

  $grouped[$n['type']]['typeAmount'] += $n['typeAmount'];
  $grouped[$n['type']]['amount'] += $n['amount'];
}

$grouped = array_values($grouped);

public function expose() {
  return array_map(fn($n) => $n instanceof Text ? $n->expose() : $n, get_object_vars($this));
}

$grouped = [];

foreach ($arr as [ 'parent' => $p, 'sub' => $s ]) {
  $grouped[$p] ??= [ 'parent' => $p, 'sub' => [] ];
  $grouped[$p]['sub'][] = $s;
}

$grouped = array_values($grouped);

function merge($idKey, $mergeKeys, ...$data) {
  $merged = [];

  foreach (array_merge(...$data) as $item) {
    $id = $item[$idKey];

    if (!array_key_exists($id, $merged)) {
      $merged[$id] = [
        'unique' => true,
        'item' => $item,
      ];
    } else {
      if ($merged[$id]['unique']) {
        $merged[$id]['unique'] = false;
        foreach ($mergeKeys as $k) {
          $merged[$id]['item'][$k] = [ $merged[$id]['item'][$k] ];
        }
      }

      foreach ($mergeKeys as $k) {
        $merged[$id]['item'][$k][] = $item[$k];
      }
    }
  }

  return array_column($merged, 'item');
}


$merged = merge('code', [ 'quantity', 'city' ], $arr1, $arr2);

preg_replace_callback('#\[~(.*?)~\]#', fn($m) => $arr['person'][$m[1]], $str)

$values = array_combine(
  array_column($example2, 'SECTION_ID'),
  array_column($example2, 'VALUE')
);

$example3 = array_map(function($n) use($values) {
  $n['VALUE'] = $values[$n['SECTION_ID']];
  return $n;
}, $example1);

function generator($str, $params) {
  $result = [];

  if (count($params)) {
    $key = key($params);
    $values = is_array($params[$key]) ? $params[$key] : [ $params[$key] ];
    unset($params[$key]);

    foreach ($values as $val) {
      array_push($result, ...generator(str_replace("{{$key}}", $val, $str), $params));
    }
  } else {
    $result[] = $str;
  }

  return $result;
}