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<?php $body = <<<HTML <body class="old_attr"> <div>(content)</div> </body> HTML; $new_class = "new_class_attr"; $body = preg_replace('~<body[^>]*class *= *["\']\K~', "$new_class ", $body, 1, $count); if (!$count) { $body = preg_replace('~<body\K~', " class=\"$new_class\"", $body, 1); } echo $body;
Output for 5.6.38, 7.1.0 - 7.1.25, 7.2.0 - 7.2.33, 7.3.0 - 7.3.33, 7.4.0 - 7.4.26, 8.0.0 - 8.0.13, 8.1rc1 - rc3
<body class="new_class_attr old_attr"> <div>(content)</div> </body>

preferences:
18.01 ms | 460 KiB | 5 Q