- preg_replace: documentation ( source)
<?php
$string = '[gallery columns="1" ids="137,138,139,140,141,142,143,144,145,146,147,148,149,150,151,152,153,154,155,156,157,158,159,160,161"]';
$pattern = '/\[(.*?)\]/i';
$replacement = '';
echo preg_replace($pattern, $replacement, $string);