<?
$string = '//[langStart-en] This is a test //[langEnd-en]';
$pattern = '/\/\/\[langStart-en\][^n]*\/\/\[langEnd-en\]/';
$replacement = '//[langStart-en]//[langEnd-en]';
$my_string = preg_replace($pattern, $replacement, $string);
echo $my_string;
?>