$re = '/(?:\[|\G(?!^))\K[^][](?=[^][]*])/m';
$str = '$lang[\'activate_deactivate\'] = "Activate / Deactivate";
$lang[\'invalid_username\'] = "Invalid username";
$lang[\'you_must_enter_username\'] = "You must enter username";
$lang[\'invalid_user_name\'] = "Invalid username";
$lang[\'user_deactivated\'] = "User has been deactivated";
$lang[\'user_not_deactivated\'] = "User could not be deactivated";';
$subst = "$0█";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php