如何在正则表达式中匹配“（”和“）”括号字符？

Question

I am very new to regex expression.

I want to validate a number like this : 123456789012345678 (123)

I know how to match the first 18 digits for number, but don't know how to allow next ( and numbers inside it)

I am using this function currently that is incomplete, please help me

$string = "123456789012345678 (123)";
$pattern = '/^[0-9]{18}$/';
  if(!preg_match($pattern, $string)){
      echo 'valid';
  }

Answer 1

(和)是正则表达式的元字符，因此需要转义：

$pattern = '/^[0-9]{18} \([0-9]+\)$/';