If i edit core wordpress code? would it be same after wordpress update?

I want to write the following code to the file: wp-includes/formatting.php
In order to Swap out all of the dashes/hyphens (-) for underscores (_):

$title = strtolower($title);
$title = preg_replace(‘/&.+?;/’, ”, $title); // kill entities
$title = str_replace(‘.’, ‘-‘, $title);
$title = preg_replace(‘/[^%a-z0-9 _-]/’, ”, $title);
$title = preg_replace(‘/\s+/’, ‘-‘, $title);
$title = preg_replace(‘|-+|’, ‘-‘, $title);
$title = trim($title, ‘-‘);

The code to be replaced:

$title = strtolower($title);
$title = preg_replace(‘/&.+?;/’, ”, $title); // kill entities
$title = str_replace(‘.’, ‘_’, $title);
$title = preg_replace(‘/[^%a-z0-9 _-]/’, ”, $title);
$title = preg_replace(‘/\s+/’, ‘_’, $title);
$title = preg_replace(‘|-+|’, ‘_’, $title);
$title = trim($title, ‘_’);