How to parse without changing the characters case (lower and upper) in wordpress the_content?

Kitefr, (this isn’t really a WordPress question, but a php one) but you could usepreg_replace_callback, in which you define your own function to handle the the replacement.

For instance,

    $text = "The quick brown fox jumped over the Quick dog.";
    $text = preg_replace_callback( "/quick/si", 'my_replace' , $text );
    function my_replace($matches){
         return '<strong>'.$matches[0].'</strong>';
    }

This replaces all instances of ‘quick’ (in any case) and wraps them with strong tags, leaving the case of the letters unchanged. You could do something similar with link tags