Sentence Comparison: Ignore Words - php

Question

I need some help with sentence comparison.
$answer = "This is the (correct and) acceptable answer. Content inside the parenthesis are ignored if not present in the user's answer. If it is present, it should not count against them.";
$response = "This is the correct and acceptable answer. Content inside the parenthesis are ignored if not present in the user's answer. If it is present, it should not count against them.";
echo "<strong>Acceptable Answer:</strong>";
echo "<pre style='white-space:normal;'>$answer</pre><hr/>";
echo "<strong>User's Answer:</strong>";
echo "<pre>".$response."</pre>";
// strip content in brackets
$answer = preg_replace("/\([^)]*\)|[()]/", "", $answer);
// strip punctuation
$answer = preg_replace("/[^a-zA-Z 0-9]+/", " ", $answer);
$response = preg_replace("/[^a-zA-Z 0-9]+/", " ", $response);
$common = similar_text($answer, $response, $percent);
$orgcount = strlen($answer);
printf("The user's response has %d/$orgcount characters in common (%.2f%%).", $common, $percent);
Basically what I want to do is ignore parenthiseised words. For example, in the $answer string, correct and are in parenthesis - because of this, I don't want these words to count agains the user's response. So if the user has these words, it doesn't count against them. And if the user doesn't have these words, it doesn't count against them.
Is this possible?

Answer 1

Thanks to the comments, I've wrote a solution, since it's a "long" process i though to put it in a function.
EDIT: After debugging it came out that strpos() was causing some trouble if the position was 0, so i added an OR statement:
$answer = "(This) is the (correct and) acceptable answer. (random this will not count) Content inside the parenthesis are ignored if not present in the user's answer. If it is present, it should not count against them.";
$response = "This is the correct and acceptable answer. Content inside the parenthesis are ignored if not present in the user's answer. If it is present, it should not count against them.";
echo 'The user\'s response has '.round(compare($answer, $response),2).'% characters in common'; // The user's response has 100% characters in common
function compare($answer, $response){
preg_match_all('/\((?P<parenthesis>[^\)]+)\)/', $answer, $parenthesis);
$catch = $parenthesis['parenthesis'];
foreach($catch as $words){
if(!strpos($response, $words) === false || strpos($response, $words) === 0){ // if it does exist then remove brackets
$answer = str_replace('('.$words.')', $words, $answer);
}else{ //if it does not exist remove the brackets with the words
$answer = str_replace('('.$words.')', '', $answer);
}
}
/* To sanitize */
$answer = preg_replace(array('/[^a-zA-Z0-9]+/', '/ +/'), array(' ', ' '), $answer);
$response = preg_replace(array('/[^a-zA-Z 0-9]+/', '/ +/'), array(' ', ' '), $response);
$common = similar_text($answer, $response, $percent);
return($percent);
}