What are the limitations of using regular expressions to filter out specific characters and extract only letters in PHP?

Using regular expressions to filter out specific characters and extract only letters in PHP can be limited because it may not be able to handle all possible edge cases or variations in input. Additionally, it may not be the most efficient solution for simple tasks like filtering out non-letter characters. To overcome these limitations, a more straightforward approach like using built-in PHP functions such as `preg_replace` or `ctype_alpha` can be considered.

$input = "Hello123World!";
$filteredString = preg_replace('/[^a-zA-Z]/', '', $input);

echo $filteredString;