Restrict characters in comment section

I am using Travelify theme and I am having some trouble restricting the characters in the comment section (textarea) or where the name or mail goes into the input tags.

I read some articles about changing the core files from wp-includes like post.php / post-formats.php / formatting.php, for changing the preg_match function and only allow a-zA-Z0-9 and some other like .,?characters or others to be used.

I know that is not the best practice to change the core files, but some times it can help you.