Submitting form via Ajax and inserting into DB

I checked a couple of tutorials but couldn’t find any validation for especially comment/message field. Any specific tips for that? The comment I expect is normal text in a couple of sentences or paragraphs with regular punctuation (a-zA-Z0-9 .,;?!:-()&'"tab,new line etc.). Shall I use a preg_replace to filter out anything else? What approach would you suggest?