- preg_match: documentation ( source)
<?php
$not_allowed_symbols = '[^a-z0-9:;,!\'\s*.?()"-]';
//$letter = nl2br("aaa\nbbb\nccc");
//print_r($letter);
if(preg_match("/$not_allowed_symbols/i", $letter)){
echo "There are not allowed symbols in the text";
}else{
echo "There are allowed symbols in the text";
}