Validating User Input

I am trying to validate an user's input. I can get the ereg function to work if I just type in the pattern I'm searching for but my problem is I want to build the pattern through a variable first and then use that variable in the ereg function. For example, I want to check an input that's only text and only between 5 to 20 characters in length. Is this possible? function check_input($user_input, $min=0, $max=0, $text=false, $number=false, $special=false, $default="") { if ($text) $pattern .= "a-zA-Z"; if ($number) $pattern .= "0-9"; if ($special) $pattern .= "[:space:]"; if (ereg("^[$pattern]{$min,$max}$", $user_input)) return $user_input; else return $default; } Also, which of the following special characters is considered safe to accept? I am just allowing spaces now but would like as many of the below characters to be included. ~ ` ! @ # $ % ^ & * ( ) _ + - = [ ] \ { } | : " ; ' < > ? , . /