Question

Allow letters, numbers and spaces (3 spaces maximum). How can i do it using regular expression?

I will the regular expression in PHP.

if(eregi("...HERE...", $_POST['txt_username']))
{
   //do something
}

Answer 1

You could use:

if(preg_match('@^(\w*\s){0,3}\w*$@', $_POST['txt_username)) {
    // do something
}

See it in action on: rubular.com

---

Note: \w includes the underscore (_). If you don’t want it, you can use:

if(preg_match('@^([^\W_]*\s){0,3}[^\W_]*$@', $_POST['txt_username)) {
    // do something
}

Instead.

EDIT: Since the OP decided to accept my answer, I added Justin’s improvements.