Regular Expressions Help

[Namadoor]

Hello, I want to create a regular expression that allows users to only enter letters, numbers, underscores and dashes in their username. I used the ereg function like this:

if (!ereg("[a-zA-Z0-9_\-]", "$_POST[username]")

This however only returns false if all the characters entered are illegal characters. What did I do wrong, and what should it be?

Thank you for your time.

[LordShryku]

Try this:

if(eregi("[^_a-z0-9-]", $_POST['username'])) {
   echo "Invalid";
}
else {
   echo "Valid";
}