Restricting characters on registration form

[Mutley]

How would I restrict characters on a registration form then it redirect to the form with the error displayed at the top?

[papaface]

Look into ereg http://php.net/manual/en/function.ereg.php

[Mutley]

I've got the basic idea but how would I tie this to a form?

[code]if (ereg ("(~`!_@#$%^&*()=+{}|:;"/?,)", $)) {
  echo "$";
} else {
  echo "Punctuation is not allowed.";
}[/code]