Is it better to display an error if user input is invalid or automatically fix it?

[Samishii23]

Making a login script.
Should I throw an error if a user puts invalid characters into the form and submits it, or trim them automatically?

[Markus]

You should error and inform them *why*.

[Samishii23]

Thanks. But now after a few hours of searching google... I haven't really been able to find a code example of this logic: If theres anything else BESIDES [a-zA-Z0-9] = true

[Markus]

That's simply done with a regular expression:

if (preg_match('/[^a-zA-Z0-9]/', $text))
{
  // Found a non-alphanumeric character
}

The caret (^) in the above character-set means 'not in this character set'. So the above expression says 'match anything that is not a-z, A-Z or 0-9 - alphanumeric.

[Samishii23]

Forgive the n00b questions. But what are the / for?

[JKing]

The / is the delimiter or start and end of the regex. You can use any non alpha numeric character as the delimiter. In some cases it may make more sense to use a % or other symbol if your regex contains several forward slashes then you don't have to escape all of them using a backslash.