easy RegEx question: validating an alphanumeric-only username

[marcnyc]

Hello, I need to validate a username which can only be alphanumeric and maybe contain an underscore.

I wrote this:

function validate_username($v_username) {
return (eregi("[a-zA-Z0-9]",$v_username));
}

which does not validate "#$%" but does validate "Ann#$%"

Can somebody help me and tell me which modfier I am supposed to add where to make the username accept only alphanumeric characters and underscores?

Thansk a lot.

[LordShryku]

function validate_username($v_username) {
   return eregi('[^a-z0-9_]', $v_username) ? 'Invalid' : 'Valid';
}

True = Invalid in this.

[marcnyc]

Is there any way to have it written so that it is valid if it is true so that I don't have to change the rest of my code where I have already called this function?
Thanks

[LordShryku]

function validate_username($v_username) {
   return eregi('[^a-z0-9_]', $v_username) ? FALSE : TRUE;
} 

[marcnyc]

I don't see any changes in what you just posted from what you posted before... I am trying to find out how to match a-z A-Z 0-9 and the underscore so that if it matches then it is true, if it doesn't, it is false

[Weedpacket]

I see the changes.

[marcnyc]

Maybe I am crazy but I see the same pattern: [^a-z0-9_]

I am looking to have a function that returns true if the pattern is matched, this one returns false if the pattern is matched, or am I wrong?

[bpat1434]

It does. He's using the ternary operator.

IF the username contains any characters that are not [a-zA-Z0-9_], then it returns a FALSE (i.e. Invalid), otherwise TRUE (i.e. valid).

If you want it written out:

function validate_username($v_username) {
   return eregi('[^a-z0-9_]', $v_username) ? FALSE : TRUE;
}
// That is the same as this:
function validate_username($v_username){
   if(eregi('[^a-zA-Z0-9_]', $v_username))
   {
      // THe username contains an inavlid character
      return FALSE;
   }
   else
   {
      return TRUE;
   }
}

~Brett

[bradgrafelman]

Whoa... wait a sec, isn't this flipped?

function validate_username($v_username) { 
   return eregi('[^a-z0-9_]', $v_username) ? FALSE : TRUE; 
} 

Shouldn't it be 'TRUE : FALSE', i.e. the part before the colon in a ternary op is executed if the condition is true, the part after the colon is used if the condition results to FALSE?

[marcnyc]

Thanks for the explanation...
I was aware of the ternary operator, but I guess I was reading it wrong and thought that if it DOES match that pattern it'd be false... I guess the ^ negates the whole thing turing it around.
Thanks again.

[bpat1434]

Yes. If you read up on RegEx (which I'm no expert) it tells you that if the carrot "^" character is in a span field "[]", then it means NOT.

~Brett