Here’s something that I implement in passwords, verification emails and if I programmed captcha I would without fail do. Remove all ambiguous characters from the captchas you generate. It is ridiculously simple to do. Less than 30 seconds editing of an array and it will save hundreds of mistyped/misinterpreted verifications and user frustration.  For example under some fonts the letter O and the number 0 look the same.

Characters to exclude are the letters o, b, z, q, g, s, i and the numbers to exclude are 0, 1, 2, 5

Yes, you can use these characters if they look sufficiently different in the font, but sometimes down the line you might change the font forgetting the consequences.

Posted on June 10th, 2007 by Ian Cheung
Filed under: Web Dev/IA