Changes in configuration file
Copy recaptchalib.php from the recaptcha download into the /includes/ucp directory

Find this code in ucp_register.php:

}
$confirm_image = '';
$s_hidden_fields .= '';
}

Unlike phpBB3 Captcha, reCAPTCHA does not require any extra authentication box.

...and add this directly underneath:

include_once('recaptchalib.php');
     $publickey = '6LcKEggAAAAAAIjIdnGkyP7xrex_DPK-UCITnft4';
     $confirm_image = recaptcha_get_html($publickey);
 Find this code ...in ucp_register.php
else
{
$error[] = $user->lang['CONFIRM_CODE_WRONG'};
$wrong_confirm = true;
}
}
else
{
$error[] = $user->lang['CONFIRM_CODE_WRONG'];
$wrong_confirm = true;
}
}
}
*/
...and then add the following code just below the previous code. This does reCAPTCHA authentication.

         require_once('recaptchalib.php');
             $privatekey = '6LcKEggAAAAAAEYH6L6FWP1LnZY27t5X5CTFD779';
             $resp = recaptcha_check_answer ($privatekey,
             $_SERVER["REMOTE_ADDR"],
             $_POST["recaptcha_challenge_field"],
             $_POST["recaptcha_response_field"]);
  if (!$resp->is_valid)
{
//die ("The reCAPTCHA wasn't entered correctly. Go back and try it again." ."(reCAPTCHA said: " . $resp->error . ")");

$error[] = $user->lang['CONFIRM_CODE_WRONG'];

  wrong_confirm = true;   }

Lastly comment the following

//'confirm_code' => array('string', !$config['enable_confirm'], CAPTCHA_MIN_CHARS, CAPTCHA_MAX_CHARS),

This will help you in installing reCAPTCHA easily.

To understand how reCAPTCHA works, please register with http://forums.pcquest.com