Graham Keyarn This can be done using a xt:include with a default for 'non existing' files

<form action="<xt:url>" method="post"> <input name="user" value="{_$user|}" required="required" /> <input type="submit"> </form> <xt:include default="/members/no_user" file="/members/{_$user|default}" />

You then create files for each username
plus
'default' for the empty form
'no_user' for incorrect entries
http://auldphart.yn.lt/members/

demo http://auldphart.yn.lt/0000
graham and keyarn are valid entries