Hello kh99,
I have a similar requirement where in I want to customize the phrase - "usernametaken".
1)For an already existing user name the error should be -
That username is already in use. If you are {1} and you have forgotten your password, click <a href="login.php?{2}do=lostpw">here</a>.
2)For special characters and space the error should be -
That username does not meet the administrator's standards.
Any idea how I do it? Have also raised it here -
https://vborg.vbsupport.ru/showthread.php?t=304430
Thanks in advance.