Remove or transform user names before authentication?

[Peter Schober]

* Brian Mathis <brian.mathis at gmail.com> [2016-12-02 06:42]:
> I am using IdP 3 to authenticate against an LDAP backend.  The LDAP
> backend has a restriction on characters allowed in usernames, so I
> would like to be able to transform or remove those characters before
> submitting to LDAP.

How are people with such characters in their userid/netid/login name
suppose to authenticate against that directory (and why do they issue
such identifuers then if they cannot handle them)? Or does the IDP (or
the LDAP DSA) fall over dead and you merely want to avoid software
errors?
Confused,
-peter