Basic syntax checking can be done with the 'filter_username' policy. From the FreeRADIUS documents:

<code>
#       Filter the username
#
#  Force some sanity on User-Name. This helps to avoid issues
#  issues where the back-end database is "forgiving" about
#  what constitutes a user name.
</code>

Example usage:
<code>
authorize {
        filter_username
        loop_prevent
        operator-name
        suffix
}
</code>

To filter out unwanted realms, such as 'hotmail.com' can be done simply by using the 'realm' command:

<code>
## Filter out NULL realms e.g. Username = fred

realm NULL {
}

## Filter out realms that aren't every going to be valid Govroam realms e.g. Username = fred@hotmail.com

realm "~hotmail\\.com$" {
}

realm "~hotmail\\.co\\.uk$" {
}

realm "~.*\\.3gppnetworks\\.org$" {
}

realm "~.*\\.3gppnetwork\\.org$" {
}

realm "~gmail\\.com" {
}

realm "~googlemail\\.com" {
}

realm "~live\\.com" {
}

realm "~outlook\\.com" {
}

realm "~yahoo\\.com" {
}

realm "~yahoo\\.cn" {
}

realm "~unimail\\.com" {
}

realm "~yahoo\\.co\\.uk" {
}

realm "~myabc\\.com" {
}
</code>

alternatively

<code>
check_bad_realms {
        if (&User-Name) {
                ## reject usernames that end with realms that are never going to be govroam/eduroam realms
                ## is there a way to automate this??!

                ## stuff here will be matched for just eduroam
                if (&control:Proxy-To-Realm =~ /^PROXY-JANET-EDUROAM$/)

                        if (&User-Name =~ /test\.net$/) {
                                update request {
                                        &Module-Failure-Message += 'Rejected: Bad Realm'
                                }
                                reject
                        }


                        if (&User-Name =~ /@.*bathnes/) {
                                update request {
                                        &Module-Failure-Message += 'Rejected: Bad Realm'
                                }
                                reject
                        }
                }


                ## stuff here will be matched for just govroam
                if (&control:Proxy-To-Realm =~ /^PROXY-JANET-GOVROAM$/)

                        if (&User-Name =~ /that\.host$/) {
                                update request {
                                        &Module-Failure-Message += 'Rejected: Bad Realm'
                                }
                                reject
                        }
                }

                ## stuff here will be matched for both govroam and eduroam
                ## Common realm provided by phone manufacturers by default
                if (&User-Name =~ /3gppnetwork\.org$/) {
                        update request {
                                &Module-Failure-Message += 'Rejected: Bad Realm'
                        }
                        reject
                }

                ## Username is not an email address.
                if (&User-Name =~ /gmail\.com$/) {
                        update request {
                                &Module-Failure-Message += 'Rejected: Bad Realm'
                        }
                        reject
                }
                ## Ends in '.'
                if (&User-Name =~ /bath\.ac\.uk.+$/) {
                        update request {
                                &Module-Failure-Message += 'Rejected: Bad Realm'
                        }
                        reject
                }


            
                ## Ends in a single character - all TLDs are 2+ characters.
                if (&User-Name =~ /\.[a-z0-9]$/) {
                        update request {
                                &Module-Failure-Message += 'Rejected: Bad Realm'
                        }
                        reject
                }

                ## All subrealms should have more than one character
                if (&User-Name =~ /\.[a-z0-9]\.uk$/) {
                        update request {
                                &Module-Failure-Message += 'Rejected: Bad Realm'
                        }
                        reject
                }


                ## Misspellings and misunderstandings
                if (&User-Name =~ /ac\.uk\.com$/) {
                        update request {
                                &Module-Failure-Message += 'Rejected: Bad Realm'
                        }
                        reject
                }

                if (&User-Name =~ /\.acuk$/) {
                        update request {
                                &Module-Failure-Message += 'Rejected: Bad Realm'
                        }
                        reject
                }


                if (&User-Name =~ /@ac\.uk$/) {
                        update request {
                                &Module-Failure-Message += 'Rejected: Bad Realm'
                        }
                        reject
                }

                ## Starts with a '.'
                if (&User-Name =~ /\.bath\.ac\.uk$/) {
                        update request {
                                &Module-Failure-Message += 'Rejected: Bad Realm'
                        }
                        reject
                }
                
                ## Some typical inappropriate realms.
                if (&User-Name =~ /@myabc\.com$/) {
                        update request {
                                &Module-Failure-Message += 'Rejected: Bad Realm'
                        }
                        reject
                }

                if (&User-Name =~ /@.*hotmail\.co/) {
                        update request {
                                &Module-Failure-Message += 'Rejected: Bad Realm'
                        }
                        reject
                }

                if (&User-Name =~ /@.*live\.co$/) {
                        update request {
                                &Module-Failure-Message += 'Rejected: Bad Realm'
                        }
                        reject
                }

                if (&User-Name =~ /\.local$/) {
                        update request {
                                &Module-Failure-Message += 'Rejected: Bad Realm'
                        }
                        reject
                }

                if (&User-Name =~ /\.guest$/) {
                        update request {
                                &Module-Failure-Message += 'Rejected: Bad Realm'
                        }
                        reject
                }

                if (&User-Name =~ /@.*gmail\.co/) {
                        update request {
                                &Module-Failure-Message += 'Rejected: Bad Realm'
                        }
                        reject
                }

                if (&User-Name =~ /googlemail\.com$/) {
                        update request {
                                &Module-Failure-Message += 'Rejected: Bad Realm'
                        }
                        reject
                }

                if (&User-Name =~ /outlook\.com$/) {
                        update request {
                                &Module-Failure-Message += 'Rejected: Bad Realm'
                        }
                        reject
                }

                if (&User-Name =~ /@.*yahoo\.c/) {
                        update request {
                                &Module-Failure-Message += 'Rejected: Bad Realm'
                        }
                        reject
                }

                if (&User-Name =~ /unimail\.com$/) {
                        update request {
                                &Module-Failure-Message += 'Rejected: Bad Realm'
                        }
                        reject
                }

                if (&User-Name =~ /blueyonder\.co\.uk$/) {
                        update request {
                                &Module-Failure-Message += 'Rejected: Bad Realm'
                        }
                        reject
                }



       } else {
                update request {
                        &Module-Failure-Message += 'Rejected: erm... WTF?!'
                }
                reject
        }
}

</code>

Many thanks to Matt Richards from Bath University for this code.