I'd suggest that you verify that it only contains valid characters. So if your usernames can contain alpha and '.' only, you could use something like (untested): 
if($display =~ /[^a-b\.]/i){
    # error condition here
}

[download]
Modify the regex to add additional conditions to suit your exact needs.

In reply to Re: validation of posted data. by fs
in thread validation of posted data. by hostux

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