perl -ne '$_=lc;s/\W+/ /g;@w{split /\s+/}=();END{$,="\n";print sort keys %w}'

##</code><code>##

while (<>) { # for each input line
 $_=lc;      # lowercase
 s/\W+/ /g;  # maps all (seqs of) non-word chars to space
 @wl=split /\s+/; # take the words of this line
 @w{@wl}=()  # put them as keys into a hash (undef values)
}
$,="\n";     # separate 'print' args with a newline
print sort keys %w; # print the sorted keys

##</code><code>##

tr -cs '[:alnum:]' '\n' < textfile |tr '[:upper:]' '[:lower:]'|sort|uniq