#! /bin/bash

printf 'bus\nit\nthe\nend\nis\nnigh\n' |
perl -0777ne '
    BEGIN {use re "debug"}
    /\A
        (?= .*? ^ (\w) (?!\1)(\w) (?!\1|\2)(\w) $)
        (?= .*? ^ \3 (?!\1|\2|\3)(\w) (?!\1|\2|\3|\4)(\w) $)
        (?= .*? ^ (?!\1|\2|\3|\4|\5)(\w) (?!\1|\2|\3|\4|\5|\6)(\w) $)
        (?= .*? ^ \4 \6 (?!\1|\2|\3|\4|\5|\6|\7)(\w) \2 $)
    /xsm and
    print "$1,$2,$3,$4,$5,$6,$7,$8.\n"
' 2>&1