===== Remove lines with duplicate letters =====
==== Task ====
Find five letter words that are made up of letters {h, i, k, e, r, s} where each letter appears only once.
==== Issue ====
<code>
grep -Ei "^[hikers]{5}$" /usr/share/dict/american-english
</code>
gives
<code>
Essie
Hesse
Irish
Kerri
Reese
Sheri
...
</code>
where some letters are duplicated in some of the words. For example, the letter 'i' appears twice in the word 'Irish'. We need to filter out all such words.

==== Solution ====
Pipe the output to ''grep -viE '(.).*\1' ''. For example

<code>
% grep -Ei "^[hikers]{5}$" /usr/share/dict/american-english | grep -viE "(.).*\1"
Sheri
Shrek
heirs
hiker
hikes
hires
sheik
shire
shirk
skier
</code>

==== Ref ====
  * https://unix.stackexchange.com/questions/352927/delete-all-lines-that-contain-duplicate-letters -> comment by Stéphane Chazelas
    * I got the idea from here.
  * (Mastering Regular Expressions, Jeffrey E.F. Friedl, 2006) book -> Chapter 1 "Introduction to Regular Expressions" -> section "Parentheses and Backreferences" -> pages 20 through 22 go over this concept.