I've been trying to figure out a regex to output only three letters and also to delete the word "not"

What I've tried so far is:

* `.gsub(/^[A-z]+$/)` (still outputs the same as below)
* I also tried everything on [this post about Regex](https://stackoverflow.com/questions/3617797/regex-to-match-only-letters)

Here's what I need regexed: 

```
bash: line 1: drs: command not found
bash: line 2: tep: command not found
bash: line 3: ldo: command not found
bash: line 4: tep: command not found
bash: line 5: txw: command not found
bash: line 6: tep: command not found
bash: line 7: jfp: command not found
bash: line 8: mys: command not found
bash: line 9: jhf: command not found
bash: line 10: mjw: command not found
bash: line 11: czw: command not found
bash: line 12: txh: command not found
bash: line 13: krn: command not found
bash: line 14: sct: command not found
bash: line 15: jad: command not found

```

I want it to only output:

```
drs
tep
ldo
tep
txw
tep
jfp
mys
jhf
mjw
czw
txh
krn
sct
jad

```

Is there a way I can do this? Please keep in mind I have multiple other three letter combinations, with all letters of the alphabet.