Objective: Find all lines in a file that only contain alpha characters and delete or replace.

Sample contents of file.

 Z2j2NH23
 VTQnPwSS
 hFbxgvFt
 VSVR8v3F
 GPrP4zo

The following sed command works for our objective.

sed s/[[:alpha:]]\{8\}/ALPHAONLY/g file.txt

The part in the [] tells sed to search for any alpha characters a-Z, the part in bold \{8\} tells it to search 8 spaces out (Change if needed) and ALPHAONLY is what alpha line will get substituted to.

sed s/[[:alpha:]]\{8\}/ALPHAONLY/g

Returns

Z2j2NH23
ALPHAONLY 
ALPHAONLY 
VSVR8v3F
GPrP4zo9

You can run the same basic syntax in VI

Search and replace

:%s/[[:alpha:]]\{8\}/ALPHAONLY/g 

Or to delete the lines

:%d/[[:alpha:]]\{8\}/d

You can also change [[:alpha:]] for [[:digit:]] if you want to search for numbers instead.