grep - why do brackets in grep pattern remove the grep process from ps results?

Question

Why do brackets in a grep pattern remove the grep process from ps results?

$ ps -ef | grep XXXX

[...] XXXX
[...] grep XXXX


$ ps -ef | grep [X]XXX

[...] XXXX

Answer 1

When you run ps -ef | grep string, grep is displayed in the output because string matches [...] grep string.

But, when you run ps -ef | grep [s]tring the line isn't displayed, because grep translates [s]tring to string, while ps outputs [...] grep [s]tring, and that doesn't match string

Answer 2

Because the brackets need to be escaped, for bash once and for grep again:

$ ps -ef | grep \\[X\\]XXX

[...] XXXX
[...] grep XXXX


$ ps -ef | grep "\[X\]XXX"

[...] XXXX
[...] grep XXXX