R regex: end anchors ($) stops working when preceeded by optional character (?)

Question

Why does my regex stops working when the end anchor ($) is preceeded by an optional caracter anchor (?)? For example

library(tidyverse
s   <- '[S|s][EG|eg]?'
s2  <- '^[S|s][EG|eg]?$'

[1] TRUE TRUE
c('s','Seg') %>% str_detect(s2) 
[1]  TRUE FALSE

Wrapping the regex with [ ] also did not work

s3  <- '^[[S|s][EG|eg]?]$'
c('s','Seg') %>% str_detect(s) 
c('s','Seg') %>% str_detect(s3) 
[1]  TRUE FALSE

Answer 1

^[[S|s][EG|eg]?]$ matches

• a single character in the list:

[ S | s

• and one optionnal character in the list:

E G | e g

• and a closing square bracket ]

https://regex101.com/r/Pr4IUq/1