How to extract part from string?

If I have this string:

bb=c("\\dat\\Bjkjgf_Yloiut_dsezr_proj_111_999.txt","\\dat\\Bjkjgf_Yloezr_proj_111_999.txt")

I need to extract whatever between \\ and _proj

i can do this, but will work for one and not the other:

substr(bb, 1, 15)

the expected output

Bjkjgf_Yloiut_dsezr,Bjkjgf_Yloezr

Using gsub

gsub(".*\\\\|_proj.*", "", bb)
[1] "Bjkjgf_Yloiut_dsezr" "Bjkjgf_Yloezr"

With regmatches:

sapply(regmatches(bb, regexec("dat\\\\(.*)_proj", bb)), `[`, 2)
#[1] "Bjkjgf_Yloiut_dsezr" "Bjkjgf_Yloezr"   

Or stringr::str_match:

stringr::str_match(bb, "dat\\\\(.*)_proj")[, 2]