a <- (cbind(c(1,2,3,4,5,6,7),c("a,z","b","c,z","d,z","e","f,z","g")))
a <- as.data.frame(a)
colnames(a) <- c("num","abc")
# Your example using dplyr::filter to find "b"
dplyr::filter(a, abc == "b")
#> num abc
#> 1 2 b
# using base::grepl to repeat that search "b"
a[grepl("b",a$abc), ]
#> num abc
#> 2 2 b
# and, your request, to find any "z"s
a[grepl("z",a$abc), ]
#> num abc
#> 1 1 a,z
#> 3 3 c,z
#> 4 4 d,z
#> 6 6 f,z