a <- c("it is ZZ10ASDJN123 and ZZ100DD22")
gsub("\\s*\\b[A-Za-z]{2}\\d{2}(?!\\d)\\w*", "", a, perl=TRUE)
gsub("\\s*\\b[A-Za-z]{2}\\d{2}[A-Za-z_]\\w*\\b", "", a)