character_vector <- c("<br>Hello</br>", "I want to keep this <important text> and <string?>")
exclude <- c("important text", "string?")
regex.escape <- function(string) {
  gsub("([][{}()+*^$|\\\\?.])", "\\\\\\1", string)
}
gsub(paste0("<(?!(?:", paste(regex.escape(exclude), collapse="|"), ")>)[^>]*>"), "", character_vector, perl=TRUE)