It was added after 4.3.9.
bindkey '^R' history-incremental-search-backward
# Also enable my usual use of Ctrl-P/Ctrl-N to get the previous/next matching
# history entry.
-bindkey -M isearch '^P' history-incremental-search-backward
-bindkey -M isearch '^N' history-incremental-search-forward
+if [[ $ZSH_VERSION == (4.<4->*|<5->*) ]]; then
+ bindkey -M isearch '^P' history-incremental-search-backward
+ bindkey -M isearch '^N' history-incremental-search-forward
+fi
# Automatically push cd-ed directories on the directory stack.
setopt autopushd