regex - Getting 2 substrings/groups before and after last nth "_" -

lets @ exemple:

abc_def_ghi_jkl 

if choose n = 1, want output be:

group1 = abc_def_ghi group2 = jkl 

if choose n = 2 want output be:

group1 = abc_def group2 = ghi_jkl 

note _ seperated 2 groups removed.

for figured out how select last group select _:

(?:.(?!(?=\_)))+$ 

note2 focusing on regex part code used in r if helps solution.

a possibility split on nth occurrence of _ end of string:

strsplit("abc_def_ghi_jkl", "_(?=([^_]*_){0}[^_]*$)", perl = t)                                      #    ^                                      #  can modify quantifier here #[[1]]                                          #[1] "abc_def_ghi" "jkl"                    # split on 1st  strsplit("abc_def_ghi_jkl", "_(?=([^_]*_){1}[^_]*$)", perl = t) #[[1]] #[1] "abc_def" "ghi_jkl"                    # split on 2nd  strsplit("abc_def_ghi_jkl", "_(?=([^_]*_){2}[^_]*$)", perl = t) #[[1]] #[1] "abc"         "def_ghi_jkl"            # split on 3rd 

_(?=([^_]*_){2}[^_]*$) looks _ before pattern ([^_]*_){2}[^_]*$ via ?= ahead syntax , pattern starts end of string $ , skips non _ patterns [^_]* , matches ([^_]*_) number of occurrences , after split on specified _.

update str_match stringr package:

str_match("abc_def_ghi_jkl", "(.*)_((?:[^_]*_){0}[^_]*$)")[,2:3] # [1] "abc_def_ghi" "jkl"       str_match("abc_def_ghi_jkl", "(.*)_((?:[^_]*_){1}[^_]*$)")[,2:3] # [1] "abc_def" "ghi_jkl"  str_match("abc_def_ghi_jkl", "(.*)_((?:[^_]*_){2}[^_]*$)")[,2:3] # [1] "abc"         "def_ghi_jkl"