regex - Replace nth occurence of a character by another -

i hope isn't duplicated, didn't find answer , need regexp wizards.

i have string , replace second space found in \n, don't know how use indices (this way) in regular expression :

for example :

# have : "a b c d e f" # want : > "a b/nc d e f"

also know how can "repeat" replacement: each 2 occurences of space replace \n. example :

"a b c d e f" > "a b\nc d\ne f"

(\\s+\\s+\\s+)\\s+

you can use , replace \1\n or $1\n.see demo.

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