FINDBACK() is a little awkward to implement without regex in a plain formula, but not impossible.
Here’s how the final formula is built up, exploded in digestable steps:
First find how many non-spaces there are:
=LEN(SUBSTITUTE(A1," “,”"))
Find how many spaces there are:
=LEN(A1)-LEN(SUBSTITUTE(A1," “,”"))
Replace the space at the [number of spaces]-th occurrence with a marker (choose an unused character):
=SUBSTITUTE(A1," “,“☿”,(LEN(A1)-LEN(SUBSTITUTE(A1,” “,”"))))
Find the index of the marker:
=FIND(“☿”,SUBSTITUTE(A1,“0”,“☿”,(LEN(A1)-LEN(SUBSTITUTE(A1,“0”,"")))))
Credits: https://stackoverflow.com/a/25820019/1052284 and yours truly
Note that this crashes with “Err:502” if there are no spaces. To cover such cases it gets even more unwieldy:
=IF(LEN(SUBSTITUTE(A1," “,”"))=LEN(A1),LEN(A1),FIND(“☿”,SUBSTITUTE(A1," “,“☿”,(LEN(A1)-LEN(SUBSTITUTE(A1,” “,”"))))))