Check if each non-blank line of text inside a cell ends with a date string. "(Completed mm/dd/yy)"

I assume this involves some REGEX work. Each separate line of text in a cell is separated by a blank line.

Here is a copy of my sheet

You could try:


=BYROW(C4:C,LAMBDA(in,IF(in="",,LET(_in,SPLIT(in,CHAR(10)),COUNTA(IFNA(FILTER(_in,REGEXMATCH(_in,"Completed \d{1,2}/\d{1,2}/\d{2}"))))=COUNTA(_in)))))


This is essentially splitting by the newline CHAR(10), filtering the values that contain Completed mm/d/yy and checking if the count of the filtered values is the same as the count of the unfiltered values.