# If you'd like to omit non-matching lines from the result; add ';d' to the end of the expression.
sed -E 's/^((?:[^+\n]*\+){7})\d{8}(?=\+(?:\d{8})?\+)/\g<1>20200101/gm;t' <<< "UNA:+,? '
UNB+UNOC:3+000000000+000000000+20190801:1115+00001+DDMP190001'
UNH+00001+BRKE:01+00+0'
INV+ED Format 1+Brustkrebs+19880117+E000000001+20080702+++1+0'
FAL+087897044+0000000++Trägername+000000000+0+20080702++1+++J+N+N+N+N+N+++0'
INL+181095200+385762115+++0'
BEE+20080702++++0'
BAA+++J+J++++++J+++++++J++0'
BBA++++++++J++++++J+J++++++J+++++J+++J+J++++++++J+0'
BHP+J+++++J+++++J+++++0'
BLA+++J+++++++++0'
BFA++++++++++++J++0'
BSA++J+++J+J+++0'
BAT+20190801+0'
DAT+20080702++++0'
UNT+000014+00001'
UNZ+00001+00001'
"
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for SED, please visit: https://www.gnu.org/software/sed/manual/html_node/The-_0022s_0022-Command.html