sed + mark line in case of marched word

Question

we want to mark with "#" the matched line by sed or perl line liner

for example

we want to mark all lines in file that include the DatePattern word

log4j.appender.DRFA.DatePattern=.yyyy-MM-dd

expected output

#log4j.appender.DRFA.DatePattern

note - in case line already marked then it will not add another "#" before the line

Answer 1

In case you don't want to remove the trailing date pattern (your spec is not quite clear), try

sed '/^[^#].*DatePattern/ s/^/#/' file

Answer 2

Prepending # to the line and also removing the = and whatever comes after it:

sed 's/^\([^#].*DatePattern\)=.*/#\1/' file

Alternatively, not removing the =:

sed 's/^[^#].*DatePattern/#&/' file