while read -r line;
do
sub1=$(echo $line|cut -f 1 -d ',');
sub2=$(echo $line|cut -f 2 -d ',');
sub3=$(echo $line|cut -f 3 -d ',');
sub4=$(echo $sub2'.*,'$sub3|sed -e "s/(.*)r/'1'/");
echo $sub1;
echo $sub2;
echo $sub3;
echo $sub4;
grep $sub4 modded_file.csv.dat;
done<input.csv

141
ps7
TILL WILL
'ps7.*,TILL WILL'
grep: WILL': No such file or directory

grep $sub4 modded_file.csv.dat;

grep -- "$sub4" modded_file.csv.dat;

awk -F "," 'NR==FNR { key[$1]=$2; value[$1]=$3; next }
    ($1 in key) && ($0 !~ "^" $1 ",.*," key[$1] "," value[$1] ",")' input.csv modded_file.csv.dat