First Re may not be empty

Question

I have written a small piece of code, which it take regex expression from variable and put into sed. When I run the script I get something like this.

sed: first RE may not be empty

The script delete lines according to specifications from a variable. variable could be empty.

NR=$1

sed '/'"${NR}"'/d' in.txt > out.txt

How can I fix it or is it some better function which I can use it? :)

Answer 1

You can use:

# re is set to $1 or $^ if $1 is not provided
re="${1:-\$^}"

# $^ being an invalid regex will NOT delete anything 
sed "/$re/d" in.txt > out.txt