This is not regarding a specific stage.
The command echo -n “<input-string>” | ./your_program.sh -E “\d” does not work for some inputs like $$ or $?, because these are special characters and are being interpreted and hence echoed differently. Wondering if the command should be replaced with single quotes in the challenge (or perhaps this could be an additional stage where the user debugs this on their own? :p).
Just wanted to point this out in case this was unintended.