Use sed to replace every character by itself followed by $n times a char?

Question

I'm trying to run the command below to replace every char in DECEMBER by itself followed by $n question marks. I tried both escaping {$n} like so {$n} and leaving it as is. Yet my output just keeps being D?{$n}E?{$n}... Is it just not possible to do this with a sed?

How should i got about this.

echo 'DECEMBER' > a.txt
sed -i "s%\(.\)%\1\(?\){$n}%g" a.txt
cat a.txt

Answer 1

Range (also, interval or limiting quantifiers), like {3} / {3,} / {3,6}, are part of regex, and not replacement patterns.

You can use

sed -i "s/./&$(for i in {1..7}; do echo -n '?'; done)/g" a.txt

See the online demo:

#!/bin/bash
sed "s/./&$(for i in {1..7}; do echo -n '?'; done)/g" <<< "DECEMBER"
# => D???????E???????C???????E???????M???????B???????E???????R???????

Here, . matches any char, and & in the replacement pattern puts it back and $(for i in {1..7}; do echo -n '?'; done) adds seven question marks right after it.