Print POSIX character class

Question

Given a class, such as

[:digit:]

I would like the output to be

0123456789

Note, the method should work for all POSIX character classes. Here is what I have tried

$ printf %s '[:digit:]'
[:digit:]

§ Character classes

Answer 1

I'm sure there's a better way but here's a brute force method:

for i in {0..127}; do 
    char=$(printf \\$(printf '%03o' "$i"))
    [[ $char =~ [[:alpha:]] ]] && echo "$char"
done

Loop through all the decimal character values, convert them to the corresponding ASCII character and test them against the character class.

The range might be wrong but the check seems to work.

As others have mentioned in the comments, it is also possible to use the == operator instead of the =~ in this case, which may be slightly faster.

Answer 2

$ seq 126 | awk '{printf "%c", $0}' | grep -o '[[:digit:]]'
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