Bash extended glob pattern negation is not working in parameter substitution

Question

Bash 3.2.57 (on macOS 12.6.8) extended glob pattern negation is not working in parameter substitution:

$ shopt -s extglob
$ s=abc
$ echo ${s##!(a)*}

I expected the last command to output abc, but it didn't output anything.

How can I make the third line output nothing if the first character of $s is not 'a', but output $s if the first character is 'a'?

Answer 1

See

$ shopt -s extglob
$ [[ abc == a!(b)c ]] && echo yes || echo no
no
$ [[ ac == a!(b)c ]] && echo yes || echo no
yes

And from man bash:

• !(pattern-list)

  Matches anything except one of the given patterns

So here the anything can be a null string. ^{(I think Bash's manual could be more explicit about this.)}

Answer 2

How can I make the third line output nothing if the first character of $s is not 'a', but output $s if the first character is 'a'?

case $s in a*) echo $s;; esac  # conditional execution

or

[[ $s =~ ^a ]] && echo $s      # conditional execution

or

[[ ${s:0:1} == a ]] && echo $s # conditional execution

or, more like what you tried,

echo ${s/#[^a]*/}              # string parsed to scrub nonmatch

Extended globbing not needed.