Insert a text with indentation persevered on the next line of matched string

I have a file with a yaml data as below:

cat x.yml
foo:
 - bar: 1
 - zoo: 2

I am able to insert the text but this is messing the indentation(see 2nd line):

sed -r '/^[ ]+- bar:/a- hoo: 3' x.yml
foo:
 - bar: 1
- hoo: 3
 - zoo: 2

Then, I tried to backreference the leading spaces but seems like it is not working with /a flag.

sed -r '/^([ ]+)- bar:/a\1- hoo: 3' x.yml
foo:
 - bar: 1
1- hoo: 3
 - zoo: 2

Any help to get the following using a one-liner ?

foo:
 - bar: 1
 - hoo: 3
 - zoo: 2

>Solution :

I suggest to switch to GNU sed’s s command:

sed -E 's/( *)- bar:.*/&\n\1- hoo: 3/' file

Output:

foo:
 - bar: 1
 - hoo: 3
 - zoo: 2

See: man sed and The Stack Overflow Regular Expressions FAQ