///, 15 14 characters

/a/{":1234}/aa

(At least the output is 1 2 characters longer than the code.)

Try it online!

Thanks to:

• ETHproductions for reusing the object delimiters as part of key (-1 character)

Sample run:

bash-4.3$ slashes.pl <<< '/a/{":1234}/aa'
{":1234}{":1234}

Just to make it more readable:

bash-4.3$ slashes.pl <<< '/a/{":1234}/aa' | jq ''
{
  ":1234}{": 1234
}