regex replacement for whole object tree / reverse operation to `tostring` - jq

Question

So I have big json, where I need to take some subtree and copy it to other place, but with some properties updated (a lot of them). So for example:
{
"items": [
{ "id": 1, "other": "abc"},
{ "id": 2, "other": "def"},
{ "id": 3, "other": "ghi"}
]
}
and say, that i'd like to duplicate record having id == 2, and replace char e in other field with char x using regex. That could go (I'm sure there is a better way, but I'm beginner) something like:
jq '.items |= . + [.[]|select (.id == 2) as $orig | .id=4 | .other=($orig.other | sub("e";"x"))]'<sample.json
producing
{
"items": [
{
"id": 1,
"other": "abc"
},
{
"id": 2,
"other": "def"
},
{
"id": 3,
"other": "ghi"
},
{
"id": 4,
"other": "dxf"
}
]
}
Now that's great. But suppose, that there ins't just one other field. There are multitude of them, and over deep tree. Well I can issue multiple sub operations, but assuming, that replacement pattern is sufficiently selective, maybe we can turn the whole JSON subtree to string (trivial, tostring method) and replace all occurences using singe sub call. But how to turn that substituted string back to — is it call object? — to be able to add it back to items array?

Answer 1

Here's a program that might be a solution to the general problem you are describing, but if not at least illustrates how problems of this type can be solved. Note in particular that there is no explicit reference to a field named "other", and that (thanks to walk) the update function is applied to all candidate JSON objects in the input.
def update($n):
if .items | length > 0
then ((.items[0]|keys_unsorted) - ["id"]) as $keys
| if ($keys | length) == 1
then $keys[0] as $key
| (.items|map(.id) | max + 1) as $newid
| .items |= . + [.[] | select(.id == $n) as $orig | .id=$newid | .[$key]=($orig[$key] | sub("e";"x"))]
else .
end
else .
end;
walk(if type == "object" and has("items") then update(2) else . end)