I see your purpose is to make the map value contain colons.
But I wonder why not use a light mode to fix it, for example, use strings.IndexRune to find the first colon and split the string into two parts, a key and a value.

Hello @sdghchj, initially I went with this approach to allow escaped double colons in the key value, but thinking better about it it does not make much sense and I can't think of a valid use case for it.

I've refactored the code using strings.SplitN to split at the first ocurrence of a :, this makes the change much simpler and from my tests it won't be necessary to escape the remaining double colons in the value, so for example the struct from my issue would work without any changes:

type Foo struct {
  Attributes map[string]string `example:"bucket_arn:arn:aws:::foobar-bucket"`
}