Remove invalid UTF-8 characters from a string - json

Question

I get this on json.Marshal of a list of strings:
json: invalid UTF-8 in string: "...ole\xc5\"
The reason is obvious, but how can I delete/replace such strings in Go? I've been reading docst on unicode and unicode/utf8 packages and there seems no obvious/quick way to do it.
In Python for example you have methods for it where the invalid characters can be deleted, replaced by a specified character or strict setting which raises exception on invalid chars. How can I do equivalent thing in Go?
UPDATE: I meant the reason for getting an exception (panic?) - illegal char in what json.Marshal expects to be valid UTF-8 string.
(how the illegal byte sequence got into that string is not important, the usual way - bugs, file corruption, other programs that do not conform to unicode, etc)

Answer 1

In Go 1.13+, you can do this:
strings.ToValidUTF8("a\xc5z", "")
In Go 1.11+, it's also very easy to do the same using the Map function and utf8.RuneError like this:
fixUtf := func(r rune) rune {
if r == utf8.RuneError {
return -1
}
return r
}
fmt.Println(strings.Map(fixUtf, "a\xc5z"))
fmt.Println(strings.Map(fixUtf, "posic�o"))
Output:
az
posico
Playground: Here.

Answer 2

For example,
package main
import (
"fmt"
"unicode/utf8"
)
func main() {
s := "a\xc5z"
fmt.Printf("%q\n", s)
if !utf8.ValidString(s) {
v := make([]rune, 0, len(s))
for i, r := range s {
if r == utf8.RuneError {
_, size := utf8.DecodeRuneInString(s[i:])
if size == 1 {
continue
}
}
v = append(v, r)
}
s = string(v)
}
fmt.Printf("%q\n", s)
}
Output:
"a\xc5z"
"az"
Unicode Standard
FAQ - UTF-8, UTF-16, UTF-32 & BOM
Q: Are there any byte sequences that are not generated by a UTF? How
should I interpret them?
A: None of the UTFs can generate every arbitrary byte sequence. For
example, in UTF-8 every byte of the form 110xxxxx2 must be followed
with a byte of the form 10xxxxxx2. A sequence such as <110xxxxx2
0xxxxxxx2> is illegal, and must never be generated. When faced with
this illegal byte sequence while transforming or interpreting, a UTF-8
conformant process must treat the first byte 110xxxxx2 as an illegal
termination error: for example, either signaling an error, filtering
the byte out, or representing the byte with a marker such as FFFD
(REPLACEMENT CHARACTER). In the latter two cases, it will continue
processing at the second byte 0xxxxxxx2.
A conformant process must not interpret illegal or ill-formed byte
sequences as characters, however, it may take error recovery actions.
No conformant process may use irregular byte sequences to encode
out-of-band information.

Answer 3

Another way to do this, according to this answer, could be
s = string([]rune(s))
Example:
package main
import (
"fmt"
"unicode/utf8"
)
func main() {
s := "...ole\xc5"
fmt.Println(s, utf8.Valid([]byte(s)))
// Output: ...ole� false
s = string([]rune(s))
fmt.Println(s, utf8.Valid([]byte(s)))
// Output: ...ole� true
}
Even though the result doesn't look "pretty", it still nevertheless converts the string into a valid UTF-8 encoding.