Remove invalid UTF-8 characters from a string

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.

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.

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.