Convert unicode code point to literal character in Go

Let's say I have a text file like this.

\u0053
\u0075
\u006E

Is there a way I can convert that to this?

S
u
n

Currently, I'm using ioutil.ReadFile("data.txt"), but when I print the data, I get the unicode code points instead of the string literals. I realize this is the correct behavior for ReadFile, it's just not want I want.

I'm aiming for a substitution of the code points with their literal characters.

You can use the strconv.Unquote() and strconv.UnquoteChar() functions to do the conversion.

One thing you should be aware of is that strconv.Unquote() can only unquote strings that are in quotes (e.g. start and end with a quote char " or a back quote char `), so we have to manually append that.

See this example:

lines := []string{
	`\u0053`,
	`\u0075`,
	`\u006E`,
}
fmt.Println(lines)

for i, v := range lines {
	var err error
	lines[i], err = strconv.Unquote(`"` + v + `"`)
	if err != nil {
		fmt.Println(err)
	}
}
fmt.Println(lines)

fmt.Println(strconv.Unquote(`"Go\u0070\x68\x65\x72"`))

Output (try it on the Go Playground):

[\u0053 \u0075 \u006E]
[S u n]
Gopher <nil>

If the strings you want to unquote contain the escape sequence of a single rune (or you just want to unquote the first rune), you may use strconv.UnquoteChar(). This is how it looks like (note: no quoting of the input is needed in this case, like it was needed for strconv.Unquote()):

runes := []string{
	`\u0053`,
	`\u0075`,
	`\u006E`,
}
fmt.Println(runes)

for _, v := range runes {
	var err error
	value, _, _, err := strconv.UnquoteChar(v, 0)
	if err != nil {
		fmt.Println(err)
	}
	fmt.Printf("%c\n", value)
}

This will output (try it on the Go Playground):

[\u0053 \u0075 \u006E]
S
u
n

A slightly different approach is using strconv.ParseInt, this generates less garbage and uses less internal logic (Unquote does a lot of other checks) for parsing the lines:

for i, v := range lines {
	if len(v) != 6 {
		continue
	}

	if r, err := strconv.ParseInt(v[2:], 16, 32); err == nil {
		lines[i] = string(r)
	}
}

<kbd>playground</kbd>

You can use this:

import "github.com/chzyer/readline/runes"

// unicodeUnquote converts unicode points such as \u0053 to UTF8 encoding.
func unicodeUnquote(bs []byte) []byte {
	unicodeEscapeRx := regexp.MustCompile(`\\u[0-9a-fA-F]{4}`)
	return unicodeEscapeRx.ReplaceAllFunc(bs, func(code []byte) []byte {
		rune, _, _, _ := strconv.UnquoteChar(string(code), 0)
		width := runes.Width(rune)
		runeBytes := make([]byte, width)
		utf8.EncodeRune(runeBytes, rune)
		return runeBytes
	})
}

A full example is at https://go.dev/play/p/ElIGZvJNyEF.