Package utf8

Constants

Numbers fundamental to the encoding.

const (
    RuneError = '\uFFFD'     // the "error" Rune or "Unicode replacement character"
    RuneSelf  = 0x80         // characters below RuneSelf are represented as themselves in a single byte.
    MaxRune   = '\U0010FFFF' // Maximum valid Unicode code point.
    UTFMax    = 4            // maximum number of bytes of a UTF-8 encoded Unicode character.
)

func AppendRune ¶ 1.18

func AppendRune(p []byte, r rune) []byte

AppendRune appends the UTF-8 encoding of r to the end of p and returns the extended buffer. If the rune is out of range, it appends the encoding of RuneError.

buf1 := utf8.AppendRune(nil, 0x10000)
buf2 := utf8.AppendRune([]byte("init"), 0x10000)
fmt.Println(string(buf1))
fmt.Println(string(buf2))

𐀀
init𐀀

func DecodeLastRune ¶

func DecodeLastRune(p []byte) (r rune, size int)

DecodeLastRune unpacks the last UTF-8 encoding in p and returns the rune and its width in bytes. If p is empty it returns (RuneError, 0). Otherwise, if the encoding is invalid, it returns (RuneError, 1). Both are impossible results for correct, non-empty UTF-8.

An encoding is invalid if it is incorrect UTF-8, encodes a rune that is out of range, or is not the shortest possible UTF-8 encoding for the value. No other validation is performed.

b := []byte("Hello, 世界")

for len(b) > 0 {
    r, size := utf8.DecodeLastRune(b)
    fmt.Printf("%c %v\n", r, size)

    b = b[:len(b)-size]
}

界 3
世 3
  1
, 1
o 1
l 1
l 1
e 1
H 1

func DecodeLastRuneInString ¶

func DecodeLastRuneInString(s string) (r rune, size int)

DecodeLastRuneInString is like DecodeLastRune but its input is a string. If s is empty it returns (RuneError, 0). Otherwise, if the encoding is invalid, it returns (RuneError, 1). Both are impossible results for correct, non-empty UTF-8.

An encoding is invalid if it is incorrect UTF-8, encodes a rune that is out of range, or is not the shortest possible UTF-8 encoding for the value. No other validation is performed.

str := "Hello, 世界"

for len(str) > 0 {
    r, size := utf8.DecodeLastRuneInString(str)
    fmt.Printf("%c %v\n", r, size)

    str = str[:len(str)-size]
}

界 3
世 3
  1
, 1
o 1
l 1
l 1
e 1
H 1

func DecodeRune ¶

func DecodeRune(p []byte) (r rune, size int)

DecodeRune unpacks the first UTF-8 encoding in p and returns the rune and its width in bytes. If p is empty it returns (RuneError, 0). Otherwise, if the encoding is invalid, it returns (RuneError, 1). Both are impossible results for correct, non-empty UTF-8.

An encoding is invalid if it is incorrect UTF-8, encodes a rune that is out of range, or is not the shortest possible UTF-8 encoding for the value. No other validation is performed.

b := []byte("Hello, 世界")

for len(b) > 0 {
    r, size := utf8.DecodeRune(b)
    fmt.Printf("%c %v\n", r, size)

    b = b[size:]
}

H 1
e 1
l 1
l 1
o 1
, 1
  1
世 3
界 3

func DecodeRuneInString ¶

func DecodeRuneInString(s string) (r rune, size int)

DecodeRuneInString is like DecodeRune but its input is a string. If s is empty it returns (RuneError, 0). Otherwise, if the encoding is invalid, it returns (RuneError, 1). Both are impossible results for correct, non-empty UTF-8.

An encoding is invalid if it is incorrect UTF-8, encodes a rune that is out of range, or is not the shortest possible UTF-8 encoding for the value. No other validation is performed.

str := "Hello, 世界"

for len(str) > 0 {
    r, size := utf8.DecodeRuneInString(str)
    fmt.Printf("%c %v\n", r, size)

    str = str[size:]
}

H 1
e 1
l 1
l 1
o 1
, 1
  1
世 3
界 3

func EncodeRune ¶

func EncodeRune(p []byte, r rune) int

EncodeRune writes into p (which must be large enough) the UTF-8 encoding of the rune. If the rune is out of range, it writes the encoding of RuneError. It returns the number of bytes written.

r := '世'
buf := make([]byte, 3)

n := utf8.EncodeRune(buf, r)

fmt.Println(buf)
fmt.Println(n)

[228 184 150]
3

runes := []rune{
    // Less than 0, out of range.
    -1,
    // Greater than 0x10FFFF, out of range.
    0x110000,
    // The Unicode replacement character.
    utf8.RuneError,
}
for i, c := range runes {
    buf := make([]byte, 3)
    size := utf8.EncodeRune(buf, c)
    fmt.Printf("%d: %d %[2]s %d\n", i, buf, size)
}

0: [239 191 189] � 3
1: [239 191 189] � 3
2: [239 191 189] � 3

func FullRune ¶

func FullRune(p []byte) bool

FullRune reports whether the bytes in p begin with a full UTF-8 encoding of a rune. An invalid encoding is considered a full Rune since it will convert as a width-1 error rune.

buf := []byte{228, 184, 150} // 世
fmt.Println(utf8.FullRune(buf))
fmt.Println(utf8.FullRune(buf[:2]))

true
false

func FullRuneInString ¶

func FullRuneInString(s string) bool

FullRuneInString is like FullRune but its input is a string.

str := "世"
fmt.Println(utf8.FullRuneInString(str))
fmt.Println(utf8.FullRuneInString(str[:2]))

true
false

func RuneCount ¶

func RuneCount(p []byte) int

RuneCount returns the number of runes in p. Erroneous and short encodings are treated as single runes of width 1 byte.

buf := []byte("Hello, 世界")
fmt.Println("bytes =", len(buf))
fmt.Println("runes =", utf8.RuneCount(buf))

bytes = 13
runes = 9

func RuneCountInString ¶

func RuneCountInString(s string) (n int)

RuneCountInString is like RuneCount but its input is a string.

str := "Hello, 世界"
fmt.Println("bytes =", len(str))
fmt.Println("runes =", utf8.RuneCountInString(str))

bytes = 13
runes = 9

func RuneLen ¶

func RuneLen(r rune) int

RuneLen returns the number of bytes in the UTF-8 encoding of the rune. It returns -1 if the rune is not a valid value to encode in UTF-8.

fmt.Println(utf8.RuneLen('a'))
fmt.Println(utf8.RuneLen('界'))

1
3

func RuneStart ¶

func RuneStart(b byte) bool

RuneStart reports whether the byte could be the first byte of an encoded, possibly invalid rune. Second and subsequent bytes always have the top two bits set to 10.

buf := []byte("a界")
fmt.Println(utf8.RuneStart(buf[0]))
fmt.Println(utf8.RuneStart(buf[1]))
fmt.Println(utf8.RuneStart(buf[2]))

true
true
false

func Valid ¶

func Valid(p []byte) bool

Valid reports whether p consists entirely of valid UTF-8-encoded runes.

valid := []byte("Hello, 世界")
invalid := []byte{0xff, 0xfe, 0xfd}

fmt.Println(utf8.Valid(valid))
fmt.Println(utf8.Valid(invalid))

true
false

func ValidRune ¶ 1.1

func ValidRune(r rune) bool

ValidRune reports whether r can be legally encoded as UTF-8. Code points that are out of range or a surrogate half are illegal.

valid := 'a'
invalid := rune(0xfffffff)

fmt.Println(utf8.ValidRune(valid))
fmt.Println(utf8.ValidRune(invalid))

true
false

func ValidString ¶

func ValidString(s string) bool

ValidString reports whether s consists entirely of valid UTF-8-encoded runes.

valid := "Hello, 世界"
invalid := string([]byte{0xff, 0xfe, 0xfd})

fmt.Println(utf8.ValidString(valid))
fmt.Println(utf8.ValidString(invalid))

true
false