Package utf8

Constants

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.
)

Numbers fundamental to the encoding.

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 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 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 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 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. 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

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 required to encode 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 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

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