src/strconv/quote.go - go - Git at Google

 // Copyright 2009 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 //go:generate go run makeisprint.go -output isprint.go

 package strconv

 import (
 	"unicode/utf8"
 )

 const (
 	lowerhex = "0123456789abcdef"
 	upperhex = "0123456789ABCDEF"
 )

 func quoteWith(s string, quote byte, ASCIIonly, graphicOnly bool) string {
 	return string(appendQuotedWith(make([]byte, 0, 3*len(s)/2), s, quote, ASCIIonly, graphicOnly))
 }

 func quoteRuneWith(r rune, quote byte, ASCIIonly, graphicOnly bool) string {
 	return string(appendQuotedRuneWith(nil, r, quote, ASCIIonly, graphicOnly))
 }

 func appendQuotedWith(buf []byte, s string, quote byte, ASCIIonly, graphicOnly bool) []byte {
 	// Often called with big strings, so preallocate. If there's quoting,
 	// this is conservative but still helps a lot.
 	if cap(buf)-len(buf) < len(s) {
 		nBuf := make([]byte, len(buf), len(buf)+1+len(s)+1)
 		copy(nBuf, buf)
 		buf = nBuf
 	}
 	buf = append(buf, quote)
 	for width := 0; len(s) > 0; s = s[width:] {
 		r := rune(s[0])
 		width = 1
 		if r >= utf8.RuneSelf {
 			r, width = utf8.DecodeRuneInString(s)
 		}
 		if width == 1 && r == utf8.RuneError {
 			buf = append(buf, `\x`...)
 			buf = append(buf, lowerhex[s[0]>>4])
 			buf = append(buf, lowerhex[s[0]&0xF])
 			continue
 		}
 		buf = appendEscapedRune(buf, r, quote, ASCIIonly, graphicOnly)
 	}
 	buf = append(buf, quote)
 	return buf
 }

 func appendQuotedRuneWith(buf []byte, r rune, quote byte, ASCIIonly, graphicOnly bool) []byte {
 	buf = append(buf, quote)
 	if !utf8.ValidRune(r) {
 		r = utf8.RuneError
 	}
 	buf = appendEscapedRune(buf, r, quote, ASCIIonly, graphicOnly)
 	buf = append(buf, quote)
 	return buf
 }

 func appendEscapedRune(buf []byte, r rune, quote byte, ASCIIonly, graphicOnly bool) []byte {
 	var runeTmp [utf8.UTFMax]byte
 	if r == rune(quote) || r == '\\' { // always backslashed
 		buf = append(buf, '\\')
 		buf = append(buf, byte(r))
 		return buf
 	}
 	if ASCIIonly {
 		if r < utf8.RuneSelf && IsPrint(r) {
 			buf = append(buf, byte(r))
 			return buf
 		}
 	} else if IsPrint(r) || graphicOnly && isInGraphicList(r) {
 		n := utf8.EncodeRune(runeTmp[:], r)
 		buf = append(buf, runeTmp[:n]...)
 		return buf
 	}
 	switch r {
 	case '\a':
 		buf = append(buf, `\a`...)
 	case '\b':
 		buf = append(buf, `\b`...)
 	case '\f':
 		buf = append(buf, `\f`...)
 	case '\n':
 		buf = append(buf, `\n`...)
 	case '\r':
 		buf = append(buf, `\r`...)
 	case '\t':
 		buf = append(buf, `\t`...)
 	case '\v':
 		buf = append(buf, `\v`...)
 	default:
 		switch {
 		case r < ' ':
 			buf = append(buf, `\x`...)
 			buf = append(buf, lowerhex[byte(r)>>4])
 			buf = append(buf, lowerhex[byte(r)&0xF])
 		case r > utf8.MaxRune:
 			r = 0xFFFD
 			fallthrough
 		case r < 0x10000:
 			buf = append(buf, `\u`...)
 			for s := 12; s >= 0; s -= 4 {
 				buf = append(buf, lowerhex[r>>uint(s)&0xF])
 			}
 		default:
 			buf = append(buf, `\U`...)
 			for s := 28; s >= 0; s -= 4 {
 				buf = append(buf, lowerhex[r>>uint(s)&0xF])
 			}
 		}
 	}
 	return buf
 }

 // Quote returns a double-quoted Go string literal representing s. The
 // returned string uses Go escape sequences (\t, \n, \xFF, \u0100) for
 // control characters and non-printable characters as defined by
 // IsPrint.
 func Quote(s string) string {
 	return quoteWith(s, '"', false, false)
 }

 // AppendQuote appends a double-quoted Go string literal representing s,
 // as generated by Quote, to dst and returns the extended buffer.
 func AppendQuote(dst []byte, s string) []byte {
 	return appendQuotedWith(dst, s, '"', false, false)
 }

 // QuoteToASCII returns a double-quoted Go string literal representing s.
 // The returned string uses Go escape sequences (\t, \n, \xFF, \u0100) for
 // non-ASCII characters and non-printable characters as defined by IsPrint.
 func QuoteToASCII(s string) string {
 	return quoteWith(s, '"', true, false)
 }

 // AppendQuoteToASCII appends a double-quoted Go string literal representing s,
 // as generated by QuoteToASCII, to dst and returns the extended buffer.
 func AppendQuoteToASCII(dst []byte, s string) []byte {
 	return appendQuotedWith(dst, s, '"', true, false)
 }

 // QuoteToGraphic returns a double-quoted Go string literal representing s.
 // The returned string leaves Unicode graphic characters, as defined by
 // IsGraphic, unchanged and uses Go escape sequences (\t, \n, \xFF, \u0100)
 // for non-graphic characters.
 func QuoteToGraphic(s string) string {
 	return quoteWith(s, '"', false, true)
 }

 // AppendQuoteToGraphic appends a double-quoted Go string literal representing s,
 // as generated by QuoteToGraphic, to dst and returns the extended buffer.
 func AppendQuoteToGraphic(dst []byte, s string) []byte {
 	return appendQuotedWith(dst, s, '"', false, true)
 }

 // QuoteRune returns a single-quoted Go character literal representing the
 // rune. The returned string uses Go escape sequences (\t, \n, \xFF, \u0100)
 // for control characters and non-printable characters as defined by IsPrint.
 func QuoteRune(r rune) string {
 	return quoteRuneWith(r, '\'', false, false)
 }

 // AppendQuoteRune appends a single-quoted Go character literal representing the rune,
 // as generated by QuoteRune, to dst and returns the extended buffer.
 func AppendQuoteRune(dst []byte, r rune) []byte {
 	return appendQuotedRuneWith(dst, r, '\'', false, false)
 }

 // QuoteRuneToASCII returns a single-quoted Go character literal representing
 // the rune. The returned string uses Go escape sequences (\t, \n, \xFF,
 // \u0100) for non-ASCII characters and non-printable characters as defined
 // by IsPrint.
 func QuoteRuneToASCII(r rune) string {
 	return quoteRuneWith(r, '\'', true, false)
 }

 // AppendQuoteRuneToASCII appends a single-quoted Go character literal representing the rune,
 // as generated by QuoteRuneToASCII, to dst and returns the extended buffer.
 func AppendQuoteRuneToASCII(dst []byte, r rune) []byte {
 	return appendQuotedRuneWith(dst, r, '\'', true, false)
 }

 // QuoteRuneToGraphic returns a single-quoted Go character literal representing
 // the rune. If the rune is not a Unicode graphic character,
 // as defined by IsGraphic, the returned string will use a Go escape sequence
 // (\t, \n, \xFF, \u0100).
 func QuoteRuneToGraphic(r rune) string {
 	return quoteRuneWith(r, '\'', false, true)
 }

 // AppendQuoteRuneToGraphic appends a single-quoted Go character literal representing the rune,
 // as generated by QuoteRuneToGraphic, to dst and returns the extended buffer.
 func AppendQuoteRuneToGraphic(dst []byte, r rune) []byte {
 	return appendQuotedRuneWith(dst, r, '\'', false, true)
 }

 // CanBackquote reports whether the string s can be represented
 // unchanged as a single-line backquoted string without control
 // characters other than tab.
 func CanBackquote(s string) bool {
 	for len(s) > 0 {
 		r, wid := utf8.DecodeRuneInString(s)
 		s = s[wid:]
 		if wid > 1 {
 			if r == '\ufeff' {
 				return false // BOMs are invisible and should not be quoted.
 			}
 			continue // All other multibyte runes are correctly encoded and assumed printable.
 		}
 		if r == utf8.RuneError {
 			return false
 		}
 		if (r < ' ' && r != '\t') || r == '`' || r == '\u007F' {
 			return false
 		}
 	}
 	return true
 }

 func unhex(b byte) (v rune, ok bool) {
 	c := rune(b)
 	switch {
 	case '0' <= c && c <= '9':
 		return c - '0', true
 	case 'a' <= c && c <= 'f':
 		return c - 'a' + 10, true
 	case 'A' <= c && c <= 'F':
 		return c - 'A' + 10, true
 	}
 	return
 }

 // UnquoteChar decodes the first character or byte in the escaped string
 // or character literal represented by the string s.
 // It returns four values:
 //
 //	1) value, the decoded Unicode code point or byte value;
 //	2) multibyte, a boolean indicating whether the decoded character requires a multibyte UTF-8 representation;
 //	3) tail, the remainder of the string after the character; and
 //	4) an error that will be nil if the character is syntactically valid.
 //
 // The second argument, quote, specifies the type of literal being parsed
 // and therefore which escaped quote character is permitted.
 // If set to a single quote, it permits the sequence \' and disallows unescaped '.
 // If set to a double quote, it permits \" and disallows unescaped ".
 // If set to zero, it does not permit either escape and allows both quote characters to appear unescaped.
 func UnquoteChar(s string, quote byte) (value rune, multibyte bool, tail string, err error) {
 	// easy cases
 	if len(s) == 0 {
 		err = ErrSyntax
 		return
 	}
 	switch c := s[0]; {
 	case c == quote && (quote == '\'' || quote == '"'):
 		err = ErrSyntax
 		return
 	case c >= utf8.RuneSelf:
 		r, size := utf8.DecodeRuneInString(s)
 		return r, true, s[size:], nil
 	case c != '\\':
 		return rune(s[0]), false, s[1:], nil
 	}

 	// hard case: c is backslash
 	if len(s) <= 1 {
 		err = ErrSyntax
 		return
 	}
 	c := s[1]
 	s = s[2:]

 	switch c {
 	case 'a':
 		value = '\a'
 	case 'b':
 		value = '\b'
 	case 'f':
 		value = '\f'
 	case 'n':
 		value = '\n'
 	case 'r':
 		value = '\r'
 	case 't':
 		value = '\t'
 	case 'v':
 		value = '\v'
 	case 'x', 'u', 'U':
 		n := 0
 		switch c {
 		case 'x':
 			n = 2
 		case 'u':
 			n = 4
 		case 'U':
 			n = 8
 		}
 		var v rune
 		if len(s) < n {
 			err = ErrSyntax
 			return
 		}
 		for j := 0; j < n; j++ {
 			x, ok := unhex(s[j])
 			if !ok {
 				err = ErrSyntax
 				return
 			}
 			v = v<<4 | x
 		}
 		s = s[n:]
 		if c == 'x' {
 			// single-byte string, possibly not UTF-8
 			value = v
 			break
 		}
 		if v > utf8.MaxRune {
 			err = ErrSyntax
 			return
 		}
 		value = v
 		multibyte = true
 	case '0', '1', '2', '3', '4', '5', '6', '7':
 		v := rune(c) - '0'
 		if len(s) < 2 {
 			err = ErrSyntax
 			return
 		}
 		for j := 0; j < 2; j++ { // one digit already; two more
 			x := rune(s[j]) - '0'
 			if x < 0 || x > 7 {
 				err = ErrSyntax
 				return
 			}
 			v = (v << 3) | x
 		}
 		s = s[2:]
 		if v > 255 {
 			err = ErrSyntax
 			return
 		}
 		value = v
 	case '\\':
 		value = '\\'
 	case '\'', '"':
 		if c != quote {
 			err = ErrSyntax
 			return
 		}
 		value = rune(c)
 	default:
 		err = ErrSyntax
 		return
 	}
 	tail = s
 	return
 }

 // Unquote interprets s as a single-quoted, double-quoted,
 // or backquoted Go string literal, returning the string value
 // that s quotes.  (If s is single-quoted, it would be a Go
 // character literal; Unquote returns the corresponding
 // one-character string.)
 func Unquote(s string) (string, error) {
 	n := len(s)
 	if n < 2 {
 		return "", ErrSyntax
 	}
 	quote := s[0]
 	if quote != s[n-1] {
 		return "", ErrSyntax
 	}
 	s = s[1 : n-1]

 	if quote == '`' {
 		if contains(s, '`') {
 			return "", ErrSyntax
 		}
 		if contains(s, '\r') {
 			// -1 because we know there is at least one \r to remove.
 			buf := make([]byte, 0, len(s)-1)
 			for i := 0; i < len(s); i++ {
 				if s[i] != '\r' {
 					buf = append(buf, s[i])
 				}
 			}
 			return string(buf), nil
 		}
 		return s, nil
 	}
 	if quote != '"' && quote != '\'' {
 		return "", ErrSyntax
 	}
 	if contains(s, '\n') {
 		return "", ErrSyntax
 	}

 	// Is it trivial? Avoid allocation.
 	if !contains(s, '\\') && !contains(s, quote) {
 		switch quote {
 		case '"':
 			if utf8.ValidString(s) {
 				return s, nil
 			}
 		case '\'':
 			r, size := utf8.DecodeRuneInString(s)
 			if size == len(s) && (r != utf8.RuneError || size != 1) {
 				return s, nil
 			}
 		}
 	}

 	var runeTmp [utf8.UTFMax]byte
 	buf := make([]byte, 0, 3*len(s)/2) // Try to avoid more allocations.
 	for len(s) > 0 {
 		c, multibyte, ss, err := UnquoteChar(s, quote)
 		if err != nil {
 			return "", err
 		}
 		s = ss
 		if c < utf8.RuneSelf || !multibyte {
 			buf = append(buf, byte(c))
 		} else {
 			n := utf8.EncodeRune(runeTmp[:], c)
 			buf = append(buf, runeTmp[:n]...)
 		}
 		if quote == '\'' && len(s) != 0 {
 			// single-quoted must be single character
 			return "", ErrSyntax
 		}
 	}
 	return string(buf), nil
 }

 // bsearch16 returns the smallest i such that a[i] >= x.
 // If there is no such i, bsearch16 returns len(a).
 func bsearch16(a []uint16, x uint16) int {
 	i, j := 0, len(a)
 	for i < j {
 		h := i + (j-i)/2
 		if a[h] < x {
 			i = h + 1
 		} else {
 			j = h
 		}
 	}
 	return i
 }

 // bsearch32 returns the smallest i such that a[i] >= x.
 // If there is no such i, bsearch32 returns len(a).
 func bsearch32(a []uint32, x uint32) int {
 	i, j := 0, len(a)
 	for i < j {
 		h := i + (j-i)/2
 		if a[h] < x {
 			i = h + 1
 		} else {
 			j = h
 		}
 	}
 	return i
 }

 // TODO: IsPrint is a local implementation of unicode.IsPrint, verified by the tests
 // to give the same answer. It allows this package not to depend on unicode,
 // and therefore not pull in all the Unicode tables. If the linker were better
 // at tossing unused tables, we could get rid of this implementation.
 // That would be nice.

 // IsPrint reports whether the rune is defined as printable by Go, with
 // the same definition as unicode.IsPrint: letters, numbers, punctuation,
 // symbols and ASCII space.
 func IsPrint(r rune) bool {
 	// Fast check for Latin-1
 	if r <= 0xFF {
 		if 0x20 <= r && r <= 0x7E {
 			// All the ASCII is printable from space through DEL-1.
 			return true
 		}
 		if 0xA1 <= r && r <= 0xFF {
 			// Similarly for ¡ through ÿ...
 			return r != 0xAD // ...except for the bizarre soft hyphen.
 		}
 		return false
 	}

 	// Same algorithm, either on uint16 or uint32 value.
 	// First, find first i such that isPrint[i] >= x.
 	// This is the index of either the start or end of a pair that might span x.
 	// The start is even (isPrint[i&^1]) and the end is odd (isPrint[i|1]).
 	// If we find x in a range, make sure x is not in isNotPrint list.

 	if 0 <= r && r < 1<<16 {
 		rr, isPrint, isNotPrint := uint16(r), isPrint16, isNotPrint16
 		i := bsearch16(isPrint, rr)
 		if i >= len(isPrint) || rr < isPrint[i&^1] || isPrint[i|1] < rr {
 			return false
 		}
 		j := bsearch16(isNotPrint, rr)
 		return j >= len(isNotPrint) || isNotPrint[j] != rr
 	}

 	rr, isPrint, isNotPrint := uint32(r), isPrint32, isNotPrint32
 	i := bsearch32(isPrint, rr)
 	if i >= len(isPrint) || rr < isPrint[i&^1] || isPrint[i|1] < rr {
 		return false
 	}
 	if r >= 0x20000 {
 		return true
 	}
 	r -= 0x10000
 	j := bsearch16(isNotPrint, uint16(r))
 	return j >= len(isNotPrint) || isNotPrint[j] != uint16(r)
 }

 // IsGraphic reports whether the rune is defined as a Graphic by Unicode. Such
 // characters include letters, marks, numbers, punctuation, symbols, and
 // spaces, from categories L, M, N, P, S, and Zs.
 func IsGraphic(r rune) bool {
 	if IsPrint(r) {
 		return true
 	}
 	return isInGraphicList(r)
 }

 // isInGraphicList reports whether the rune is in the isGraphic list. This separation
 // from IsGraphic allows quoteWith to avoid two calls to IsPrint.
 // Should be called only if IsPrint fails.
 func isInGraphicList(r rune) bool {
 	// We know r must fit in 16 bits - see makeisprint.go.
 	if r > 0xFFFF {
 		return false
 	}
 	rr := uint16(r)
 	i := bsearch16(isGraphic, rr)
 	return i < len(isGraphic) && rr == isGraphic[i]
 }
	// Copyright 2009 The Go Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	//go:generate go run makeisprint.go -output isprint.go

	package strconv

	import (
	"unicode/utf8"
	)

	const (
	lowerhex = "0123456789abcdef"
	upperhex = "0123456789ABCDEF"
	)

	func quoteWith(s string, quote byte, ASCIIonly, graphicOnly bool) string {
	return string(appendQuotedWith(make([]byte, 0, 3*len(s)/2), s, quote, ASCIIonly, graphicOnly))
	}

	func quoteRuneWith(r rune, quote byte, ASCIIonly, graphicOnly bool) string {
	return string(appendQuotedRuneWith(nil, r, quote, ASCIIonly, graphicOnly))
	}

	func appendQuotedWith(buf []byte, s string, quote byte, ASCIIonly, graphicOnly bool) []byte {
	// Often called with big strings, so preallocate. If there's quoting,
	// this is conservative but still helps a lot.
	if cap(buf)-len(buf) < len(s) {
	nBuf := make([]byte, len(buf), len(buf)+1+len(s)+1)
	copy(nBuf, buf)
	buf = nBuf
	}
	buf = append(buf, quote)
	for width := 0; len(s) > 0; s = s[width:] {
	r := rune(s[0])
	width = 1
	if r >= utf8.RuneSelf {
	r, width = utf8.DecodeRuneInString(s)
	}
	if width == 1 && r == utf8.RuneError {
	buf = append(buf, `\x`...)
	buf = append(buf, lowerhex[s[0]>>4])
	buf = append(buf, lowerhex[s[0]&0xF])
	continue
	}
	buf = appendEscapedRune(buf, r, quote, ASCIIonly, graphicOnly)
	}
	buf = append(buf, quote)
	return buf
	}

	func appendQuotedRuneWith(buf []byte, r rune, quote byte, ASCIIonly, graphicOnly bool) []byte {
	buf = append(buf, quote)
	if !utf8.ValidRune(r) {
	r = utf8.RuneError
	}
	buf = appendEscapedRune(buf, r, quote, ASCIIonly, graphicOnly)
	buf = append(buf, quote)
	return buf
	}

	func appendEscapedRune(buf []byte, r rune, quote byte, ASCIIonly, graphicOnly bool) []byte {
	var runeTmp [utf8.UTFMax]byte
	if r == rune(quote) \|\| r == '\\' { // always backslashed
	buf = append(buf, '\\')
	buf = append(buf, byte(r))
	return buf
	}
	if ASCIIonly {
	if r < utf8.RuneSelf && IsPrint(r) {
	buf = append(buf, byte(r))
	return buf
	}
	} else if IsPrint(r) \|\| graphicOnly && isInGraphicList(r) {
	n := utf8.EncodeRune(runeTmp[:], r)
	buf = append(buf, runeTmp[:n]...)
	return buf
	}
	switch r {
	case '\a':
	buf = append(buf, `\a`...)
	case '\b':
	buf = append(buf, `\b`...)
	case '\f':
	buf = append(buf, `\f`...)
	case '\n':
	buf = append(buf, `\n`...)
	case '\r':
	buf = append(buf, `\r`...)
	case '\t':
	buf = append(buf, `\t`...)
	case '\v':
	buf = append(buf, `\v`...)
	default:
	switch {
	case r < ' ':
	buf = append(buf, `\x`...)
	buf = append(buf, lowerhex[byte(r)>>4])
	buf = append(buf, lowerhex[byte(r)&0xF])
	case r > utf8.MaxRune:
	r = 0xFFFD
	fallthrough
	case r < 0x10000:
	buf = append(buf, `\u`...)
	for s := 12; s >= 0; s -= 4 {
	buf = append(buf, lowerhex[r>>uint(s)&0xF])
	}
	default:
	buf = append(buf, `\U`...)
	for s := 28; s >= 0; s -= 4 {
	buf = append(buf, lowerhex[r>>uint(s)&0xF])
	}
	}
	}
	return buf
	}

	// Quote returns a double-quoted Go string literal representing s. The
	// returned string uses Go escape sequences (\t, \n, \xFF, \u0100) for
	// control characters and non-printable characters as defined by
	// IsPrint.
	func Quote(s string) string {
	return quoteWith(s, '"', false, false)
	}

	// AppendQuote appends a double-quoted Go string literal representing s,
	// as generated by Quote, to dst and returns the extended buffer.
	func AppendQuote(dst []byte, s string) []byte {
	return appendQuotedWith(dst, s, '"', false, false)
	}

	// QuoteToASCII returns a double-quoted Go string literal representing s.
	// The returned string uses Go escape sequences (\t, \n, \xFF, \u0100) for
	// non-ASCII characters and non-printable characters as defined by IsPrint.
	func QuoteToASCII(s string) string {
	return quoteWith(s, '"', true, false)
	}

	// AppendQuoteToASCII appends a double-quoted Go string literal representing s,
	// as generated by QuoteToASCII, to dst and returns the extended buffer.
	func AppendQuoteToASCII(dst []byte, s string) []byte {
	return appendQuotedWith(dst, s, '"', true, false)
	}

	// QuoteToGraphic returns a double-quoted Go string literal representing s.
	// The returned string leaves Unicode graphic characters, as defined by
	// IsGraphic, unchanged and uses Go escape sequences (\t, \n, \xFF, \u0100)
	// for non-graphic characters.
	func QuoteToGraphic(s string) string {
	return quoteWith(s, '"', false, true)
	}

	// AppendQuoteToGraphic appends a double-quoted Go string literal representing s,
	// as generated by QuoteToGraphic, to dst and returns the extended buffer.
	func AppendQuoteToGraphic(dst []byte, s string) []byte {
	return appendQuotedWith(dst, s, '"', false, true)
	}

	// QuoteRune returns a single-quoted Go character literal representing the
	// rune. The returned string uses Go escape sequences (\t, \n, \xFF, \u0100)
	// for control characters and non-printable characters as defined by IsPrint.
	func QuoteRune(r rune) string {
	return quoteRuneWith(r, '\'', false, false)
	}

	// AppendQuoteRune appends a single-quoted Go character literal representing the rune,
	// as generated by QuoteRune, to dst and returns the extended buffer.
	func AppendQuoteRune(dst []byte, r rune) []byte {
	return appendQuotedRuneWith(dst, r, '\'', false, false)
	}

	// QuoteRuneToASCII returns a single-quoted Go character literal representing
	// the rune. The returned string uses Go escape sequences (\t, \n, \xFF,
	// \u0100) for non-ASCII characters and non-printable characters as defined
	// by IsPrint.
	func QuoteRuneToASCII(r rune) string {
	return quoteRuneWith(r, '\'', true, false)
	}

	// AppendQuoteRuneToASCII appends a single-quoted Go character literal representing the rune,
	// as generated by QuoteRuneToASCII, to dst and returns the extended buffer.
	func AppendQuoteRuneToASCII(dst []byte, r rune) []byte {
	return appendQuotedRuneWith(dst, r, '\'', true, false)
	}

	// QuoteRuneToGraphic returns a single-quoted Go character literal representing
	// the rune. If the rune is not a Unicode graphic character,
	// as defined by IsGraphic, the returned string will use a Go escape sequence
	// (\t, \n, \xFF, \u0100).
	func QuoteRuneToGraphic(r rune) string {
	return quoteRuneWith(r, '\'', false, true)
	}

	// AppendQuoteRuneToGraphic appends a single-quoted Go character literal representing the rune,
	// as generated by QuoteRuneToGraphic, to dst and returns the extended buffer.
	func AppendQuoteRuneToGraphic(dst []byte, r rune) []byte {
	return appendQuotedRuneWith(dst, r, '\'', false, true)
	}

	// CanBackquote reports whether the string s can be represented
	// unchanged as a single-line backquoted string without control
	// characters other than tab.
	func CanBackquote(s string) bool {
	for len(s) > 0 {
	r, wid := utf8.DecodeRuneInString(s)
	s = s[wid:]
	if wid > 1 {
	if r == '\ufeff' {
	return false // BOMs are invisible and should not be quoted.
	}
	continue // All other multibyte runes are correctly encoded and assumed printable.
	}
	if r == utf8.RuneError {
	return false
	}
	if (r < ' ' && r != '\t') \|\| r == '`' \|\| r == '\u007F' {
	return false
	}
	}
	return true
	}

	func unhex(b byte) (v rune, ok bool) {
	c := rune(b)
	switch {
	case '0' <= c && c <= '9':
	return c - '0', true
	case 'a' <= c && c <= 'f':
	return c - 'a' + 10, true
	case 'A' <= c && c <= 'F':
	return c - 'A' + 10, true
	}
	return
	}

	// UnquoteChar decodes the first character or byte in the escaped string
	// or character literal represented by the string s.
	// It returns four values:
	//
	// 1) value, the decoded Unicode code point or byte value;
	// 2) multibyte, a boolean indicating whether the decoded character requires a multibyte UTF-8 representation;
	// 3) tail, the remainder of the string after the character; and
	// 4) an error that will be nil if the character is syntactically valid.
	//
	// The second argument, quote, specifies the type of literal being parsed
	// and therefore which escaped quote character is permitted.
	// If set to a single quote, it permits the sequence \' and disallows unescaped '.
	// If set to a double quote, it permits \" and disallows unescaped ".
	// If set to zero, it does not permit either escape and allows both quote characters to appear unescaped.
	func UnquoteChar(s string, quote byte) (value rune, multibyte bool, tail string, err error) {
	// easy cases
	if len(s) == 0 {
	err = ErrSyntax
	return
	}
	switch c := s[0]; {
	case c == quote && (quote == '\'' \|\| quote == '"'):
	err = ErrSyntax
	return
	case c >= utf8.RuneSelf:
	r, size := utf8.DecodeRuneInString(s)
	return r, true, s[size:], nil
	case c != '\\':
	return rune(s[0]), false, s[1:], nil
	}

	// hard case: c is backslash
	if len(s) <= 1 {
	err = ErrSyntax
	return
	}
	c := s[1]
	s = s[2:]

	switch c {
	case 'a':
	value = '\a'
	case 'b':
	value = '\b'
	case 'f':
	value = '\f'
	case 'n':
	value = '\n'
	case 'r':
	value = '\r'
	case 't':
	value = '\t'
	case 'v':
	value = '\v'
	case 'x', 'u', 'U':
	n := 0
	switch c {
	case 'x':
	n = 2
	case 'u':
	n = 4
	case 'U':
	n = 8
	}
	var v rune
	if len(s) < n {
	err = ErrSyntax
	return
	}
	for j := 0; j < n; j++ {
	x, ok := unhex(s[j])
	if !ok {
	err = ErrSyntax
	return
	}
	v = v<<4 \| x
	}
	s = s[n:]
	if c == 'x' {
	// single-byte string, possibly not UTF-8
	value = v
	break
	}
	if v > utf8.MaxRune {
	err = ErrSyntax
	return
	}
	value = v
	multibyte = true
	case '0', '1', '2', '3', '4', '5', '6', '7':
	v := rune(c) - '0'
	if len(s) < 2 {
	err = ErrSyntax
	return
	}
	for j := 0; j < 2; j++ { // one digit already; two more
	x := rune(s[j]) - '0'
	if x < 0 \|\| x > 7 {
	err = ErrSyntax
	return
	}
	v = (v << 3) \| x
	}
	s = s[2:]
	if v > 255 {
	err = ErrSyntax
	return
	}
	value = v
	case '\\':
	value = '\\'
	case '\'', '"':
	if c != quote {
	err = ErrSyntax
	return
	}
	value = rune(c)
	default:
	err = ErrSyntax
	return
	}
	tail = s
	return
	}

	// Unquote interprets s as a single-quoted, double-quoted,
	// or backquoted Go string literal, returning the string value
	// that s quotes. (If s is single-quoted, it would be a Go
	// character literal; Unquote returns the corresponding
	// one-character string.)
	func Unquote(s string) (string, error) {
	n := len(s)
	if n < 2 {
	return "", ErrSyntax
	}
	quote := s[0]
	if quote != s[n-1] {
	return "", ErrSyntax
	}
	s = s[1 : n-1]

	if quote == '`' {
	if contains(s, '`') {
	return "", ErrSyntax
	}
	if contains(s, '\r') {
	// -1 because we know there is at least one \r to remove.
	buf := make([]byte, 0, len(s)-1)
	for i := 0; i < len(s); i++ {
	if s[i] != '\r' {
	buf = append(buf, s[i])
	}
	}
	return string(buf), nil
	}
	return s, nil
	}
	if quote != '"' && quote != '\'' {
	return "", ErrSyntax
	}
	if contains(s, '\n') {
	return "", ErrSyntax
	}

	// Is it trivial? Avoid allocation.
	if !contains(s, '\\') && !contains(s, quote) {
	switch quote {
	case '"':
	if utf8.ValidString(s) {
	return s, nil
	}
	case '\'':
	r, size := utf8.DecodeRuneInString(s)
	if size == len(s) && (r != utf8.RuneError \|\| size != 1) {
	return s, nil
	}
	}
	}

	var runeTmp [utf8.UTFMax]byte
	buf := make([]byte, 0, 3*len(s)/2) // Try to avoid more allocations.
	for len(s) > 0 {
	c, multibyte, ss, err := UnquoteChar(s, quote)
	if err != nil {
	return "", err
	}
	s = ss
	if c < utf8.RuneSelf \|\| !multibyte {
	buf = append(buf, byte(c))
	} else {
	n := utf8.EncodeRune(runeTmp[:], c)
	buf = append(buf, runeTmp[:n]...)
	}
	if quote == '\'' && len(s) != 0 {
	// single-quoted must be single character
	return "", ErrSyntax
	}
	}
	return string(buf), nil
	}

	// bsearch16 returns the smallest i such that a[i] >= x.
	// If there is no such i, bsearch16 returns len(a).
	func bsearch16(a []uint16, x uint16) int {
	i, j := 0, len(a)
	for i < j {
	h := i + (j-i)/2
	if a[h] < x {
	i = h + 1
	} else {
	j = h
	}
	}
	return i
	}

	// bsearch32 returns the smallest i such that a[i] >= x.
	// If there is no such i, bsearch32 returns len(a).
	func bsearch32(a []uint32, x uint32) int {
	i, j := 0, len(a)
	for i < j {
	h := i + (j-i)/2
	if a[h] < x {
	i = h + 1
	} else {
	j = h
	}
	}
	return i
	}

	// TODO: IsPrint is a local implementation of unicode.IsPrint, verified by the tests
	// to give the same answer. It allows this package not to depend on unicode,
	// and therefore not pull in all the Unicode tables. If the linker were better
	// at tossing unused tables, we could get rid of this implementation.
	// That would be nice.

	// IsPrint reports whether the rune is defined as printable by Go, with
	// the same definition as unicode.IsPrint: letters, numbers, punctuation,
	// symbols and ASCII space.
	func IsPrint(r rune) bool {
	// Fast check for Latin-1
	if r <= 0xFF {
	if 0x20 <= r && r <= 0x7E {
	// All the ASCII is printable from space through DEL-1.
	return true
	}
	if 0xA1 <= r && r <= 0xFF {
	// Similarly for ¡ through ÿ...
	return r != 0xAD // ...except for the bizarre soft hyphen.
	}
	return false
	}

	// Same algorithm, either on uint16 or uint32 value.
	// First, find first i such that isPrint[i] >= x.
	// This is the index of either the start or end of a pair that might span x.
	// The start is even (isPrint[i&^1]) and the end is odd (isPrint[i\|1]).
	// If we find x in a range, make sure x is not in isNotPrint list.

	if 0 <= r && r < 1<<16 {
	rr, isPrint, isNotPrint := uint16(r), isPrint16, isNotPrint16
	i := bsearch16(isPrint, rr)
	if i >= len(isPrint) \|\| rr < isPrint[i&^1] \|\| isPrint[i\|1] < rr {
	return false
	}
	j := bsearch16(isNotPrint, rr)
	return j >= len(isNotPrint) \|\| isNotPrint[j] != rr
	}

	rr, isPrint, isNotPrint := uint32(r), isPrint32, isNotPrint32
	i := bsearch32(isPrint, rr)
	if i >= len(isPrint) \|\| rr < isPrint[i&^1] \|\| isPrint[i\|1] < rr {
	return false
	}
	if r >= 0x20000 {
	return true
	}
	r -= 0x10000
	j := bsearch16(isNotPrint, uint16(r))
	return j >= len(isNotPrint) \|\| isNotPrint[j] != uint16(r)
	}

	// IsGraphic reports whether the rune is defined as a Graphic by Unicode. Such
	// characters include letters, marks, numbers, punctuation, symbols, and
	// spaces, from categories L, M, N, P, S, and Zs.
	func IsGraphic(r rune) bool {
	if IsPrint(r) {
	return true
	}
	return isInGraphicList(r)
	}

	// isInGraphicList reports whether the rune is in the isGraphic list. This separation
	// from IsGraphic allows quoteWith to avoid two calls to IsPrint.
	// Should be called only if IsPrint fails.
	func isInGraphicList(r rune) bool {
	// We know r must fit in 16 bits - see makeisprint.go.
	if r > 0xFFFF {
	return false
	}
	rr := uint16(r)
	i := bsearch16(isGraphic, rr)
	return i < len(isGraphic) && rr == isGraphic[i]
	}