func unquoteChar(s string, quote byte) (value rune, multibyte bool, tail string, err error) {
	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
	}
	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' {
			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++ {  
			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
}
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.
package strutil
import (
	"errors"
	"unicode/utf8"
)
var ErrSyntax = errors.New("invalid syntax")
func Unquote(s string) (t string, err 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
		}
		return s, nil
	}
	if quote != '"' && quote != '\'' {
		return "", ErrSyntax
	}
	if contains(s, '\n') {
		return "", ErrSyntax
	}
	if !contains(s, '\\') && !contains(s, quote) {
		return s, nil
	}
	var runeTmp [utf8.UTFMax]byte
	buf := make([]byte, 0, 3*len(s)/2)  
	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]...)
		}
	}
	return string(buf), nil
}