You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: utils.go
    path of file: ./repos/expr/parser/lexer
    
    the code of the file until where you have to start completion: package lexer

import (
	"fmt"
	"math"
	"strings"
	"unicode/utf8"
)

var (
	newlineNormalizer = strings.NewReplacer("\r\n", "\n", "\r", "\n")
)

// Unescape takes a quoted string, unquotes, and unescapes it.
func unescape(value string) (string, e
    
func unescapeChar(s string) (value rune, multibyte bool, tail string, err error) {
	// 1. Character is not an escape sequence.
	switch c := s[0]; {
	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
	}

	// 2. Last character is the start of an escape sequence.
	if len(s) <= 1 {
		err = fmt.Errorf("unable to unescape string, found '\\' as last character")
		return
	}

	c := s[1]
	s = s[2:]
	// 3. Common escape sequences shared with Google SQL
	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 '\\':
		value = '\\'
	case '\'':
		value = '\''
	case '"':
		value = '"'
	case '`':
		value = '`'
	case '?':
		value = '?'

	// 4. Unicode escape sequences, reproduced from `strconv/quote.go`
	case 'x', 'X', 'u', 'U':
		n := 0
		switch c {
		case 'x', 'X':
			n = 2
		case 'u':
			n = 4
		case 'U':
			n = 8
		}
		var v rune
		if len(s) < n {
			err = fmt.Errorf("unable to unescape string")
			return
		}
		for j := 0; j < n; j++ {
			x, ok := unhex(s[j])
			if !ok {
				err = fmt.Errorf("unable to unescape string")
				return
			}
			v = v<<4 | x
		}
		s = s[n:]
		if v > utf8.MaxRune {
			err = fmt.Errorf("unable to unescape string")
			return
		}
		value = v
		multibyte = true

	// 5. Octal escape sequences, must be three digits \[0-3][0-7][0-7]
	case '0', '1', '2', '3':
		if len(s) < 2 {
			err = fmt.Errorf("unable to unescape octal sequence in string")
			return
		}
		v := rune(c - '0')
		for j := 0; j < 2; j++ {
			x := s[j]
			if x < '0' || x > '7' {
				err = fmt.Errorf("unable to unescape octal sequence in string")
				return
			}
			v = v*8 + rune(x-'0')
		}
		if v > utf8.MaxRune {
			err = fmt.Errorf("unable to unescape string")
			return
		}
		value = v
		s = s[2:]
		multibyte = true

		// Unknown escape sequence.
	default:
		err = fmt.Errorf("unable to unescape string")
	}

	tail = s
	return
}