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: lexer.go
    path of file: ./repos/goruby/lexer
    
    the code of the file until where you have to start completion: package lexer

import (
	"bytes"
	"fmt"
	"strings"
	"unicode"
	"unicode/utf8"

	"github.com/goruby/goruby/token"
)

const (
	eof = -1
)

// LexStartFn represents the entrypoint the Lexer uses to start processing the
// input.
var LexStartF
    
func lexCharacterLiteral(l *Lexer) StateFn {
	l.ignore()
	r := l.next()
	if isWhitespace(r) && r != '\t' && r != '\v' && r != '\f' && r != '\r' {
		return l.errorf("invalid character syntax; use ?\\s")
	}
	if r == '\\' {
		r = l.next()
	}
	if p := l.peek(); !isWhitespace(p) && !isExpressionDelimiter(p) {
		return l.errorf("unexpected '?'")
	}
	l.emit(token.STRING)
	return startLexer
}