}

func (s *Scanner) isIdentRune(ch rune, i int) bool {
	if s.IsIdentRune != nil {
		return ch != EOF && s.IsIdentRune(ch, i)
	}
	return ch == '_' || unicode.IsLetter(ch) || unicode.IsDigit(ch) && i > 0
}

func (s *Scanner) scanIdentifier() rune {
	// we know the zero'th rune is OK; start scanning at the next one
	ch := s.next()
	for i := 1; s.isIdentRune(ch, i); i++ {
		ch = s.next()
	}

	const src = "faab12345 a12b123 a12 3b"
	s := new(Scanner).Init(strings.NewReader(src))
	// ident = ( 'a' | 'b' ) { digit } .
	// digit = '0' .. '3' .
	// with a maximum length of 4
	s.IsIdentRune = func(ch rune, i int) bool {
		return i == 0 && (ch == 'a' || ch == 'b') || 0 < i && i < 4 && '0' <= ch && ch <= '3'
	}
	checkTok(t, s, 1, s.Scan(), 'f', "f")
	checkTok(t, s, 1, s.Scan(), Ident, "a")

pkg testing, method (*M) Run() int
pkg testing, type M struct

# CL 108030044 text/scanner: provide facility for custom identifiers, Robert Griesemer <******@****.***>
pkg text/scanner, type Scanner struct, IsIdentRune func(int32, int) bool

# CL 130620043 text/template: add back pointer to Nodes for better error generation, Rob Pike <******@****.***>
pkg text/template/parse, type DotNode struct, embedded NodeType