func lexNumber(lexer *Lexer) {
	isSymbol := lexer.accept("+-")
	digits := "0123456789"
	width := lexer.acceptRun(digits)
	if isSymbol && width == 0 {
		lexer.back()
		return
	}
	if lexer.accept(".") {
		lexer.acceptRun(digits)
	}
	lexer.emit(ItemNumber)
}
lexNumber emits the next number by accepting numbers like -14.50 or +192
package lexer