diff --git a/ex_28.py b/ex_28.py
new file mode 100644
index 0000000..d03b6f4
--- /dev/null
+++ b/ex_28.py
@@ -0,0 +1,186 @@
+from collections import deque
+import string
+from io import StringIO
+
+TOKEN_NUMERIC_LITERAL = "0123456789"
+TOKEN_DOT = "."
+TOKEN_PARENTHESIS_OPEN = "("
+TOKEN_PARENTHESIS_CLOSE = ")"
+TOKEN_BINARY_OPERATOR = "+-*/"
+TOKEN_SIGN = "-"
+TOKEN_WHITESPACE = string.whitespace
+
+STATE_BEGIN_EXPRESSION = 0
+STATE_INSIDE_TERM = 1
+STATE_EXPECT_NEXT_ARGUMENT = 2
+STATE_PARENTHESIS_CLOSED = 4
+
+class CharacterDevice(object):
+	def __init__(self, file_):
+		self._file = file_
+		self._offset = 0
+		self.character = deque([0])
+		self.line = 0
+
+	def getc(self):
+		self._offset += 1
+		character = self._file.read(1)
+		if(character == "\n"):
+			self.line += 1
+			self.character.append(0)
+		else:
+			self.character[-1] += 1
+		return character
+
+	def ungetc(self, c):
+		if(c == "\n"):
+			self.line -= 1
+			self.character.pop()
+		else:
+			self.character[-1] -= 1
+		self._offset -= 1
+		self._file.seek(self._offset)
+
+class ParsingError(Exception): 
+	pass
+
+class InternalError(Exception): 
+	pass
+
+class ArithmeticParser(object):
+	def __init__(self, file_):
+		self._file = CharacterDevice(file_)
+		self._stack = deque()
+		self._current_expr = deque()
+
+	def parse_numeric_literal(self, carry = ""):
+		literal = carry
+		dot = False
+		character = None
+		while True:
+			character = self._file.getc()
+			if(not character):
+				break
+			if(character in TOKEN_NUMERIC_LITERAL):
+				literal += character
+				continue
+			elif(character in TOKEN_DOT):
+				if(dot):
+					raise self.build_parsing_error("Unexpected dot while parsing literal")
+				dot = True
+				literal += character
+				continue
+			break
+		self._file.ungetc(character)
+
+		self._current_expr.append(literal)
+
+	def parse_parenthesis(self):
+		parenthesis = self._file.getc()
+		if(parenthesis not in TOKEN_PARENTHESIS_OPEN):
+			raise InternalError("parse_parenthesis did not recieve an opening parenthesis")
+		
+		self._stack.append(self._current_expr)
+		self._current_expr = deque()
+		state = self.parse_expression()
+		parenthesis = self._file.getc()
+		if(parenthesis not in TOKEN_PARENTHESIS_CLOSE):
+			raise self.build_parsing_error("Expected closing parenthesis")
+		if(not parenthesis):
+			raise self.build_parsing_error("Unexpected EOF while parsing")
+		nested = self._current_expr
+		self._current_expr = self._stack.pop()
+		self._current_expr.append(list(nested))
+
+	def parse_expression(self):
+		state = STATE_BEGIN_EXPRESSION
+		while True:
+			peek = self._file.getc()
+			if(not peek):
+				break
+			if(peek in TOKEN_WHITESPACE):
+				continue
+			if(state == STATE_BEGIN_EXPRESSION):
+				if(peek in TOKEN_NUMERIC_LITERAL):
+					self._file.ungetc(peek)
+					self.parse_numeric_literal()
+					state = STATE_INSIDE_TERM
+					continue
+				if(peek in TOKEN_SIGN):
+					self.parse_numeric_literal(carry=peek)
+					state = STATE_INSIDE_TERM
+					continue
+				if(peek in TOKEN_PARENTHESIS_OPEN):
+					self._file.ungetc(peek)
+					self.parse_parenthesis()
+					state = STATE_INSIDE_TERM
+					continue
+
+				raise self.build_parsing_error("Unexpected token: '{}'".format(peek))
+			if(state == STATE_INSIDE_TERM):
+				if(peek in TOKEN_BINARY_OPERATOR):
+					self._current_expr.append(peek)
+					state = STATE_EXPECT_NEXT_ARGUMENT
+					continue
+				if(peek in TOKEN_PARENTHESIS_CLOSE):
+					state = STATE_PARENTHESIS_CLOSED
+					self._file.ungetc(peek)
+					break
+				raise self.build_parsing_error("Unexpected token: '{}', expected binary operator".format(peek))
+			if(state == STATE_EXPECT_NEXT_ARGUMENT):
+				if(peek in TOKEN_NUMERIC_LITERAL):
+					self._file.ungetc(peek)
+					self.parse_numeric_literal()
+					state = STATE_INSIDE_TERM
+					continue
+				if(peek in TOKEN_SIGN):
+					self.parse_numeric_literal(carry=peek)
+					state = STATE_INSIDE_TERM
+					continue
+				if(peek in TOKEN_PARENTHESIS_OPEN):
+					self._file.ungetc(peek)
+					self.parse_parenthesis()
+					state = STATE_INSIDE_TERM
+					continue
+
+				raise self.build_parsing_error("Unexpected token: '{}'".format(peek))
+		if(state not in (STATE_BEGIN_EXPRESSION
+				, STATE_INSIDE_TERM
+				, STATE_PARENTHESIS_CLOSED)):
+			raise self.build_parsing_error("Unexpected EOF while parsing")
+		return state
+
+
+	def parse(self):
+		state = self.parse_expression()
+		if(state not in (STATE_BEGIN_EXPRESSION
+				, STATE_INSIDE_TERM)):
+			raise self.build_parsing_error("Unexpected EOF while parsing")
+
+		return list(self._current_expr)
+
+			
+
+
+
+	def build_parsing_error(self, msg):
+		return ParsingError("Error: Line {}, Characer {}: {}".format(self._file.line, self._file.character, msg))
+		
+
+def check_expression(string):
+	parser = ArithmeticParser(StringIO(string))
+	try:
+		parser.parse()
+		return True
+	except ParsingError:
+		return False
+
+
+if( __name__ == "__main__"):
+
+	print(ArithmeticParser(StringIO("((3.2+4)*(5)+7)")).parse())
+	print(check_expression("((3+4)*(5)+7)"))
+	print(check_expression("(-3+4)*(5/7)"))
+	print(check_expression("(-3+4)*(5/(7+5))"))
+	print(check_expression("(-3.3+4)*(5/(7+5)"))
+	print(ArithmeticParser(StringIO("((3+4)*(5)+7")).parse())