diff --git a/enriclluelles/python/string_calculator.py b/enriclluelles/python/string_calculator.py
new file mode 100644
index 0000000..f4c2e69
--- /dev/null
+++ b/enriclluelles/python/string_calculator.py
@@ -0,0 +1,51 @@
+import re
+from nose.tools import *
+
+class StringCalculator:
+    def add(self,s):
+        if s == "": return 0
+        (delimiter,st) = self.__find_delimiter(s)
+        numbers = [int(x) for x in re.split(delimiter, st)]
+        negatives = [x for x in numbers if x < 0] #collect the negatives to show them after
+        if negatives:
+            raise Exception("Tried to add negatives: %s" % negatives)
+        to_sum = [x for x in numbers if x <= 1000]
+        return sum(to_sum)
+
+    def __find_delimiter(self,s):
+        pattern = "^//(\[.+\])+\n"
+        scan = re.search(pattern, s)
+        if scan: #find out if we have custom delimiters
+            #we make a regexp with delimiters: (ex) *|,|??
+            delimiters = "|".join([ re.escape(dl) for dl in re.findall("\[([^\[\]]+)\]",
+                scan.group(1)) ])
+            return delimiters, re.sub(pattern,"",s)
+        return ",|\n",s
+
+class TestStringCalculator:
+    def setUp(self):
+        self.s = StringCalculator()
+
+    def test_empty(self):
+        assert self.s.add("") == 0
+
+    def test_simple(self):
+        assert self.s.add("1,2,3") == 6
+
+    def test_with_carriage(self):
+        assert self.s.add("1,2\n3") == 6
+
+    def test_big_stuff(self):
+        assert self.s.add("1"+",1"*1000) == 1001
+
+    def test_custom_delimiters(self):
+        assert self.s.add("//[o][hai]\n1o2o34hai0") == 37
+
+    def test_custom_special_delimiters(self):
+        assert self.s.add("//[??][*]\n1*2*34??0") == 37
+
+    def test_negatives(self):
+        assert_raises(Exception, self.s.add, "//*\n-1*-1*3")
+
+    def test_no_more_than_one_thousand(self):
+        assert self.s.add("1,2,3,1001") == 6