diff --git a/.gitignore b/.gitignore
index b4aea5a..b218332 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,3 +1,4 @@
 dist/
 *.egg-info/
 __pycache__/
+test_*.srt
diff --git a/src/fsub/__init__.py b/src/fsub/__init__.py
index e69de29..6b60bcf 100644
--- a/src/fsub/__init__.py
+++ b/src/fsub/__init__.py
@@ -0,0 +1,6 @@
+import fsub
+
+
+# External interface
+def run(args):
+    fsub.run(args)
diff --git a/src/fsub/fsub.py b/src/fsub/fsub.py
index 1049b7f..2ab5799 100755
--- a/src/fsub/fsub.py
+++ b/src/fsub/fsub.py
@@ -37,37 +37,35 @@ def panic(message, code):
 
 class TimeStamp:
     def __init__(self, time_str):
-        parsed_time = time_str.split(':')
-        h = int(parsed_time[0])
-        m = int(parsed_time[1])
-        ms = int(parsed_time[2].replace(',', ''))
-        self.time = h * 3600000 + m * 60000 + ms
+        m = re.match(r'(\d{2,}):(\d{2}):(\d{2}),(\d{3})', time_str)
+        if not m:
+            raise Exception
+
+        h, m, s, ms = map(int, m.groups())
+        self.time = h * 3600000 + m * 60000 + s * 1000 + ms
 
     def getmilliseconds(self):
         return self.time % 1000
 
     def getseconds(self):
-        return (self.time % 60000) / 1000
+        return int((self.time % 60000) / 1000)
 
     def getminutes(self):
-        return (self.time / 60000) % 60
+        return int((self.time / 60000) % 60)
 
     def gethours(self):
-        return self.time / 3600000
+        return int(self.time / 3600000)
 
     millisecods = property(getmilliseconds)
     seconds = property(getseconds)
     minutes = property(getminutes)
     hours = property(gethours)
 
+    def __int__(self):
+        return self.time
+
     def __iadd__(self, other):
-        t = type(other)
-        if t is int:
-            self.time += other
-        elif t is type(self):
-            self.time += other.time
-        else:
-            raise TypeError
+        self.time += int(other)
         return self
 
     def __neg__(self):
@@ -79,19 +77,19 @@ class TimeStamp:
         return self.__iadd__(-other)
 
     def __lt__(self, other):
-        return self.time < other.time
+        return self.time < int(other)
 
     def __le__(self, other):
-        return self.time <= other.time
+        return self.time <= int(other)
 
     def __eq__(self, other):
-        return self.time == other.time
+        return self.time == int(other)
 
     def __gt__(self, other):
-        return self.time > other.time
+        return self.time > int(other)
 
     def __ge__(self, other):
-        return self.time >= other.time
+        return self.time >= int(other)
 
     def __repr__(self):
         return '%02d:%02d:%02d,%03d' % \
@@ -107,6 +105,7 @@ class Subtitle:
         try:
             # This is mostly ignored, as the subtitles are renumbered later
             self.number = int(lines.pop(0))
+            assert self.number > 0
         except Exception:
             panic('Invalid line number detected ({}:{})'
                   .format(file_name, line_number), 1)
@@ -140,12 +139,12 @@ class Subtitle:
         self.time_end += ms
 
     def replace(self, pattern, new_content):
-        for line in self.content:
-            line = pattern.replace(new_content, line)
+        self.content = \
+         list(map(lambda line: pattern.sub(new_content, line), self.content))
 
     def matches(self, regexp):
         for line in self.content:
-            if regexp.findall(line):
+            if regexp.search(line):
                 return True
         return False
 
@@ -272,14 +271,15 @@ class SubripFile:
     def write_file(self):
         output = open(self.file_name, 'w', encoding='utf-8')
         output.write(repr(self))
-        output.write('\n')
+        if len(self.subs) > 0:
+            output.write('\n')
         output.close()
 
     def __repr__(self):
         return '\n\n'.join(map(repr, self.subs))
 
 
-def main():
+def parse_args(args):
     parser = argparse.ArgumentParser(
         prog='fsub',
         description='Fix, edit and clean SubRip (.srt) files.',
@@ -326,7 +326,7 @@ def main():
         nargs='+'
     )
 
-    args = parser.parse_args()
+    args = parser.parse_args(args)
 
     # Make sure --clean is the default
     if not args.shift and not args.no_html:
@@ -336,6 +336,11 @@ def main():
     if not args.clean and args.config_file:
         panic('-f requires -c', 1)
 
+    return args
+
+
+def run(args):
+    args = parse_args(args)
     config = ConfigFile(args)
 
     parsed_files = []
@@ -348,5 +353,9 @@ def main():
         file.process(args, config)
 
 
+def main():
+    run(list(iter(sys.argv).next()))
+
+
 if __name__ == '__main__':
     main()
diff --git a/tests/__init__.py b/tests/__init__.py
new file mode 100644
index 0000000..f56aafe
--- /dev/null
+++ b/tests/__init__.py
@@ -0,0 +1,6 @@
+from tests.unit import *
+from tests.integration import *
+import unittest
+
+if __name__ == '__main__':
+    unittest.main()
diff --git a/tests/integration.py b/tests/integration.py
new file mode 100644
index 0000000..0f870db
--- /dev/null
+++ b/tests/integration.py
@@ -0,0 +1,62 @@
+import unittest
+import src.fsub.fsub as fsub
+import shutil
+import os
+import inspect
+from pathlib import Path
+
+
+class TestFsub(unittest.TestCase):
+    samples = Path('tests/samples')
+    maxDiff = None
+
+    def run_on(self, args, sample, ofile):
+        ifile = inspect.stack()[1][3] + '.srt'
+
+        sample = str(self.samples / sample) + '.srt'
+        shutil.copy(sample, ifile)
+        args.append(ifile)
+
+        fsub.run(args)
+
+        out = open(ifile)
+        result = out.read()
+        out.close()
+
+        ofile = str(self.samples / ofile) + '.srt'
+        cmp_file = open(ofile)
+        cmp = cmp_file.read()
+        cmp_file.close()
+
+        self.assertEqual(result, cmp)
+        os.remove(ifile)
+
+    def test_cleaned(self):
+        args = ['-f', str(self.samples / 'blacklist')]
+        self.run_on(args, 'sample1', 'sample1-cleaned')
+
+    def test_stripped(self):
+        self.run_on(['-n'], 'sample1', 'sample1-stripped')
+
+    def test_cleaned_stripped(self):
+        args = ['-c', '-f', str(self.samples / 'blacklist'), '-n']
+        self.run_on(args, 'sample1', 'sample1-cleaned-stripped')
+
+    def test_cleaned_stripped_shifted_1h(self):
+        args = ['-c',
+                '-f', str(self.samples / 'blacklist'),
+                '-n',
+                '-s', '3600000']
+        self.run_on(args, 'sample1', 'sample1-cleaned-stripped-shifted-1h')
+
+    def test_shifted_minus_1h(self):
+        args = ['-s', '-3600000']
+        self.run_on(args, 'sample1', 'sample1-shifted-minus-1h')
+
+    def test_shifted_minus_52s(self):
+        args = ['-s', '-52000']
+        self.run_on(args, 'sample1', 'sample1-shifted-minus-52s')
+
+
+if __name__ == '__main__':
+    unittest.main()
diff --git a/tests/samples/blacklist b/tests/samples/blacklist
new file mode 100644
index 0000000..b317114
--- /dev/null
+++ b/tests/samples/blacklist
@@ -0,0 +1 @@
+Even <a>
diff --git a/tests/samples/sample1-cleaned-stripped-shifted-1h.srt b/tests/samples/sample1-cleaned-stripped-shifted-1h.srt
new file mode 100644
index 0000000..e76e652
--- /dev/null
+++ b/tests/samples/sample1-cleaned-stripped-shifted-1h.srt
@@ -0,0 +1,19 @@
+1
+01:00:48,900 --> 01:00:49,800
+This one is full of HTML tags.
+Above, below, everywhere
+
+2
+01:00:53,500 --> 01:00:55,200
+The script should not
+care whether the tag is
+valid or not
+
+3
+01:00:56,000 --> 01:00:57,000
+It should just strip all of
+them mercilessly
+
+4
+01:00:58,100 --> 01:00:59,600
+Including this one!
diff --git a/tests/samples/sample1-cleaned-stripped.srt b/tests/samples/sample1-cleaned-stripped.srt
new file mode 100644
index 0000000..3d658d6
--- /dev/null
+++ b/tests/samples/sample1-cleaned-stripped.srt
@@ -0,0 +1,19 @@
+1
+00:00:48,900 --> 00:00:49,800
+This one is full of HTML tags.
+Above, below, everywhere
+
+2
+00:00:53,500 --> 00:00:55,200
+The script should not
+care whether the tag is
+valid or not
+
+3
+00:00:56,000 --> 00:00:57,000
+It should just strip all of
+them mercilessly
+
+4
+00:00:58,100 --> 00:00:59,600
+Including this one!
diff --git a/tests/samples/sample1-cleaned.srt b/tests/samples/sample1-cleaned.srt
new file mode 100644
index 0000000..f3abc62
--- /dev/null
+++ b/tests/samples/sample1-cleaned.srt
@@ -0,0 +1,19 @@
+1
+00:00:48,900 --> 00:00:49,800
+<b>This one is full of HTML tags.</b>
+<i>Above, below, everywhere</i>
+
+2
+00:00:53,500 --> 00:00:55,200
+<html>The script should not
+care whether the tag is
+valid or not</html>
+
+3
+00:00:56,000 --> 00:00:57,000
+<p>It should just strip all of
+them mercilessly</p>
+
+4
+00:00:58,100 --> 00:00:59,600
+<ul>Including this one!</ul>
diff --git a/tests/samples/sample1-shifted-minus-1h.srt b/tests/samples/sample1-shifted-minus-1h.srt
new file mode 100644
index 0000000..e69de29
diff --git a/tests/samples/sample1-shifted-minus-52s.srt b/tests/samples/sample1-shifted-minus-52s.srt
new file mode 100644
index 0000000..b8cd2b2
--- /dev/null
+++ b/tests/samples/sample1-shifted-minus-52s.srt
@@ -0,0 +1,14 @@
+1
+00:00:01,500 --> 00:00:03,200
+<html>The script should not
+care whether the tag is
+valid or not</html>
+
+2
+00:00:04,000 --> 00:00:05,000
+<p>It should just strip all of
+them mercilessly</p>
+
+3
+00:00:06,100 --> 00:00:07,600
+<ul>Including this one!</ul>
diff --git a/tests/samples/sample1-stripped.srt b/tests/samples/sample1-stripped.srt
new file mode 100644
index 0000000..5d9385c
--- /dev/null
+++ b/tests/samples/sample1-stripped.srt
@@ -0,0 +1,23 @@
+1
+00:00:48,900 --> 00:00:49,800
+This one is full of HTML tags.
+Above, below, everywhere
+
+2
+00:00:51,800 --> 00:00:52,700
+Even 's!
+
+3
+00:00:53,500 --> 00:00:55,200
+The script should not
+care whether the tag is
+valid or not
+
+4
+00:00:56,000 --> 00:00:57,000
+It should just strip all of
+them mercilessly
+
+5
+00:00:58,100 --> 00:00:59,600
+Including this one!
diff --git a/tests/samples/sample1.srt b/tests/samples/sample1.srt
new file mode 100644
index 0000000..05156ed
--- /dev/null
+++ b/tests/samples/sample1.srt
@@ -0,0 +1,23 @@
+1
+00:00:48,900 --> 00:00:49,800
+<b>This one is full of HTML tags.</b>
+<i>Above, below, everywhere</i>
+
+2
+00:00:51,800 --> 00:00:52,700
+<a href='dummy'>Even <a>'s!</a>
+
+3
+00:00:53,500 --> 00:00:55,200
+<html>The script should not
+care whether the tag is
+valid or not</html>
+
+4
+00:00:56,000 --> 00:00:57,000
+<p>It should just strip all of
+them mercilessly</p>
+
+5
+00:00:58,100 --> 00:00:59,600
+<ul>Including this one!</ul>
diff --git a/tests/samples/sample2.srt b/tests/samples/sample2.srt
new file mode 100644
index 0000000..85d42e8
--- /dev/null
+++ b/tests/samples/sample2.srt
@@ -0,0 +1,6 @@
+
+1
+00:01:00,000 --> 00:01:01,000
+Just a dummy line, I'm sorry
+But there's whitespace!
+
diff --git a/tests/unit.py b/tests/unit.py
new file mode 100755
index 0000000..aa18f3d
--- /dev/null
+++ b/tests/unit.py
@@ -0,0 +1,198 @@
+import unittest
+import re
+import io
+import sys
+import src.fsub.fsub as fsub
+
+
+class TestTimeStamp(unittest.TestCase):
+    def test_parse(self):
+        # 3 h = 10800000 ms
+        # 46 min = 2760000 ms
+        # 13 s = 13000 ms
+        # 93 ms
+        # summed up: 13573093 ms
+        t = fsub.TimeStamp('03:46:13,093')
+        self.assertEqual(t.time, 13573093)
+        self.assertEqual(t.hours, 3)
+        self.assertEqual(t.minutes, 46)
+        self.assertEqual(t.seconds, 13)
+        self.assertEqual(t.millisecods, 93)
+
+    @unittest.expectedFailure
+    def test_missing_comma(self):
+        fsub.TimeStamp('00:00:00000')
+
+    @unittest.expectedFailure
+    def test_missing_zeros(self):
+        fsub.TimeStamp('0:0:00,00')
+
+    def test_repr(self):
+        time = '03:46:13,093'
+        t = fsub.TimeStamp(time)
+        self.assertEqual(repr(t), time)
+
+    def test_operations(self):
+        t1_str = '03:46:13,093'
+        t1 = fsub.TimeStamp(t1_str)
+        t2 = fsub.TimeStamp('07:39:50,920')
+        res = fsub.TimeStamp('11:26:04,013')
+        zero = fsub.TimeStamp('00:00:00,000')
+
+        self.assertNotEqual(t1, t2)
+        self.assertLess(t1, t2)
+        self.assertGreater(t2, t1)
+
+        t1 += t2
+
+        self.assertEqual(t1, res)
+        self.assertGreater(t1, t2)
+        self.assertLess(t2, t1)
+
+        t1 += -t2
+
+        self.assertEqual(t1, fsub.TimeStamp(t1_str))
+        self.assertLess(t1, t2)
+        self.assertGreater(t2, t1)
+
+        t1 -= t1
+        t = t2.time
+        t2 += t
+        t2 -= t
+        t2 -= t
+
+        self.assertEqual(t1, zero)
+        self.assertEqual(t2, zero)
+
+
+class TestSubtitle(unittest.TestCase):
+    sample_n = 10
+    sample_start = '02:01:02,000'
+    sample_end = '02:02:00,000'
+    sample_content = \
+        'This is a test subtitle, which\n' + \
+        'may contain line breaks'
+    sample_sub = '{}\n{} --> {}\n{}' \
+        .format(sample_n, sample_start, sample_end, sample_content)
+    sample_fname = 'some_file.srt'
+    sample_line = 30
+
+    def test_parse(self):
+        sub = fsub.Subtitle(self.sample_sub,
+                            self.sample_fname,
+                            self.sample_line)
+
+        self.assertEqual(sub.number, self.sample_n)
+        self.assertEqual(repr(sub.time_start), self.sample_start)
+        self.assertEqual(repr(sub.time_end), self.sample_end)
+        self.assertEqual(len(sub), 4)
+
+        for line in zip(self.sample_content.splitlines(), sub.content):
+            self.assertEqual(line[0], line[1])
+
+    def test_repr(self):
+        sub = fsub.Subtitle(self.sample_sub,
+                            self.sample_fname,
+                            self.sample_line)
+        self.assertEqual(repr(sub), self.sample_sub)
+
+    def test_shift(self):
+        sub = fsub.Subtitle(self.sample_sub,
+                            self.sample_fname,
+                            self.sample_line)
+        start = fsub.TimeStamp(self.sample_start)
+        end = fsub.TimeStamp(self.sample_end)
+        # Some random amount
+        shift_by = 2327291392
+
+        sub.shift(shift_by)
+        start += shift_by
+        end += shift_by
+        self.assertEqual(sub.time_start, start)
+        self.assertEqual(sub.time_end, end)
+
+    def test_replace(self):
+        sub = fsub.Subtitle(self.sample_sub,
+                            self.sample_fname,
+                            self.sample_line)
+
+        sub.replace(re.compile('dummy str not in sub'), '')
+
+        self.assertEqual(repr(sub), self.sample_sub)
+
+        sub.replace(re.compile('is a test'), 'is not a test')
+
+        self.assertNotEqual(repr(sub), self.sample_sub)
+
+    def test_matches(self):
+        sub = fsub.Subtitle(self.sample_sub,
+                            self.sample_fname,
+                            self.sample_line)
+
+        m1 = sub.matches(re.compile('dummy str not in sub'))
+
+        self.assertFalse(m1)
+
+        m2 = sub.matches(re.compile('is a test'))
+
+        self.assertTrue(m2)
+
+    @unittest.expectedFailure
+    def test_bad_number(self):
+        sub_str = """badnumber
+02:01:02,000 --> 02:02:00,000
+This is a test subtitle, which
+may contain line breaks"""
+        sys.stderr = io.StringIO()
+
+        fsub.Subtitle(sub_str,
+                      self.sample_fname,
+                      self.sample_line)
+
+        sys.stderr = sys.__stderr__
+
+    @unittest.expectedFailure
+    def test_neg_number(self):
+        sub_str = """-1
+02:01:02,000 --> 02:02:00,000
+This is a test subtitle, which
+may contain line breaks"""
+        sys.stderr = io.StringIO()
+
+        fsub.Subtitle(sub_str,
+                      self.sample_fname,
+                      self.sample_line)
+
+        sys.stderr = sys.__stderr__
+
+    @unittest.expectedFailure
+    def test_bad_time_span(self):
+        sub_str = """1
+02:01:02,000 <-- 02:02:00,000
+This is a test subtitle, which
+may contain line breaks"""
+        sys.stderr = io.StringIO()
+
+        fsub.Subtitle(sub_str,
+                      self.sample_fname,
+                      self.sample_line)
+
+        sys.stderr = sys.__stderr__
+
+    @unittest.expectedFailure
+    def test_inverted_time(self):
+        sub_str = """1
+12:01:02,000 --> 02:02:00,000
+This is a test subtitle, which
+may contain line breaks"""
+        sys.stderr = io.StringIO()
+
+        fsub.Subtitle(sub_str,
+                      self.sample_fname,
+                      self.sample_line)
+
+        sys.stderr = sys.__stderr__
+
+
+if __name__ == '__main__':
+    unittest.main()