diff --git a/soundchanger/change.py b/soundchanger/change.py
index c4d1fcd..a9c7f79 100644
--- a/soundchanger/change.py
+++ b/soundchanger/change.py
@@ -1,7 +1,7 @@
 import re
 
 
-def apply(change, string, categories={}, apply=True):
+def apply(change, string, categories={}, apply=True, zero_characters=['∅']):
     """Apply a sound change to a given string"""
 
     if not apply:
@@ -10,14 +10,13 @@ def apply(change, string, categories={}, apply=True):
     # Check validity of change
     if change.count('>') != 1:
         raise ValueError(f"Change {change} is not a valid sound change. (Missing character '>')")
-
     if change.count('/') > 1:
         raise ValueError(f"Change {change} is not a valid sound change. (More than one '/' character)")
 
     # Prepare change for regex
-    for k, v in {'{': '(', '}': ')', ',': '|'}.items():
+    for k, v in {' ': '', '{': '(', '}': ')', ',': '|'}.items():
         change = change.replace(k, v)
-    
+
     # Replacements for categories
     for k, v in categories.items():
         # Replacement of categories inside brackets (before or after a comma)
@@ -30,15 +29,24 @@ def apply(change, string, categories={}, apply=True):
     else:
         environment = '_'
 
+    # Collaplse multiple underscores
+    environment = re.sub('(_+)', '_', environment)
+
     if environment.count('_') != 1:
         raise ValueError(f"Environment {environment} is not a valid environment. (Character '_' should exist exactly once)")
 
     original, change_to = change.split('>')
     before, after = environment.split('_')
 
-    n = before.count('(') + original.count('(') + 3
+    if not original:
+        raise ValueError(f"Nothing to change from.")
+
+    # Remove zero characters
+    for char in zero_characters:
+        change_to = change_to.replace(char, '')
+
     pattern = f"({before})({original})({after})"
-    repl = f"\\1{change_to}\\{n}"
-
-    return re.sub(pattern, repl, f"#{string}#").strip('#')
+    last_group_index = pattern.count('(') - after.count('(')
+    replacement = f"\\1{change_to}\\{last_group_index}"
 
+    return re.sub(pattern, replacement, f"#{string}#").strip('#')
diff --git a/tests/test_change.py b/tests/test_change.py
index 8d594ad..fba8203 100644
--- a/tests/test_change.py
+++ b/tests/test_change.py
@@ -1,13 +1,43 @@
 from soundchanger.change import apply
 
 
-def test_apply_without_environment():
+def test_without_environment():
     assert apply('p>h', 'pana') == 'hana'
 
-def test_apply_with_environment():
+def test_with_environment():
     assert apply('p>f/#_u', 'pune') == 'fune'
 
-def test_apply_with_complex_environment():
+def test_with_partial_environment():
+    assert apply('p>h/#_', 'pi') == 'hi'
+    assert apply('p>h/_i', 'pi') == 'hi'
+
+def test_change_to_nothing():
+    assert apply('u>', 'fun') == 'fn'
+
+def test_change_to_nothing_with_environment():
+    assert apply('u>/_n#', 'fun') == 'fn'
+
+def test_change_to_zero_character_with_environment():
+    assert apply('w>∅', 'kawa') == 'kaa'
+
+def test_custom_zero_character():
+    assert apply('w>-', 'kawa', zero_characters=['-']) == 'kaa'
+
+def test_whitespace_in_change():
+    assert apply(' a > b / c _ d ', 'cad') == 'cbd'
+
+def test_multiple_underscores_in_environment():
+    assert apply('a>b/c___d ', 'cad') == 'cbd'
+
+def test_categories_as_string():
+    assert apply('V>o', 'ha', categories={'V': 'aiu'}) == 'ho'
+    assert apply('V>o', 'he', categories={'V': 'aiu'}) == 'he'
+
+def test_categories_as_list():
+    assert apply('V>o', 'ha', categories={'V': ['a', 'i', 'u']}) == 'ho'
+    assert apply('V>o', 'he', categories={'V': ['a', 'i', 'u']}) == 'he'
+
+def test_complex_environment():
     inputs =  ['pana', 'pina', 'puna', 'pama', 'pima', 'puma']
     outputs = ['pana', 'hina', 'huna', 'pama', 'hima', 'huma']
     for string, output in zip(inputs, outputs):
@@ -17,14 +47,14 @@ def test_apply_with_complex_environment():
                 categories={'N': 'nm'}
             ) == output
 
-def test_apply_with_complex_group_and_categories():
+def test_complex_group_and_categories():
     assert apply('e>i/{#,p,t,k}V{m,n,h}_#', 'pane', categories={
         'V': 'aiu',
         'P': 'ptk',
         'N': 'mn',
         }) == 'pani'
 
-def test_apply_with_complex_group_and_categories_and_digraphs():
+def test_with_complex_group_and_categories_and_digraphs():
     assert apply('u>o/#V{ts,pf,t}_k{V,e,o}#', 'atsuka', categories={
         'V': ['a', 'i', 'u']
     }) == 'atsoka'
@@ -34,3 +64,14 @@ def test_apply_with_complex_group_and_categories_and_digraphs():
     assert apply('u>o/#V{ts,pf,t}_k{V,e,o}#', 'matsuka', categories={
         'V': ['a', 'i', 'u']
     }) == 'matsuka'
+
+def test_multiple_changes():
+    string = 'pana'
+    for change in ['p>f', 'n>m/fa_']:
+        string = apply(change, string)
+    assert string == 'fama'
+
+def test_change_from_category():
+    assert apply('V>o', 'tatiru', categories={
+        'V': 'aiu'
+    }) == 'totoro'
diff --git a/tests/test_change_errors.py b/tests/test_change_errors.py
index 14fb14a..da6901c 100644
--- a/tests/test_change_errors.py
+++ b/tests/test_change_errors.py
@@ -2,14 +2,18 @@ import pytest
 import re
 from soundchanger.change import apply
 
-def test_parse_change_error_change():
+def test_error_in_change():
     with pytest.raises(ValueError, match=re.escape("Change  is not a valid sound change. (Missing character '>')")):
         apply('', '')
 
-def test_apply_error_split():
+def test_error_split():
     with pytest.raises(ValueError, match=re.escape("Change a>b/c/d is not a valid sound change. (More than one '/' character)")):
         apply('a>b/c/d', '')
 
-def test_apply_error_environment():
+def test_error_in_environment():
     with pytest.raises(ValueError, match=re.escape("Environment c is not a valid environment. (Character '_' should exist exactly once)")):
         apply('a>b/c', '')
+
+def test_no_change_from():
+    with pytest.raises(ValueError, match=re.escape("Nothing to change from.")):
+        apply('>b', '')