Add split_numbers parameter to underscore function.

inflection/__init__.py

@@ -394,7 +394,7 @@ def transliterate(string: str) -> str:
     return normalized.encode('ascii', 'ignore').decode('ascii')
 
 
-def underscore(word: str) -> str:
+def underscore(word: str, split_numbers: bool = False) -> str:
     """
     Make an underscored, lowercase form from the expression in the string.
 
@@ -403,6 +403,14 @@ def underscore(word: str) -> str:
         >>> underscore("DeviceType")
         'device_type'
 
+    If ``split_numbers`` is ``True`` inserts underscores between
+    numbers and alphabet characters.
+
+    Example::
+
+        >>> underscore('xyz123', split_numbers=True)
+        'xyz_123'
+
     As a rule of thumb you can think of :func:`underscore` as the inverse of
     :func:`camelize`, though there are cases where that does not hold::
 
@@ -411,6 +419,9 @@ def underscore(word: str) -> str:
 
     """
     word = re.sub(r"([A-Z]+)([A-Z][a-z])", r'\1_\2', word)
+    if split_numbers:
+        word = re.sub(r"([a-zA-Z])(\d)", r'\1_\2', word)
+        word = re.sub(r"(\d)([a-z])", r'\1_\2', word)
     word = re.sub(r"([a-z\d])([A-Z])", r'\1_\2', word)
     word = word.replace("-", "_")
     return word.lower()

test_inflection.py

@@ -125,6 +125,13 @@
     ("HTML",                  "html"),
 )
 
+CAMEL_TO_UNDERSCORE_SPLITTING_NUMBERS = (
+    ("xyz123", "xyz_123"),
+    ("xyZ123", "xy_z_123"),
+    ("xy123z", "xy_123_z"),
+    ("xy123Z", "xy_123_z")
+)
+
 STRING_TO_PARAMETERIZED = (
     (u"Donald E. Knuth", "donald-e-knuth"),
     (
@@ -360,6 +367,14 @@ def test_underscore(camel, underscore):
     assert underscore == inflection.underscore(camel)
 
 
+@pytest.mark.parametrize(
+    ("camel", "underscore"),
+    CAMEL_TO_UNDERSCORE_SPLITTING_NUMBERS
+)
+def test_underscore_split_numbers(camel, underscore):
+    assert underscore == inflection.underscore(camel, split_numbers=True)
+
+
 @pytest.mark.parametrize(
     ("some_string", "parameterized_string"),
     STRING_TO_PARAMETERIZED