diff --git a/inflection/__init__.py b/inflection/__init__.py
index ffac5ce..a5337c0 100644
--- a/inflection/__init__.py
+++ b/inflection/__init__.py
@@ -277,7 +277,7 @@ def parameterize(string: str, separator: str = '-') -> str:
     if separator:
         re_sep = re.escape(separator)
         # No more than one of the separator in a row.
-        string = re.sub(r'%s{2,}' % re_sep, separator, string)
+        string = re.sub(r'(?:%s){2,}' % re_sep, separator, string)
         # Remove leading/trailing separator.
         string = re.sub(r"(?i)^{sep}|{sep}$".format(sep=re_sep), '', string)
 
diff --git a/test_inflection.py b/test_inflection.py
index f1e64a3..53c04d8 100644
--- a/test_inflection.py
+++ b/test_inflection.py
@@ -402,6 +402,13 @@ def test_parameterize_with_multi_character_separator(
     )
 
 
+def test_parameterize_with_multi_character_separator_partial_collapse() -> None:
+    # Regression test: multi-character separator should not consume
+    # non-separator characters that happen to overlap with separator chars.
+    # '__' is a 2-char separator, so '___' = one separator + one literal '_'.
+    assert 'test___case' == inflection.parameterize('test___case', '__')
+
+
 @pytest.mark.parametrize(
     ("some_string", "parameterized_string"),
     STRING_TO_PARAMETERIZE_WITH_NO_SEPARATOR