diff --git a/opensyndrome/filter.py b/opensyndrome/filter.py
index cbad64e..6d6100a 100644
--- a/opensyndrome/filter.py
+++ b/opensyndrome/filter.py
@@ -1,11 +1,13 @@
 from __future__ import annotations
+
+import re
 from dataclasses import dataclass, field
 from typing import Any
+
 import polars as pl
 
 from opensyndrome import schema
 
-
 _VALID_CONCEPTS = [
     format_type.value for format_type in schema.Type if format_type.value != "criterion"
 ]
@@ -166,16 +168,28 @@ def _cast(
 
 
 def _code_to_regex(code: str) -> str:
-    """Convert an OSD code to a regex pattern.
+    """Convert an OSD code to a regex pattern, ignoring dot placement.
+
+    ICD-10 codes are conventionally written with a dot after the 3-character
+    category (``A69.2``), but plenty of real definitions and datasets omit it
+    (``A692``) — SIA's own billing codes among them, and NSSP/CCDD category
+    lists the other way around. *code* and the value it is matched against
+    are compared with dots optional at that one position, so it makes no
+    difference which side, if either, carries one: ``A692``, ``A69.2`` and
+    ``A69`` all match ``A69.2`` and ``A692`` alike.
 
-    - ``%`` wildcard → ``.*`` (e.g. ``J1%`` → ``^J1.*$``)
-    - Exact code → anchored to avoid prefix collisions
-      (e.g. ``A90`` matches ``A90`` and ``A90.1`` but not ``A900``)
+    ``%`` is still a wildcard (``J1%`` → ``^J1.*$``).
     """
-    if "%" in code:
-        return f"^{code.replace('%', '.*')}$"
-    # Allow optional sub-code suffix (dot-separated), e.g. A90.1
-    return f"^{code}(\\..+)?$"
+    stripped = code.replace(".", "")
+    if "%" in stripped:
+        return f"^{re.escape(stripped).replace('%', '.*')}$"
+
+    category, sub_code = stripped[:3], stripped[3:]
+    pattern = re.escape(category)
+    if sub_code:
+        pattern += r"\.?" + re.escape(sub_code)
+    # A value with more digits than the code is still one of its sub-codes.
+    return f"^{pattern}(\\.?\\w+)?$"
 
 
 def _apply_flags(pattern: str, flags: str) -> str:
diff --git a/tests/test_filter.py b/tests/test_filter.py
index c5c83ad..d5e1ffc 100644
--- a/tests/test_filter.py
+++ b/tests/test_filter.py
@@ -1,23 +1,24 @@
 import re
-import pytest
+
 import polars as pl
+import pytest
 
 from opensyndrome.filter import (
     ColumnSpec,
-    load_profile,
-    ProfileData,
     InvalidOperator,
-    UnresolvableCriterion,
     OSDEngine,
-    _code_to_regex,
+    ProfileData,
+    UnresolvableCriterion,
     _apply_flags,
-    _cast,
-    _combine,
-    _build_code_expr,
-    _build_text_expr,
     _build_attr_expr,
+    _build_code_expr,
     _build_ontology_id_expr,
+    _build_text_expr,
+    _cast,
+    _code_to_regex,
+    _combine,
     _parse_criterion,
+    load_profile,
 )
 
 SEX_ENCODINGS = {"sex": {"male": "M", "female": "F"}}
@@ -155,7 +156,7 @@ class TestCodeToRegex:
     @pytest.mark.parametrize(
         "code, matching, non_matching",
         [
-            ("A90", ["A90", "A90.1", "A90.12"], ["A900", "A901", "B90"]),
+            ("A90", ["A90", "A90.1", "A90.12", "A900", "A901"], ["B90", "A9"]),
             ("J1%", ["J10", "J11", "J18", "J1X"], ["J20", "K10"]),
             ("A9%", ["A90", "A91", "A99"], ["B90", "A100"]),
         ],
@@ -171,15 +172,34 @@ def test_pattern_matches_and_rejects(self, code, matching, non_matching):
                 pattern, value
             ), f"Expected '{value}' NOT to match pattern for '{code}'"
 
-    def test_exact_code_does_not_match_extension(self):
-        pattern = _code_to_regex("A90")
-        assert not re.match(pattern, "A900")
-        assert not re.match(pattern, "A901")
-
-    def test_exact_code_matches_sub_code(self):
+    def test_exact_code_matches_sub_code_with_or_without_dot(self):
+        # ICD-10's own convention dots the sub-code (A90.1), but plenty of real
+        # datasets and definitions don't (SIA's billing codes, NSSP/CCDD).
         pattern = _code_to_regex("A90")
         assert re.match(pattern, "A90.1")
         assert re.match(pattern, "A90.12")
+        assert re.match(pattern, "A901")
+        assert re.match(pattern, "A9012")
+
+    @pytest.mark.parametrize(
+        "code, value",
+        [
+            ("A692", "A69.2"),  # definition undotted, value dotted (NSSP/CCDD style)
+            ("A69.2", "A692"),  # definition dotted, value undotted (SIA style)
+            ("A69.2", "A69.2"),  # both dotted
+            ("A692", "A692"),  # both undotted
+        ],
+    )
+    def test_matches_regardless_of_which_side_carries_the_dot(self, code, value):
+        pattern = _code_to_regex(code)
+        assert re.match(
+            pattern, value
+        ), f"Expected '{value}' to match pattern for '{code}'"
+
+    def test_unrelated_category_still_rejected(self):
+        pattern = _code_to_regex("A692")
+        assert not re.match(pattern, "A693")
+        assert not re.match(pattern, "B692")
 
 
 class TestApplyFlags: