diff --git a/src/interscript/expr.py b/src/interscript/expr.py
index ddbc139..78a133f 100644
--- a/src/interscript/expr.py
+++ b/src/interscript/expr.py
@@ -29,6 +29,16 @@
 
 SPACE = re.escape(" ")
 
+# Ruby's \b counts combining marks as word characters; Python's \w
+# does not (Mn is not alphanumeric). At a hamza-carrier + kasra
+# junction Ruby sees no boundary while Python does — word-final rules
+# fired wrongly and doubled vowels. Express the boundary as an
+# explicit word/non-word transition over a word class that includes
+# combining marks (Mnemonic ranges: combining diacritics 0300-036F,
+# Arabic diacritics 064B-065F, 0670, and Quranic annotation 06D6-06ED).
+_WORD = r"[\w\u0300-\u036F\u064B-\u065F\u0670\u06D6-\u06ED]"
+_BOUNDARY = "(?:(?<=" + _WORD + ")(?!" + _WORD + ")|(?<!" + _WORD + ")(?=" + _WORD + "))"
+
 
 def _unesc(s: str) -> str:
     return _UNESC.sub(lambda m: chr(int(m.group(1), 16)), s)
@@ -95,7 +105,7 @@ def expr_to_regex(expr: str) -> str:
         elif kind == "space":
             parts.append(SPACE)
         elif kind == "boundary":
-            parts.append(r"\b")
+            parts.append(_BOUNDARY)
         elif kind == "nwb":
             parts.append(r"\B")
         elif kind == "grp":
diff --git a/tests/test_engine.py b/tests/test_engine.py
index 2aa1c11..6cbc0e9 100644
--- a/tests/test_engine.py
+++ b/tests/test_engine.py
@@ -135,3 +135,20 @@ def test_parallel_selection_matches_ruby_max_length():
         '    sub "abc", "Y"\n  }\n}\n'
     )
     assert Engine(tree2).transliterate("abc") == "Y"
+
+
+def test_boundary_treats_combining_marks_as_word_chars():
+    """Ruby's \\b counts combining marks (Arabic diacritics) as word
+    characters — a word-final rule must not fire when a kasra follows
+    the hamza carrier (dā'im, not dā'aim)."""
+    tree = parse_imp(
+        'stage {\n  parallel {\n'
+        '    sub "ئ" + boundary, "\'a"\n'
+        '    sub "ئ", "\'"\n'
+        '    sub "ِ", "i"\n'
+        '    sub "d", "d"\n  }\n}\n'
+    )
+    # ئ + kasra: no boundary — the bare-ئ rule fires, not the final one.
+    assert Engine(tree).transliterate("dئِ") == "d'i"
+    # ئ at a true word end: the boundary rule fires.
+    assert Engine(tree).transliterate("dئ") == "d'a"