diff --git a/src/interscript/expr.py b/src/interscript/expr.py
index fdfd689..18cd861 100644
--- a/src/interscript/expr.py
+++ b/src/interscript/expr.py
@@ -17,6 +17,7 @@
     r'|any\(\s*"(?P<rlo>(?:[^"\\]|\\.)*)"\s*\.\.\s*"(?P<rhi>(?:[^"\\]|\\.)*)"\s*\)'
     r'|any\(\s*"(?P<cls>(?:[^"\\]|\\.)*)"\s*\)'
     r"|(?P<space>\bspace\b)|(?P<boundary>\bboundary\b)"
+    r"|(?P<stdin_kw>\b(?:alpha|digit|word|any_character)\b)"
     r"|(?P<nwb>\bnon_word_boundary\b)"
     r'|capture\(\s*(?P<grp>(?:[^()\\]|\\.|\([^()]*\))*)\s*\)' 
     r"|(?P<line_end>\bline_end\b)|(?P<line_start>\bline_start\b)"
@@ -77,6 +78,15 @@ def _read_bracketed(expr: str, pos: int) -> tuple[str, int]:
 
 SPACE = re.escape(" ")
 
+# Ruby Stdlib::ALIASES, ASCII-exact as Onigmo defines them (Python's
+# \w and \d are unicode-wide; Onigmo's are not).
+_STDLIB_REGEX = {
+    "alpha": "[a-zA-Z]",
+    "digit": "[0-9]",
+    "word": "[a-zA-Z0-9_]",
+    "any_character": ".",
+}
+
 # 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
@@ -163,6 +173,8 @@ def _scan(expr: str, want: str):
             out.append(("nwb", ""))
         elif g["grp"] is not None:
             out.append(("grp", g["grp"]))
+        elif g["stdin_kw"] is not None:
+            out.append(("stdlib", g["stdin_kw"]))
         elif g["line_end"] is not None:
             out.append(("anchor", "$"))
         elif g["line_start"] is not None:
@@ -196,6 +208,8 @@ def _lb_branches(expr: str) -> list[str]:
             expanded = [re.escape(value)]
         elif kind == "cls":
             expanded = ["[" + re.escape(value) + "]"]
+        elif kind == "stdlib":
+            expanded = [_STDLIB_REGEX[value]]
         elif kind == "range":
             lo, hi = value.split("\x00")
             expanded = ["[" + re.escape(lo) + "-" + re.escape(hi) + "]"]
@@ -240,6 +254,8 @@ def expr_to_regex(expr: str) -> str:
             parts.append("(?:" + expr_to_regex(value) + ")?")
         elif kind == "space":
             parts.append(SPACE)
+        elif kind == "stdlib":
+            parts.append(_STDLIB_REGEX[value])
         elif kind == "boundary":
             parts.append(_BOUNDARY)
         elif kind == "nwb":
@@ -281,7 +297,7 @@ def expr_max_length(expr: str) -> int:
     for kind, value in _scan(expr, "expression"):
         if kind == "lit":
             total += len(value)
-        elif kind in ("cls", "range", "space", "boundary", "nwb", "anchor"):
+        elif kind in ("cls", "range", "space", "boundary", "nwb", "anchor", "stdlib"):
             total += 1
         elif kind == "alt":
             total += max(expr_max_length(a) for a in value.split("\x00"))
diff --git a/src/interscript/isc.py b/src/interscript/isc.py
index 68f07ed..21e67bf 100644
--- a/src/interscript/isc.py
+++ b/src/interscript/isc.py
@@ -53,6 +53,9 @@ def __init__(self, subs: list[dict], capture_rules: list[dict]) -> None:
 PRIMITIVES = {"boundary", "line_start", "line_end", "word_boundary", "non_word_boundary", "space"}
 _FUNCTIONS = {"upcase", "downcase", "title_case", "reverse", "strip", "swapcase"}
 _CONSTRAINTS = {"before", "after", "not_before", "not_after"}
+# Stdlib aliases usable as bare names (Ruby Stdlib::ALIASES, resolved
+# before doc-local aliases, mirroring the interpreter's lookup order).
+_STDLIB_EXPR = {"alpha", "digit", "word", "any_character"}
 # Tokens that terminate an item inside a rule; a bare word equal to one
 # of these is a keyword, never an alias reference.
 _KEYWORDS = _CONSTRAINTS | {"to", "from", "note"}
@@ -688,6 +691,8 @@ def _render_item(item: dict, aliases: dict[str, str]) -> str:
         name = item["name"]
         if item.get("map"):
             return _qualified_expr(item["map"], name)
+        if name in _STDLIB_EXPR:
+            return name
         if name in _imported:
             return _qualified_expr(None, name)
         if name not in aliases:
@@ -750,6 +755,8 @@ def _regex_of(item: dict, aliases: dict[str, str]) -> str:
         name = item["name"]
         if item.get("map"):
             return _qualified_regex(item["map"], name)
+        if name in _STDLIB_EXPR:
+            return {"alpha": "[a-zA-Z]", "digit": "[0-9]", "word": "[a-zA-Z0-9_]", "any_character": "."}[name]
         if name in _imported:
             return _qualified_regex(None, name)
         if name not in aliases:
@@ -788,6 +795,10 @@ def _repl_of(item: dict, aliases: dict[str, str]) -> str:
         if name not in aliases:
             raise UnsupportedConstruct(f"unresolved alias {name}")
         raise UnsupportedConstruct("alias in a result")
+    if kind == "primitive":
+        if item["name"] == "space":
+            return " "
+        raise UnsupportedConstruct(f"primitive {item['name']} in a result")
     raise UnsupportedConstruct(f"item kind {kind} in a result")
 
 
diff --git a/tests/test_engine.py b/tests/test_engine.py
index 6fee48b..9ce9f01 100644
--- a/tests/test_engine.py
+++ b/tests/test_engine.py
@@ -281,3 +281,13 @@ def test_maybe_accepts_full_expressions():
     # empty maybe + "c" still matches the bare c (Ruby parallel
     # semantics: the rule consumes just "c" here).
     assert transliterate("mb", "dc") == "dQ"
+
+
+@pytest.mark.skipif(not MAPS.is_dir(), reason="interscript maps repo not present")
+def test_primitive_space_result_pads_the_string():
+    """moct-kor pads with `sub line_start space` / `sub line_end space`;
+    the subst renderer rejected primitive results and silently dropped
+    the rules, so the before-space guards on initial consonants never
+    fired: 불국사 -> ᄇulguksa instead of Bulguksa."""
+    e = _load("moct-kor-Hang-Latn-2000")
+    assert e.transliterate("불국사") == "Bulguksa"