diff --git a/phiml/math/_shape.py b/phiml/math/_shape.py
index 7429859..31d6a1b 100644
--- a/phiml/math/_shape.py
+++ b/phiml/math/_shape.py
@@ -1784,7 +1784,7 @@ def dual(*args, **dims: Union[int, str, tuple, list, Shape, 'Tensor']) -> Shape:
         raise AssertionError(f"dual() must be called either as a selector dual(Shape) or dual(Tensor) or as a constructor dual(*names, **dims). Got *args={args}, **dims={dims}")
     
 
-def auto(spec: Union[str, Shape], default_type=None) -> Shape:
+def auto(spec: Union[str, Shape], default_type: Callable = None) -> Shape:
     """
     Create a single-dimension `Shape` from a specification string.
     
@@ -1803,55 +1803,38 @@ def auto(spec: Union[str, Shape], default_type=None) -> Shape:
         return spec  # allow multi-dim Shapes as well, as the main application is stacking
     assert isinstance(spec, str), f"spec must be a Shape or str but got {type(spec)}"
     assert ',' not in spec, f"auto dim only supported for single dimensions"
-    dim_type = None
-    dim_name = spec
-    for dt, char in SUPERSCRIPT.items():
-        if spec.endswith(char):
-            dim_type = dt
-            dim_name = spec[:-1]
-            break
-    else:
-        for dt, char in CHAR.items():
-            if spec.endswith(':' + char):
-                dim_type = dt
-                dim_name = spec[:-2]
-                break
-    if spec.startswith('~'):
-        assert dim_type == DUAL_DIM or dim_type is None, f"Inconsistent dim types for '{spec}'. '~' indicates dual dimension but declared type is {dim_type}"
-        dim_type = DUAL_DIM
-        dim_name = dim_name[1:]
-    if dim_type is None:
-        assert default_type is not None, f"No dim type specified: '{spec}'"
-        if callable(default_type):
-            return default_type(spec)
-        else:
-            assert isinstance(default_type, str), f"default_type must be a dimension generator or str but got {type(default_type)}"
-            dim_type = default_type
-    return _construct_shape(dim_type, dim_name)
+    return parse_shape_spec(spec, default_type=default_type)
 
 
 class InvalidShapeSpec(ValueError):
     pass
 
 
-def parse_shape_spec(input_string) -> Shape:
-    name_type_items = re.compile(r'(~?)(\w+):(\w+)=\(([^)]*)\)')
-    name_type = re.compile(r'(~?)(\w+):(\w+)')
-    name_items = re.compile(r'(~?)(\w+)=\(([^)]*)\)')
-    items = re.compile(r'(~?)\(([^)]*)\)')
-    dual_name = re.compile(r'(~\w+)')
+SPEC_PATTERNS = {
+    'name_type_items': re.compile(r'(~?)(\w+):(\w+)=\(([^)]*)\)'),
+    'name_type': re.compile(r'(~?)(\w+):(\w+)'),
+    'name_items': re.compile(r'(~?)(\w+)=\(([^)]*)\)'),
+    'items': re.compile(r'(~?)\(([^)]*)\)'),
+    'dual_name': re.compile(r'(~\w+)'),
+    'single_letter': re.compile(r'(\w)(?=,|$)'),
+    'name_only': re.compile(r'(\w+)(?=,|$)')
+}
+
+
+def parse_shape_spec(input_string, default_type: Callable = None) -> Shape:
     results = []
     pos = 0
     while pos < len(input_string):
-        if match := name_type_items.match(input_string, pos):
+        if match := SPEC_PATTERNS['name_type_items'].match(input_string, pos):
             tilde, name, type_, values = match.groups()
             if tilde and type_ not in ('d', 'dual'):
                 raise InvalidShapeSpec(input_string, f"Dimension names starting with ~ must be of type dual. Failed at index {pos}: {input_string[pos:]}")
             elif not tilde and type_ in ('d', 'dual'):
                 raise InvalidShapeSpec(input_string, f"Dual dims must start with ~. Failed at index {pos}: {input_string[pos:]}")
-            results.append({'name': '~' + name if tilde else name, 'type': type_, 'values': values.split(',')})
+            items = [n.strip() for n in values.split(',') if n.strip()]
+            results.append({'name': '~' + name if tilde else name, 'type': type_, 'values': items})
             pos = match.end() + 1
-        elif match := name_type.match(input_string, pos):
+        elif match := SPEC_PATTERNS['name_type'].match(input_string, pos):
             tilde, name, type_ = match.groups()
             if tilde and type_ not in ('d', 'dual'):
                 raise InvalidShapeSpec(input_string, f"Dimension names starting with ~ must be of type dual. Failed at index {pos}: {input_string[pos:]}")
@@ -1863,18 +1846,28 @@ def parse_shape_spec(input_string) -> Shape:
                 raise ValueError(f"Invalid format at position {pos}: values must be inside parentheses")
             results.append({'name': '~' + name if tilde else name, 'type': type_})
             pos = match.end() + 1
-        elif match := name_items.match(input_string, pos):
+        elif match := SPEC_PATTERNS['name_items'].match(input_string, pos):
             tilde, name, values = match.groups()
-            results.append({'name': '~' + name if tilde else name, 'type': 'd' if tilde else 'c', 'values': values.split(',')})
+            items = [n.strip() for n in values.split(',') if n.strip()]
+            results.append({'name': '~' + name if tilde else name, 'type': 'd' if tilde else 'c', 'values': items})
             pos = match.end() + 1
-        elif match := items.match(input_string, pos):
+        elif match := SPEC_PATTERNS['items'].match(input_string, pos):
             tilde, values = match.groups()
             results.append({'name': '~vector' if tilde else 'vector', 'type': 'd' if tilde else 'c', 'values': values.split(',')})
             pos = match.end() + 1
-        elif match := dual_name.match(input_string, pos):
+        elif match := SPEC_PATTERNS['dual_name'].match(input_string, pos):
             name, = match.groups()
             results.append({'name': name, 'type': 'd'})
             pos = match.end() + 1
+        elif match := SPEC_PATTERNS['single_letter'].match(input_string, pos):
+            name, = match.groups()
+            results.append({'name': name, 'type': 's'})
+            pos = match.end() + 1
+        elif default_type is not None and (match := SPEC_PATTERNS['name_only'].match(input_string, pos)):
+            name, = match.groups()
+            default_type_str = TYPE_BY_FUNCTION[default_type]
+            results.append({'name': name, 'type': default_type_str})
+            pos = match.end() + 1
         else:
             raise InvalidShapeSpec(input_string, f"Failed to parse from index {pos}: '{input_string[pos:]}'. Dims must be specified as name:type or name:type=(item_names...). Names and types may only be omitted if component names are given.")
     names = [r['name'] for r in results]