diff --git a/dol/paths.py b/dol/paths.py
index 3c29d3bf..55571bfd 100644
--- a/dol/paths.py
+++ b/dol/paths.py
@@ -1152,6 +1152,10 @@ def _field_names(string_template):
     )
 
 
+def identity(x):
+    return x
+
+
 Codec = namedtuple('Codec', 'encoder decoder')
 FieldTypeNames = Literal['str', 'dict', 'tuple', 'namedtuple', 'simple_str']
 
@@ -1176,7 +1180,7 @@ class StringTemplate:
         template: A template string with fields to be extracted or filled in.
         field_patterns: A dictionary of field names and their regex patterns.
         simple_str_sep: A separator string for simple strings (i.e. strings without
-            fields). If `None`, the template string will be used as the separator.
+            fields).
 
     Examples:
         >>> st = StringTemplate(
@@ -1209,57 +1213,103 @@ def __init__(
         template: str,
         *,
         field_patterns: dict = None,
-        simple_str_sep: str = None,
+        to_str_funcs: dict = None,
+        from_str_funcs: dict = None,
+        simple_str_sep: str = ',',
         namedtuple_type_name: str = 'NamedTuple',
         dflt_pattern: str = '.*',
     ):
         self._original_template = template
-        self.field_patterns = field_patterns or {}
         self.simple_str_sep = simple_str_sep
         self.namedtuple_type_name = namedtuple_type_name
         self.dflt_pattern = dflt_pattern
-        self.template = self._normalize_template(template)
-        self.field_names = _field_names(self.template)
+
+        (
+            self.template,
+            self.field_names,
+            to_str_funcs,
+            field_patterns_,
+        ) = self._extract_template_info(template)
+
+        self.field_patterns = dict(
+            {field: self.dflt_pattern for field in self.field_names},
+            **dict(field_patterns_, **(field_patterns or {})),
+        )
+        self.to_str_funcs = dict(
+            {field: str for field in self.field_names},
+            **dict(to_str_funcs, **(to_str_funcs or {})),
+        )
+        self.from_str_funcs = dict(
+            {field: identity for field in self.field_names}, **(from_str_funcs or {})
+        )
         self.regex = self._compile_regex(self.template)
 
-    def _normalize_template(self, template):
-        r"""Normalizes the template so that each placeholder has a name and format_spec.
+    def _extract_template_info(self, template):
+        r"""Extracts information from the template. Namely:
+
+        - normalized_template: A template where each placeholder has a field name
+        (if not given, "_{index}" will be used)
 
-        >>> StringTemplate('{}.ext').template
-        '{_1:.*}.ext'
-        >>> StringTemplate('{name}.ext').template
-        '{name:.*}.ext'
-        >>> StringTemplate('{:\w+}.ext').template
-        '{_1:\w+}.ext'
-        >>> StringTemplate('{name:\w+}.ext').template
-        '{name:\w+}.ext'
+        - field_names: The tuple of field names in the order they appear in template
+
+        - to_str_funcs: A dict of field names and their corresponding to_str functions,
+        which will be used to convert the field values to strings when generating a
+        string.
+
+        - field_patterns_: A dict of field names and their corresponding regex patterns,
+        which will be used to extract the field values from a string.
+
+        These four values are used in the init to compute the parameters of the
+        instance.
+
+        >>> st = StringTemplate('{:02.0f}/{name::\w+}')
+        >>> st.template
+        '{_1}/{name}'
+        >>> st.field_names
+        ('_1', 'name')
+        >>> st.field_patterns
+        {'_1': '.*', 'name': '\\w+'}
+        >>> st.regex.pattern
+        '(?P<_1>.*)/(?P<name>\\w+)'
+        >>> to_str_funcs = st.to_str_funcs
+        >>> to_str_funcs['_1'](3)
+        '03'
+        >>> to_str_funcs['name']('Alice')
+        'Alice'
 
         """
 
-        def get_format_spec(field_name, current_format_spec):
-            if current_format_spec == '':
-                return self.field_patterns.get(field_name, self.dflt_pattern)
-            return current_format_spec
+        field_names = []
+        field_patterns_ = {}
+        to_str_funcs = {}
 
         def parse_and_transform():
             for index, (literal_text, field_name, format_spec, conversion) in enumerate(
                 self._formatter.parse(template), 1
             ):
+                field_name = f"_{index}" if field_name == '' else field_name
+                if field_name is not None:
+                    field_names.append(field_name)  # remember the field name
+                    # extract format and pattern information:
+                    if ':' not in format_spec:
+                        format_spec += ':'
+                    to_str_func_format, pattern = format_spec.split(':')
+                    if to_str_func_format:
+                        to_str_funcs[field_name] = (
+                            '{' + f":{to_str_func_format}" + '}'
+                        ).format
+                    field_patterns_[field_name] = pattern or self.dflt_pattern
+                # At this point you should have a valid field_name and empty format_spec
                 yield (
                     literal_text,
-                    f"_{index}" if field_name == '' else field_name,
-                    get_format_spec(field_name, format_spec),
+                    field_name,
+                    '',
                     conversion,
                 )
 
-        return string_unparse(parse_and_transform())
+        normalized_template = string_unparse(parse_and_transform())
+        return normalized_template, tuple(field_names), to_str_funcs, field_patterns_
 
-    # TODO: For now we harded coded the "interpret format_specs as field patterns",
-    #   but we could make this more general and allow for other types of
-    #   "interpretations" (e.g. "interpret format_specs as cast functions").
-    #   Note: We removed the cast functions (that existed in legacy StrTupleDict class)
-    #   to keep it simple, for now. The idea being that if you want to cast, you can
-    #   do it yourself by adding egress/ingress to codecs.
     def _compile_regex(self, template):
         r"""Parses the template, generating regex for matching the template.
         Essentially, it weaves together the literal text parts and the format_specs
@@ -1276,29 +1326,36 @@ def _compile_regex(self, template):
         '(?P<_1>.*)\\.ext'
         >>> StringTemplate('{name}.ext').regex.pattern
         '(?P<name>.*)\\.ext'
-        >>> StringTemplate('{:\w+}.ext').regex.pattern
+        >>> StringTemplate('{::\w+}.ext').regex.pattern
         '(?P<_1>\\w+)\\.ext'
-        >>> StringTemplate('{name:\w+}.ext').regex.pattern
+        >>> StringTemplate('{name::\w+}.ext').regex.pattern
+        '(?P<name>\\w+)\\.ext'
+        >>> StringTemplate('{:0.02f:\w+}.ext').regex.pattern
+        '(?P<_1>\\w+)\\.ext'
+        >>> StringTemplate('{name:0.02f:\w+}.ext').regex.pattern
         '(?P<name>\\w+)\\.ext'
-
         """
 
-        def mk_named_capture_group(field_name, format_spec):
-            if field_name and format_spec:
-                return f"(?P<{field_name}>{format_spec})"
+        def mk_named_capture_group(field_name):
+            if field_name:
+                return f"(?P<{field_name}>{self.field_patterns[field_name]})"
             else:
                 return ""
 
         def generate_pattern_parts(template):
             parts = self._formatter.parse(template)
-            for literal_text, field_name, format_spec, conversion in parts:
-                yield (
-                    re.escape(literal_text)
-                    + mk_named_capture_group(field_name, format_spec)
-                )
+            for literal_text, field_name, _, _ in parts:
+                yield re.escape(literal_text) + mk_named_capture_group(field_name)
 
         return re.compile(''.join(generate_pattern_parts(template)))
 
+    @staticmethod
+    def _assert_field_type(field_type: FieldTypeNames, name='field_type'):
+        if field_type not in FieldTypeNames.__args__:
+            raise ValueError(
+                f"{name} must be one of {FieldTypeNames}. Was: {field_type}"
+            )
+
     def codec(self, source: FieldTypeNames, target: FieldTypeNames):
         """Makes a ``(coder, decoder)`` pair for the given source and target types.
 
@@ -1312,10 +1369,19 @@ def codec(self, source: FieldTypeNames, target: FieldTypeNames):
         >>> encoder({'name': 'Alice', 'age': '30'})
         ('Alice', '30')
         """
+        self._assert_field_type(target, 'target')
+        self._assert_field_type(source, 'source')
         coder = getattr(self, f'{source}_to_{target}')
         decoder = getattr(self, f'{target}_to_{source}')
         return Codec(coder, decoder)
 
+    def filt_iter(self, field_type: FieldTypeNames):
+        from dol.trans import filt_iter
+
+        self._assert_field_type(field_type, 'field_type')
+        filt_func = getattr(self, f'match_{field_type}')
+        return filt_iter(filt=filt_func)
+
     # @_return_none_if_none_input
     def str_to_dict(self, s: str) -> dict:
         """Parses the input string and returns a dictionary of extracted values.
@@ -1332,7 +1398,7 @@ def str_to_dict(self, s: str) -> dict:
             return None
         match = self.regex.match(s)
         if match:
-            return match.groupdict()
+            return {k: self.from_str_funcs[k](v) for k, v in match.groupdict().items()}
         else:
             raise ValueError(f"String '{s}' does not match the template.")
 
@@ -1350,6 +1416,7 @@ def dict_to_str(self, params: dict) -> str:
         """
         if params is None:
             return None
+        params = {k: self.to_str_funcs[k](v) for k, v in params.items()}
         return self.template.format(**params)
 
     # @_return_none_if_none_input
@@ -1400,7 +1467,7 @@ def str_to_tuple(self, s: str) -> tuple:
         return self.dict_to_tuple(self.str_to_dict(s))
 
     # @_return_none_if_none_input
-    def tuple_to_str(self, params: tuple) -> str:
+    def tuple_to_str(self, param_vals: tuple) -> str:
         """Generates a string from the tuple values based on the template.
 
         >>> st = StringTemplate(
@@ -1410,9 +1477,9 @@ def tuple_to_str(self, params: tuple) -> str:
         >>> st.tuple_to_str(('Alice', '30'))
         'Alice is 30 years old.'
         """
-        if params is None:
+        if param_vals is None:
             return None
-        return self.dict_to_str(self.tuple_to_dict(params))
+        return self.dict_to_str(self.tuple_to_dict(param_vals))
 
     # @_return_none_if_none_input
     def dict_to_namedtuple(
@@ -1450,25 +1517,22 @@ def namedtuple_to_dict(self, nt):
         return dict(nt._asdict())  # TODO: Find way that doesn't involve private method
 
     # @_return_none_if_none_input
-    def str_to_simple_str(self, s: str, sep: str):
+    def str_to_simple_str(self, s: str, sep: str = None):
         """Converts a string to a simple string (i.e. a simple character-delimited string).
 
         >>> st = StringTemplate(
         ...     "{name} is {age} years old.",
         ...     field_patterns={"name": r"\w+", "age": r"\d+"}
         ... )
+        >>> st.str_to_simple_str("Alice is 30 years old.")
+        'Alice,30'
         >>> st.str_to_simple_str("Alice is 30 years old.", '-')
         'Alice-30'
         """
+        sep = sep or self.simple_str_sep
         if s is None:
             return None
-        elif sep is None:
-            if self.simple_str_sep is None:
-                raise ValueError(
-                    'Need to specify a sep (at method call time), or a simple_str_sep '
-                    '(at instiantiation time) to use str_to_simple_str'
-                )
-        return sep.join(self.str_to_tuple(s))
+        return sep.join(self.to_str_funcs[k](v) for k, v in self.str_to_dict(s).items())
 
     # @_return_none_if_none_input
     def simple_str_to_str(self, ss: str, sep: str):
@@ -1484,3 +1548,21 @@ def simple_str_to_str(self, ss: str, sep: str):
         if ss is None:
             return None
         return self.tuple_to_str(tuple(ss.split(sep)))
+
+    def match_str(self, s: str) -> bool:
+        return self.regex.match(s) is not None
+
+    def match_dict(self, params: dict) -> bool:
+        return self.match_str(self.dict_to_str(params))
+        # Note: Could do:
+        #  return all(self.field_patterns[k].match(v) for k, v in params.items())
+        # but not sure that's even quicker (given regex is compiled)
+
+    def match_tuple(self, param_vals: tuple) -> bool:
+        return self.match_str(self.tuple_to_str(param_vals))
+
+    def match_namedtuple(self, params: namedtuple) -> bool:
+        return self.match_str(self.namedtuple_to_str(params))
+
+    def match_simple_str(self, params: str) -> bool:
+        return self.match_str(self.simple_str_to_str(params))
diff --git a/dol/tests/test_paths.py b/dol/tests/test_paths.py
index c31de821..3381a64d 100644
--- a/dol/tests/test_paths.py
+++ b/dol/tests/test_paths.py
@@ -1,24 +1,51 @@
 """Tests for paths.py"""
 
+from dol import StringTemplate
 
-def test_string_template():
+
+def test_string_template_template_construction():
+    assert StringTemplate('{}.ext').template == '{_1}.ext'
+    assert StringTemplate('{name}.ext').template == '{name}.ext'
+    assert StringTemplate('{::\w+}.ext').template == '{_1}.ext'
+    assert StringTemplate('{name::\w+}.ext').template == '{name}.ext'
+    assert StringTemplate('{name::\w+}.ext').template == '{name}.ext'
+    assert StringTemplate('{name:0.02f}.ext').template == '{name}.ext'
+    assert StringTemplate('{name:0.02f:\w+}.ext').template == '{name}.ext'
+    assert StringTemplate('{:0.02f:\w+}.ext').template == '{_1}.ext'
+
+
+def test_string_template_regex():
+    assert StringTemplate('{}.ext').regex.pattern == '(?P<_1>.*)\\.ext'
+    assert StringTemplate('{name}.ext').regex.pattern == '(?P<name>.*)\\.ext'
+    assert StringTemplate('{::\w+}.ext').regex.pattern == '(?P<_1>\\w+)\\.ext'
+    assert StringTemplate('{name::\w+}.ext').regex.pattern == '(?P<name>\\w+)\\.ext'
+    assert StringTemplate('{:0.02f:\w+}.ext').regex.pattern == '(?P<_1>\\w+)\\.ext'
+    assert StringTemplate('{name:0.02f:\w+}.ext').regex.pattern == '(?P<name>\\w+)\\.ext'
+
+
+def test_string_template_simple():
     from dol.paths import StringTemplate
     from collections import namedtuple
 
     st = StringTemplate(
-        '{name} is {age} years old.', field_patterns={'name': r'\w+', 'age': r'\d+'}
+        'root/{name}/v_{version}.json', 
+        field_patterns={'name': r'\w+', 'version': r'\d+'},
+        from_str_funcs={'version': int},
     )
 
-    assert st.str_to_dict('Alice is 30 years old.') == {'name': 'Alice', 'age': '30'}
-    assert st.dict_to_str({'name': 'Alice', 'age': '30'}) == 'Alice is 30 years old.'
-    assert st.dict_to_tuple({'name': 'Alice', 'age': '30'}) == ('Alice', '30')
-    assert st.tuple_to_dict(('Alice', '30')) == {'name': 'Alice', 'age': '30'}
-    assert st.str_to_tuple('Alice is 30 years old.') == ('Alice', '30')
-    assert st.tuple_to_str(('Alice', '30')) == 'Alice is 30 years old.'
+    assert st.str_to_dict('root/Alice/v_30.json') == {'name': 'Alice', 'version': 30}
+    assert st.dict_to_str({'name': 'Alice', 'version': 30}) == 'root/Alice/v_30.json'
+    assert st.dict_to_tuple({'name': 'Alice', 'version': 30}) == ('Alice', 30)
+    assert st.tuple_to_dict(('Alice', 30)) == {'name': 'Alice', 'version': 30}
+    assert st.str_to_tuple('root/Alice/v_30.json') == ('Alice', 30)
+    assert st.tuple_to_str(('Alice', 30)) == 'root/Alice/v_30.json'
+
+    VersionedFile = st.dict_to_namedtuple({'name': 'Alice', 'version': 30})
 
-    Person = st.dict_to_namedtuple({'name': 'Alice', 'age': '30'})
-    assert Person == namedtuple('Person', ['name', 'age'])('Alice', '30')
-    assert st.namedtuple_to_dict(Person) == {'name': 'Alice', 'age': '30'}
+    from collections import namedtuple
+    assert VersionedFile == namedtuple('VersionedFile', ['name', 'version'])('Alice', 30)
+    assert st.namedtuple_to_dict(VersionedFile) == {'name': 'Alice', 'version': 30}
 
-    assert st.str_to_simple_str('Alice is 30 years old.', '-') == 'Alice-30'
-    assert st.simple_str_to_str('Alice-30', '-') == 'Alice is 30 years old.'
+    assert st.str_to_simple_str('root/Alice/v_30.json') == 'Alice,30'
+    assert st.str_to_simple_str('root/Alice/v_30.json', '-') == 'Alice-30'
+    assert st.simple_str_to_str('Alice-30', '-') == 'root/Alice/v_30.json'
\ No newline at end of file
diff --git a/dol/tools.py b/dol/tools.py
index f3106957..71ffb5bf 100644
--- a/dol/tools.py
+++ b/dol/tools.py
@@ -125,7 +125,7 @@ def __missing__(self, k):
                 user_value = value_preprocessor(user_value)
             self[k] = user_value
         else:
-            super().__missing__(k)
+            super(type(self), self).__missing__(k)
 
     store.__missing__ = __missing__
     return store