diff --git a/src/bidsi/bids_model.py b/src/bidsi/bids_model.py
index e2d95db..c35d032 100644
--- a/src/bidsi/bids_model.py
+++ b/src/bidsi/bids_model.py
@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import re
 from dataclasses import dataclass
 from functools import cached_property
 from pathlib import Path
@@ -35,6 +36,19 @@ def relative_entity_dir(self, entity: BidsEntity) -> Path:
         rel_path /= entity.datatype
         return rel_path
 
+    def _clean_bids_field(self, value: str) -> str:
+        """Clean BIDS field value.
+
+        Replaces all non-alphanumeric characters.
+
+        Args:
+            value (str): Value to clean.
+
+        Returns:
+            str: Cleaned value.
+        """
+        return re.sub(r"[^a-zA-Z0-9]", "", value)
+
     def relative_entity_path(self, entity: BidsEntity) -> Path:
         """Return relative path to entity."""
         return self.relative_entity_dir(entity) / self.entity_formatter(entity)
@@ -126,7 +140,39 @@ def subject_ids(self) -> List[str]:
         return list(set([entity.subject_id for entity in self.entities]))
 
 
-@dataclass(frozen=True)
+class EntityFieldStringDescriptor:
+    """Descriptor Object for BIDS fields.
+
+    Cleans field values using a regular expression.
+    """
+
+    def __init__(self, *, clean_regex: str, default: Optional[str] = None) -> None:
+        """Initialize EntityFieldStringDescriptor."""
+        self._default = default
+        self._clean_regex = clean_regex
+
+    def __set_name__(self, owner: type[BidsEntity], name: str) -> None:
+        """Set name with underscore prefix."""
+        self._name = "_" + name
+
+    def __get__(self, obj: BidsEntity, type: type[BidsEntity]) -> str:
+        """Get value or default."""
+        if obj is None:
+            raise AttributeError("No default value for BidsEntityStringDescriptor.")
+
+        return str(getattr(obj, self._name, self._default))
+
+    def __set__(self, obj: object, value: str) -> None:
+        """Set value after substituting clean_regex matches with empty string."""
+        value = re.sub(self._clean_regex, "", value)
+        setattr(obj, self._name, value)
+
+
+# TODO: Add support for cleaning optional fields.
+# TODO: Can BidsEntity be frozen?
+
+
+@dataclass
 class BidsEntity:
     """Model of BIDS entity, a representation of data within the BIDS structure.
 
@@ -134,10 +180,18 @@ class BidsEntity:
     Only one of file or tabular_data should be set.
     """
 
-    subject_id: str
-    datatype: str
-    task_name: str
-    suffix: str
+    subject_id: EntityFieldStringDescriptor = EntityFieldStringDescriptor(
+        clean_regex=r"[^a-zA-Z0-9]"
+    )
+    datatype: EntityFieldStringDescriptor = EntityFieldStringDescriptor(
+        clean_regex=r"[^a-zA-Z0-9]"
+    )
+    task_name: EntityFieldStringDescriptor = EntityFieldStringDescriptor(
+        clean_regex=r"[^a-zA-Z0-9]"
+    )
+    suffix: EntityFieldStringDescriptor = EntityFieldStringDescriptor(
+        clean_regex=r"[^a-zA-Z0-9]"
+    )
     session_id: Optional[str] = None
     metadata: Optional[Dict[str, str]] = None
     file_path: Optional[Path] = None
@@ -152,6 +206,13 @@ def is_tabular_data(self) -> bool:
         """Return True if entity is tabular data."""
         return self.tabular_data is not None
 
+    @classmethod
+    def bids_field(
+        cls, clean_regex: str, default: Optional[str] = None
+    ) -> EntityFieldStringDescriptor:
+        """Return BidsEntity field descriptor."""
+        return EntityFieldStringDescriptor(clean_regex=clean_regex, default=default)
+
 
 class BidsBuilder:
     """Builder for BIDS Model."""
diff --git a/src/bidsi/bids_writer.py b/src/bidsi/bids_writer.py
index e079c17..3c66dc1 100644
--- a/src/bidsi/bids_writer.py
+++ b/src/bidsi/bids_writer.py
@@ -15,7 +15,6 @@
 from .bids_model import BidsBuilder, BidsConfig, BidsEntity, BidsModel
 
 LOG = logging.getLogger(__name__)
-LOG.setLevel(logging.DEBUG)
 
 
 class MergeStrategy(Enum):
@@ -24,16 +23,21 @@ class MergeStrategy(Enum):
     NO_MERGE: Do not merge, exit with error on conflict.
     OVERWRITE: Overwrite existing files on conflict.
     KEEP: Keep existing files on conflict.
-    RENAME_FILE: Rename files on conflict using run-label increments.
-    NEW_SESSION: Write new session folder for all data additions.
+    RENAME_SEQUENTIAL: Rename files on conflict using run-label increments.
     """
 
-    UNKNOWN_MERGE = 0
-    NO_MERGE = 1
-    OVERWRITE = 2
-    KEEP = 3
-    RENAME_ENTITIES = 5
-    NEW_SESSION = 6
+    # Do not merge, only proceed with empty BIDS root.
+    # Exit with error on conflict. Default.
+    NO_MERGE = 0
+
+    # Overwrite existing files on conflict.
+    OVERWRITE = 1
+
+    # Keep existing files on conflict.
+    KEEP = 2
+
+    # Rename files on conflict using run-label increments.
+    RENAME_SEQUENTIAL = 3
 
     def __str__(self) -> str:
         """Return string representation of MergeStrategy."""
@@ -80,7 +84,7 @@ def __exit__(
         return self.write()
 
     def builder(self) -> BidsBuilder:
-        """Return BIDS builder."""
+        """Return BIDS builder, creating new if does not already exist."""
         if self._builder is None:
             self._builder = BidsBuilder()
         return self._builder
@@ -103,7 +107,7 @@ def _merge_tsv(self, path: Path, data: pd.DataFrame) -> None:
         data.to_csv(path, sep="\t", index=False)
 
     def _ensure_directory_path(self, path: Path, is_dir: bool = False) -> None:
-        """Ensure directory path exists."""
+        """Ensure directory path, or path to parent dir of file exists, or create."""
         if is_dir:
             if not path.exists():
                 path.mkdir(parents=True, exist_ok=True)
@@ -133,32 +137,26 @@ def _merge_entity(
             return
         elif self._entity_merge_strategy == MergeStrategy.KEEP:
             return
-        elif self._entity_merge_strategy == MergeStrategy.RENAME_ENTITIES:
-            raise NotImplementedError("RENAME_ENTITIES merge strategy not implemented.")
-        elif self._entity_merge_strategy == MergeStrategy.NEW_SESSION:
-            raise NotImplementedError("New session merge strategy not implemented.")
         else:
             raise ValueError(f"Unknown merge strategy {self._entity_merge_strategy}.")
 
     def write(self) -> bool:
         """Write BIDS structure to disk."""
         if self._bids is None and self._builder is None:
-            raise ValueError("No BIDS model to write.")
+            raise ValueError("No BIDS model or builder to write.")
 
         if self._bids is None and self._builder is not None:
             self._bids = self._builder.build()
 
         # Unwrap Optional value for type-checking.
         if self._bids is None:
-            raise ValueError("No BIDS model or BIDS Builder to write.")
+            raise ValueError("No BIDS model to write.")
 
         # Write BIDS structure
         # Confirm root
         LOG.info(f"Writing BIDS structure to {self._bids_root}")
-        self._bids_root.mkdir(parents=True, exist_ok=True)
+        self._ensure_directory_path(self._bids_root, is_dir=True)
         if len(list(self._bids_root.iterdir())) > 0:
-            if self._entity_merge_strategy == MergeStrategy.UNKNOWN_MERGE:
-                raise ValueError("BIDS root is not empty, merge strategy required.")
             if self._entity_merge_strategy == MergeStrategy.NO_MERGE:
                 raise ValueError("BIDS root is not empty, cannot merge.")
 
@@ -178,11 +176,12 @@ def write(self) -> bool:
 
         # Write subject folders
         for entity in self._bids.entities:
-            LOG.info(f"Writing entity {entity.subject_id}")
             if entity.file_path is not None:
+                LOG.info(f"Writing Path entity {entity.subject_id}")
                 fp = entity.file_path
                 self._merge_entity(entity, lambda path: shutil.copy2(fp, path))
             elif entity.tabular_data is not None:
+                LOG.info(f"Writing tabular data entity {entity.subject_id}")
                 tb = entity.tabular_data
                 self._merge_entity(
                     entity,
diff --git a/tests/test_bids_model.py b/tests/test_bids_model.py
new file mode 100644
index 0000000..ca71f3c
--- /dev/null
+++ b/tests/test_bids_model.py
@@ -0,0 +1,39 @@
+"""Tests for the BidsEntity model."""
+
+from bidsi import BidsEntity
+
+
+def test_bids_entity_descriptor_cleans_values() -> None:
+    """Test that BidsEntity descriptor cleans values."""
+    bids = BidsEntity(
+        subject_id="01_AZ",
+        task_name="*task*00",
+        datatype="func_",
+        run_id="1",
+        suffix=".suffix=",
+        metadata={"ke_y": "_va_lue"},
+        session_id="-01-az",
+    )
+    assert bids.subject_id == "01AZ"
+    assert bids.task_name == "task00"
+    assert bids.datatype == "func"
+    assert bids.run_id == "1"
+    assert bids.suffix == "suffix"
+
+
+def test_bids_entity_descriptor_cleans_optional_values() -> None:
+    """Test that BidsEntity descriptor cleans values."""
+    bids = BidsEntity(
+        subject_id="01_AZ",
+        task_name="*task*00",
+        datatype="func_",
+        run_id="1",
+        suffix=".suffix=",
+        metadata={"ke_y": "_va_lue"},
+        session_id="-01-az",
+    )
+    assert bids.subject_id == "01AZ"
+    assert bids.task_name == "task00"
+    assert bids.datatype == "func"
+    assert bids.run_id == "1"
+    assert bids.suffix == "suffix"