diff --git a/ukwa_api/mementos/router.py b/ukwa_api/mementos/router.py
index ecd58a0..87c4562 100644
--- a/ukwa_api/mementos/router.py
+++ b/ukwa_api/mementos/router.py
@@ -112,7 +112,6 @@ async def lookup_url(
     
     collapseToFirst: str = schemas.create_query_param_from_path(schemas.path_collapse),
     collapseToLast: str = schemas.create_query_param_from_path(schemas.path_collapse)
-
 ):
 
     # Basic validation and derived parameters:
@@ -120,23 +119,9 @@ async def lookup_url(
         raise HTTPException(status_code=400, detail="Timestamp required for Closest sort.")
     if sort.value != "closest" and closest:
         raise HTTPException(status_code=400, detail="Closest Sort required for Closest Timestamp.")
-
-
-    ALLOWED_CDX_FIELDS = ["timestamp", "statuscode"]
-    ALLOWED_LENGTHS = range(1, 15)
-
-    valid_collapse_options = [f"{cdx_field}:{length}" for cdx_field in ALLOWED_CDX_FIELDS for length in ALLOWED_LENGTHS]
-
     if collapseToFirst and collapseToLast:
         raise HTTPException(status_code=400, detail="Only one of collapseToFirst or collapseToLast can be specified")
 
-    if collapseToFirst:
-        if collapseToFirst not in valid_collapse_options:
-            raise HTTPException(status_code=400, detail="Invalid collapseToFirst option")
-    elif collapseToLast:
-        if collapseToLast not in valid_collapse_options:
-            raise HTTPException(status_code=400, detail="Invalid collapseToLast option")
-
     # Only put through allowed parameters:
     params = {
         'url': url,
diff --git a/ukwa_api/mementos/schemas.py b/ukwa_api/mementos/schemas.py
index 19aafa4..1ba4f84 100644
--- a/ukwa_api/mementos/schemas.py
+++ b/ukwa_api/mementos/schemas.py
@@ -45,16 +45,28 @@ class LookupOutputType(str, Enum):
     regex="^\d{4,14}$",  # Allow 4-14 digits
 )
 
+
+# note the pattern here is to specify the RANGE of leading characters or digits of the field value to be collapsed on
+# not to specify otherwise the pattern of the field value itself. see description below for timestamp example
+# it is more restrictive than the CDX endpoint, but meets sensible use cases following the CDX spec
 path_collapse = Path(
     ...,
     description= '''CDX Field to collapse on, optionally with :number suffix to collapse on substring of field; 
                     in other words, return only the first/last row when of the series multiple consecutive rows
                     have the same value for the supplied field. Example: "timestamp:4" 
-                    will return a single row per year (YYYY are the first 4 digits).''',
-    regex="^(timestamp|statuscode):\d{1,2}$"  # Allow 4-14 digits
+                    will return a single row per year (YYYY are the first 4 digits).''',          
+    regex = (
+        r"^(statuscode:([1-3])|digest:(?:[1-9]|[1-3][0-9]|40)|urlkey:([1-9][0-9]?)|"
+        r"timestamp:(1[0-4]|[4-9])|mimetype:([1-9][0-9]?)|"
+        r"original:([1-9][0-9]?)|redirecturl:([1-9][0-9]?)|"
+        r"filename:([1-9][0-9]?)|robotflags:([1-9])|"
+        r"offset:(?:[1-9]|1[0-2])|length:(?:[1-9]|1[0-2])|"
+        r"(urlkey|timestamp|original|mimetype|statuscode|digest|length|offset|filename|redirecturl|robotflags)?)$"
+
+    )
 )
 
-# allows us to reuse the timestamp definition as a whole 
+# allows us to reuse a basic param definition as a whole 
 # rather than having having to reference the attibutes each time
 def create_query_param_from_path(path: Path, alias: Optional[str] = None) -> Query:
     query_params = {