fix for date format

Author: FBruzzesi

narwhals/_arrow/utils.py

@@ -338,11 +338,11 @@ def convert_str_slice_to_int_slice(
 
 
 # Regex for date, time, separator and timezone components
-DATE_RE = r"^(?P<date>\d{1,4}[-/.]\d{1,2}[-/.]\d{1,4})?"
-SEP_RE = r"(?P<sep>\s|T)?"
-TIME_RE = r"(?P<time>\d{2}:\d{2}:\d{2})?"  # \s*(?P<period>[AP]M)?)?
-TZ_RE = r"(?P<tz>Z|[+-]\d{2}:?\d{2})?$"  # Matches 'Z', '+02:00', '+0200', '+02', etc.
-FULL_RE = DATE_RE + SEP_RE + TIME_RE + TZ_RE
+DATE_RE = r"(?P<date>\d{1,4}[-/.]\d{1,2}[-/.]\d{1,4})"
+SEP_RE = r"(?P<sep>\s|T)"
+TIME_RE = r"(?P<time>\d{2}:\d{2}:\d{2})"  # \s*(?P<period>[AP]M)?)?
+TZ_RE = r"(?P<tz>Z|[+-]\d{2}:?\d{2})"  # Matches 'Z', '+02:00', '+0200', '+02', etc.
+FULL_RE = rf"{DATE_RE}{SEP_RE}?{TIME_RE}?{TZ_RE}?$"
 
 # Separate regexes for different date formats
 YMD_RE = r"^(?P<year>(?:[12][0-9])?[0-9]{2})(?P<sep1>[-/.])(?P<month>0[1-9]|1[0-2])(?P<sep2>[-/.])(?P<day>0[1-9]|[12][0-9]|3[01])$"

tests/expr_and_series/str/to_datetime_test.py

@@ -73,6 +73,22 @@ def test_to_datetime_series_infer_fmt(constructor_eager: ConstructorEager) -> No
     assert str(result) == expected
 
 
+def test_to_datetime_infer_fmt_from_date(constructor: Constructor) -> None:
+    data = {"z": ["2020-01-01", "2020-01-02"]}
+    if "cudf" in str(constructor):  # pragma: no cover
+        expected = "2020-01-01T00:00:00.000000000"
+    else:
+        expected = "2020-01-01 00:00:00"
+    result = (
+        nw.from_native(constructor(data))
+        .lazy()
+        .select(y=nw.col("z").str.to_datetime())
+        .collect()
+        .item(row=0, column="y")
+    )
+    assert str(result) == expected
+
+
 @pytest.mark.parametrize("data", [["2024-01-01", "abc"], ["2024-01-01", None]])
 def test_pyarrow_infer_datetime_raise_invalid(data: list[str | None]) -> None:
     with pytest.raises(