fix: dehumanize accepts singular nouns with numeric prefix ("1 day ago")

arrow/arrow.py

@@ -1414,6 +1414,15 @@ def dehumanize(self, input_string: str, locale: str = "en_us") -> "Arrow":
                 pattern = re.compile(rf"(^|\b|\d){search_string}")
                 match = pattern.search(input_string)
 
+                # Singular forms like "a day" have no {0} placeholder, so the
+                # primary pattern never matches "1 day".  Try a numeric form
+                # using the unit key (e.g. r"\d+\s+day\b") as a fallback.
+                if not match and "{0}" not in str(time_string):
+                    alt_pattern = re.compile(
+                        rf"(^|\b)\d+\s+{re.escape(str(time_delta))}\b"
+                    )
+                    match = alt_pattern.search(input_string)
+
                 # If there is no match continue to next iteration
                 if not match:
                     continue

tests/test_arrow.py

@@ -2987,6 +2987,19 @@ def test_czech_slovak(self):
                 assert arw.dehumanize(past_string, locale=lang) == past
                 assert arw.dehumanize(future_string, locale=lang) == future
 
+    def test_singular_with_number(self):
+        # "1 day ago" must work the same as "a day ago" (issue #1150)
+        arw = arrow.Arrow(2021, 4, 20, 22, 27, 34)
+        assert arw.dehumanize("1 second ago") == arw.shift(seconds=-1)
+        assert arw.dehumanize("1 minute ago") == arw.shift(minutes=-1)
+        assert arw.dehumanize("1 hour ago") == arw.shift(hours=-1)
+        assert arw.dehumanize("1 day ago") == arw.shift(days=-1)
+        assert arw.dehumanize("1 week ago") == arw.shift(weeks=-1)
+        assert arw.dehumanize("1 month ago") == arw.shift(months=-1)
+        assert arw.dehumanize("1 year ago") == arw.shift(years=-1)
+        assert arw.dehumanize("in 1 day") == arw.shift(days=1)
+        assert arw.dehumanize("in 1 week") == arw.shift(weeks=1)
+
 
 class TestArrowIsBetween:
     def test_start_before_end(self):