Reject negative numbers in dehumanize instead of silently dropping the sign

arrow/arrow.py

@@ -1428,6 +1428,19 @@ def dehumanize(self, input_string: str, locale: str = "en_us") -> "Arrow":
                         1 if not time_delta.isnumeric() else abs(int(time_delta))
                     )
                 else:
+                    # num_pattern only matches unsigned digits, so a literal minus
+                    # sign right before the number (e.g. "-1 hours") is invisible
+                    # to it and would otherwise be silently dropped. Direction is
+                    # already conveyed by the "ago"/"in" phrasing, so treat a sign
+                    # here as invalid input rather than letting it disappear.
+                    number_start = match.start() + num_match.start()
+                    if number_start > 0 and input_string[number_start - 1] == "-":
+                        raise ValueError(
+                            "Invalid input String. Negative numbers are not "
+                            "supported by dehumanize(), as direction is already "
+                            "conveyed by phrases such as 'ago' or 'in'. Found a "
+                            f"negative value near: {match_string!r}"
+                        )
                     change_value = int(num_match.group())
 
                 # No time to update if now is the unit

tests/test_arrow.py

@@ -2960,6 +2960,25 @@ def test_slavic_locales(self, slavic_locales: List[str]):
                 assert arw.dehumanize(past_string, locale=lang) == past
                 assert arw.dehumanize(future_string, locale=lang) == future
 
+    def test_negative_numbers_rejected(self):
+        arw = arrow.Arrow(2000, 6, 18, 5, 55, 0)
+
+        # A literal minus sign is invisible to the unsigned number regex, so
+        # without a check these used to silently return the same result as
+        # their unsigned counterpart instead of raising.
+        with pytest.raises(ValueError):
+            arw.dehumanize("in -1 hours")
+
+        with pytest.raises(ValueError):
+            arw.dehumanize("-3 minutes ago")
+
+        with pytest.raises(ValueError):
+            arw.dehumanize("in -2 days")
+
+        # unsigned input must still work exactly as before
+        assert arw.dehumanize("in 1 hours") == arw.shift(hours=1)
+        assert arw.dehumanize("3 minutes ago") == arw.shift(minutes=-3)
+
     def test_czech_slovak(self):
         # Relevant units for Slavic locale plural logic
         units = [