diff --git a/spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/QueryMapper.java b/spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/QueryMapper.java
index 584081556..0127216e5 100644
--- a/spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/QueryMapper.java
+++ b/spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/QueryMapper.java
@@ -797,7 +797,6 @@ public class QueryMapper {
 	 */
 	static class Keyword {
 
-		private static final String N_OR_PATTERN = "\\$.*or";
 		private static final Set<String> NON_DBREF_CONVERTING_KEYWORDS = new HashSet<>(
 				Arrays.asList("$", "$size", "$slice", "$gt", "$lt"));
 
@@ -828,7 +827,7 @@ public class QueryMapper {
 		}
 
 		public boolean isOrOrNor() {
-			return key.matches(N_OR_PATTERN);
+			return key.equalsIgnoreCase("$or") || key.equalsIgnoreCase("$nor");
 		}
 
 		/**
diff --git a/spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/convert/QueryMapperUnitTests.java b/spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/convert/QueryMapperUnitTests.java
index dd8dd25e8..166c602be 100755
--- a/spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/convert/QueryMapperUnitTests.java
+++ b/spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/convert/QueryMapperUnitTests.java
@@ -1277,6 +1277,14 @@ public class QueryMapperUnitTests {
 		assertThat(document).isEqualTo(new org.bson.Document("stringAsOid", new org.bson.Document("$type", 10)));
 	}
 
+	@Test // GH-3635
+	void $floorKeywordDoesNotMatch$or$norPattern() {
+
+		Query query = new BasicQuery(" { $expr: { $gt: [ \"$spent\" , { $floor : \"$budget\" } ] } }");
+		assertThatNoException()
+				.isThrownBy(() -> mapper.getMappedObject(query.getQueryObject(), context.getPersistentEntity(Foo.class)));
+	}
+
 	class WithDeepArrayNesting {
 
 		List<WithNestedArray> level0;