IsValid check syntax before evaluating expression

gronx.go

@@ -174,6 +174,12 @@ func IsValid(expr string) bool {
 		return false
 	}
 
+	// First check syntax without time dependency
+	if !isSyntaxValid(segs) {
+		return false
+	}
+
+	// Then check with time dependency
 	for pos, seg := range segs {
 		if _, err := checker.CheckDue(seg, pos); err != nil {
 			return false
@@ -182,3 +188,35 @@ func IsValid(expr string) bool {
 
 	return true
 }
+
+// isSyntaxValid checks if the cron segments are syntactically valid without time dependency.
+// It returns bool.
+func isSyntaxValid(segs []string) bool {
+	for _, seg := range segs {
+		// Check for empty segments
+		if seg == "" {
+			return false
+		}
+
+		// Split by comma to check each part
+		parts := strings.Split(seg, ",")
+		for _, part := range parts {
+			// Check for empty parts
+			if part == "" {
+				return false
+			}
+
+			// Check for invalid characters
+			if strings.ContainsAny(part, "*/") {
+				// If contains /, must have a number after it
+				if strings.Contains(part, "/") {
+					parts := strings.Split(part, "/")
+					if len(parts) != 2 || parts[1] == "" {
+						return false
+					}
+				}
+			}
+		}
+	}
+	return true
+}

gronx_test.go

@@ -99,6 +99,28 @@ func TestIsValid(t *testing.T) {
 		}
 	})
 
+	t.Run("sensitivity to reference time", func(t *testing.T) {
+		originalRef := checker.ref
+		defer func() { checker.ref = originalRef }()
+
+		expr := "*/15, * * * *"
+		moments := []time.Time{
+			time.Date(2025, 4, 29, 12, 13, 0, 0, time.UTC),
+			time.Date(2025, 4, 29, 12, 14, 0, 0, time.UTC),
+			time.Date(2025, 4, 29, 12, 15, 0, 0, time.UTC),
+			time.Date(2025, 4, 29, 12, 16, 0, 0, time.UTC),
+			time.Date(2025, 4, 29, 12, 17, 0, 0, time.UTC),
+		}
+
+		for _, moment := range moments {
+			checker.ref = moment
+
+			if gron.IsValid(expr) {
+				t.Errorf("expected false, got true at %v", moment)
+			}
+		}
+
+	})
 }
 
 func TestAddTag(t *testing.T) {