Does the regurgitation of acidic fluid from the stomach to the pharynx of the esophagus invalidate the fast?

The return of stomach fluids does not invalidate the fast if they do not reach the mouth. However, if they do reach the mouth, it takes the ruling of 'qals' (regurgitation) or vomiting. 'Qals' is what emerges from the stomach cavity but does not fill the mouth, or what emerges from the entrance of the stomach when it is full. If the 'qals' reaches the mouth and the fasting person is able to expel it but then returns it to their stomach, their fast is broken. However, if they swallow it due to inability to expel it, it does not affect their fast.