  Anomalies and BRST invariance are governed, in the context of Lagrangian
Batalin-Vilkovisky quantization, by the master equation, whose classical limit
is $(S, S)=0$. Using Zimmerman's normal products and the BPHZ renormalisation
method, we obtain a corresponding local quantum operator equation, which is
valid to all orders in perturbation theory. The formulation implies a
calculational method for anomalies to all orders that is useful also outside
the BV context and that remains completely within regularised perturbation
theory. It makes no difference in principle whether the anomaly appears at one
loop or at higher loops. The method is illustrated by computing the one- and
two-loop anomalies in chiral $W_3$ gravity.
