Future communication networks are expected to be highly automated, disburdening human operators of their most complex tasks. However, while first powerful and automated network analysis tools are emerging, existing tools provide very limited (and inefficient) support of reasoning about failure scenarios. We present P-Rex, a fast what-if analysis tool, that allows us to test important reachability and policy-compliance properties even under an arbitrary number of failures, in polynomial-time, i.e., without enumerating all failure scenarios (the usual approach today, if supported at all). P-Rex targets networks based on Multiprotocol Label Switching (MPLS) and its Segment Routing (SR) extension and comes with an expressive query language based on regular expressions. It takes into account the actual router tables, and is hence well-suited for debugging. We also report on an industrial case study and demonstrate that P-Rex supports rich queries, performing what-if analyses in less than 70 minutes in most cases, in a 24-router network with about 100,000 forwarding rules.