regtest Bitcoin's "regression test" mode. Local-only chain, instant block generation, no PoW difficulty. The default for Bitcoin testing. Setup is required because the empty regtest mempool has no fee estimates. Multi-node setup Useful commands Address prefixes - P2PKH: or . - P2SH: . - Bech32: . - Bech32m: . Use cases - Unit tests that need a real bitcoind. - Integration tests for wallet / Lightning logic. - Demos / tutorials with deterministic state. - Reproducing bugs with controlled chain history. Common bugs - Forgetting → errors. - Forgetting to mine 100+ blocks before spending coinbase…