libfsm is a C library and toolchain for constructing, manipulating, and compiling finite state machines (FSMs) from regular expressions. It supports multiple regex dialects (PCRE, glob, SQL LIKE, literal, native) and compiles them into deterministic finite automata (DFAs) that guarantee linear-time matching. The library provides a rich set of FSM operations: determinisation, minimisation, union, intersection, complement, reversal, trimming, and more. It also includes a lexer generator (lx) that produces C lexers from specification files, and a pattern matcher builder (rx) that merges many patterns into a single DFA with conflict detection. The architecture is modular: an abstract data type (ADT) layer provides reusable data structures (sets, hash tables, priority queues, edge sets, interned state sets); the libfsm core implements FSM algorithms; libre handles regex parsing and AST analysis; and multiple code generation backends emit C, Rust, Go, LLVM IR, x86 assembly, awk, shell, JSON, and a custom VM bytecode. The build system uses BSD make (bmake) and the project is extensively tested with unit tests, property-based fuzzing (theft), and a cross-implementation test runner (retest) that compiles and runs generated code via dlopen. Key entry points include the fsm CLI tool for interactive FSM operations, the re CLI for regex compilation and code generation, the lx CLI for lexer generation, and the rx CLI for building pattern matchers. The library is designed for embedding in other applications, with allocator hooks and a clean public API. It is particularly suited for high-performance pattern matching, protocol parsing, and any domain requiring deterministic automata with guaranteed worst-case performance.