Defines a Thompson NFA and provides the PikeVM and BoundedBacktracker regex engines.

A Thompson NFA (non-deterministic finite automaton) is arguably the central data type in this library. It is the result of what is commonly referred to as "regex compilation." That is, turning a regex pattern from its concrete syntax string into something that can run a search looks roughly like this:

• A &str is parsed into a regex-syntax::ast::Ast.
• An Ast is translated into a regex-syntax::hir::Hir.
• An Hir is compiled into a NFA.
• The NFA is then used to build one of a few different regex engines:
  • An NFA is used directly in the PikeVM and BoundedBacktracker engines.
  • An NFA is used by a hybrid NFA/DFA to build out a DFA's transition table at search time.
  • An NFA, assuming it is one-pass, is used to build a full one-pass DFA ahead of time.
  • An NFA is used to build a full DFA ahead of time.

The meta regex engine makes all of these choices for you based on various criteria. However, if you have a lower level use case, you can build any of the above regex engines and use them directly. But you must start here by building an NFA.

Details

It is perhaps worth expanding a bit more on what it means to go through the &str->Ast->Hir->NFA process.

• Parsing a string into an Ast gives it a structured representation. Crucially, the size and amount of work done in this step is proportional to the size of the original string. No optimization or Unicode handling is done at this point. This means that parsing into an Ast has very predictable costs. Moreover, an Ast can be roundtripped back to its original pattern string as written.
• Translating an Ast into an Hir is a process by which the structured representation is simplified down to its most fundamental components. Translation deals with flags such as case insensitivity by converting things like (?i:a) to [Aa]. Translation is also where Unicode tables are consulted to resolve things like \p{Emoji} and \p{Greek}. It also flattens each character class, regardless of how deeply nested it is, into a single sequence of non-overlapping ranges. All the various literal forms are thrown out in favor of one common representation. Overall, the Hir is small enough to fit into your head and makes analysis and other tasks much simpler.
• Compiling an Hir into an NFA formulates the regex into a finite state machine whose transitions are defined over bytes. For example, an Hir might have a Unicode character class corresponding to a sequence of ranges defined in terms of char. Compilation is then responsible for turning those ranges into a UTF-8 automaton. That is, an automaton that matches the UTF-8 encoding of just the codepoints specified by those ranges. Otherwise, the main job of an NFA is to serve as a byte-code of sorts for a virtual machine. It can be seen as a sequence of instructions for how to match a regex.