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715a807289
This commit represents a ground up rewrite of the regex-syntax crate.
This commit is also an intermediate state. That is, it adds a new
regex-syntax-2 crate without making any serious changes to any other
code. Subsequent commits will cover the integration of the rewrite and
the removal of the old crate.
The rewrite is intended to be the first phase in an effort to overhaul
the entire regex crate. To that end, this rewrite takes steps in that
direction:
* The principle change in the public API is an explicit split between a
regular expression's abstract syntax (AST) and a high-level
intermediate representation (HIR) that is easier to analyze. The old
version of this crate mixes these two concepts, but leaned heavily
towards an HIR. The AST in the rewrite has a much closer
correspondence with the concrete syntax than the old `Expr` type does.
The new HIR embraces its role; all flags are now compiled away
(including the `i` flag), which will simplify subsequent passes,
including literal detection and the compiler. ASTs are produced by
ast::parse and HIR is produced by hir::translate. A top-level parser
is provided that combines these so that callers can skip straight from
concrete syntax to HIR.
* Error messages are vastly improved thanks to the span information that
is now embedded in the AST. In addition to better formatting, error
messages now also include helpful hints when trying to use features
that aren't supported (like backreferences and look-around). In
particular, octal support is now an opt-in option. (Octal support
will continue to be enabled in regex proper to support backwards
compatibility, but will be disabled in 1.0.)
* More robust support for Unicode Level 1 as described in UTS#18.
In particular, we now fully support Unicode character classes
including set notation (difference, intersection, symmetric
difference) and correct support for named general categories, scripts,
script extensions and age. That is, `\p{scx:Hira}` and `p{age:3.0}`
now work. To make this work, we introduce an internal interval set
data structure.
* With the exception of literal extraction (which will be overhauled in
a later phase), all code in the rewrite uses constant stack space,
even while performing analysis that requires structural induction over
the AST or HIR. This is done by pushing the call stack onto the heap,
and is abstracted by the `ast::Visitor` and `hir::Visitor` traits.
The point of this method is to eliminate stack overflows in the
general case.
* Empty sub-expressions are now properly supported. Expressions like
`()`, `|`, `a|` and `b|()+` are now valid syntax.
The principle downsides of these changes are parse time and binary size.
Both seemed to have increased (slower and bigger) by about 1.5x. Parse
time is generally peanuts compared to the compiler, so we mostly don't
care about that. Binary size is mildly unfortunate, and if it becomes a
serious issue, it should be possible to introduce a feature that
disables some level of Unicode support and/or work on compressing the
Unicode tables. Compile times have increased slightly, but are still a
very small fraction of the overall time it takes to compile `regex`.
Fixes #174, Fixes #424