Derivative based regex matcher

For basic introduction see Regular-expression derivatives reexamined.

For extensions, see Derivative Based Nonbacktracking Real-World Regex Matching with Backtracking Semantics and Derivative Based Extended Regular Expression Matching Supporting Intersection, Complement and Lookarounds and the sbre implementation of it.

Usage

This library uses regex-syntax for regular expression parsing. This means that currently there is no surface syntax for & and ~ operators (but the library supports it).

The library only checks if the regex matches the string from the beginning (it doesn't search for it, or in other words there's an implied \A at the beginning).

let mut rx = Regex::new("[ab]c").unwrap();
assert!(rx.is_match("ac"));
assert!(rx.is_match("bc"));
assert!(!rx.is_match("xxac"));
assert!(!rx.is_match("acxx"));

The library supports a single look-ahead at the end of the regex, written as A(?P<stop>B) where A and B are regexes without any look-arounds. You can get the length of the string matching B upon successful match.

// the syntax in other libraries would be: r"\A[abx]*(?=[xq]*y)"
let mut rx = Regex::new("[abx]*(?P<stop>[xq]*y)").unwrap();
assert!(rx.lookahead_len("axxxxxy") == Some(1));
assert!(rx.lookahead_len("axxxxxqqqy") == Some(4));
assert!(rx.lookahead_len("axxxxxqqq") == None);
assert!(rx.lookahead_len("ccqy") == None);

Code map

In recommended reading order:

simplify.rs - simplification (rewrite rules) of AST
deriv.rs - derivative computation
bytecompress.rs - alphabet compression
syntax.rs - uses regex-syntax crate for parsing
regexc.rs - top-level memoization of state transition and user interface

The rest:

ast.rs - AST for regexes
hashcons.rs - hash-consing of u32 vectors (used by AST)

TODO

more simplification rules from sbre
benchmarks
extend regex-syntax for & and ~ operators
add & valid-utf8 if there is negation somewhere
either make derivative() non-recursive (mk_*() already are?) or limit the regex depth
implement relevance check for & and ~ operators; see symbolic derivatives
add .forced_byte() method on state descriptor

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