Support lookbehind: (?<= (?<!

[halotukozak]

Currently rejected at parse time (see RegexParser.scala, group-header parsing: case '=' | '!' => unsupported("lookahead") and the lookbehind branch).

Like anchors, lookaround is a zero-width assertion that doesn't map cleanly onto plain Brzozowski derivatives — matching at a position needs to inspect surrounding text without consuming it. Needs design work on how this interacts with Subset's derivative/subset-containment model (lookaround changes what "language equivalence" even means for containment checks).

Consider starting with lookahead ((?=, (?!) since it only needs to inspect the remaining input, which fits the derivative direction; lookbehind ((?<=, (?<!) needs the consumed prefix and is harder.

[halotukozak]

Lookahead ((?=, (?!) is done — merged in #50: added a Look AST node whose derivative is folded into Concat's derive rule (re-deriving the assertion alongside the continuation, intersected with "the assertion eventually matches a prefix of what's left"), plus a distribute-then-derive case for (A|B)·b when a branch is itself a leading lookahead. Verified against java.util.regex.Pattern on ~500 pattern/string pairs with zero mismatches.

Retitled this issue to track the remaining half: lookbehind. That's the harder direction predicted in the original issue body — it needs the consumed prefix, not the remaining input, which doesn't fit the forward-derivative direction lookahead used. Likely needs either a reverse-derivative/co-derivative construction, or tracking a bounded window of consumed context (only works for fixed-width lookbehind, which is the most common real-world case — variable-width lookbehind is harder still).