Add parsing of lookarounds

[shilangyu]

No description provided.

[Copilot]

PR Overview

This pull request adds parsing, AST generation, HIR translation, and testing support for lookaround assertions (specifically lookbehinds) in regular expressions. Key changes include:

• Updating the parser (and related group parsing functions) to handle lookaround syntax.
• Implementing HIR translation for the new LookAround AST node.
• Adding and updating tests for lookaround parsing and printing.

Reviewed Changes

File	Description
regex-syntax/src/hir/print.rs	Updated lookaround tests by removing the ignore attribute and introducing a dedicated test helper function.
regex-syntax/src/ast/parse.rs	Modified group parsing (renamed to grouping) to support lookaround syntax and added error handling for unsupported lookahead.
regex-syntax/src/hir/translate.rs	Implemented HIR translation for LookAround AST nodes with separate handling for positive and negative lookbehinds.
regex-syntax/src/ast/mod.rs	Introduced a new LookAroundUnclosed error variant and updated function naming for consistency.
regex-syntax/src/hir/literal.rs	Updated tests to validate lookaround handling in literal printing.
regex-syntax/src/hir/mod.rs	Minor comment updates reflecting the new "look-around" terminology in the HIR module.

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (3)

regex-syntax/src/hir/print.rs:489

• [nitpick] Consider rephrasing the comment for clarity. For example, 'We do not perform a roundtrip for lookarounds because they cannot contain capture groups, which are unsupported by the parser.'

// we do not want to do a roundtrip: printed lookarounds are not
// can contain capture groups which are unsupported by the parser.

regex-syntax/src/hir/translate.rs:449

• [nitpick] Ensure that the translation logic for LookAround covers both positive and negative lookbehinds in all edge cases; consider adding a clarifying comment to explain the branching logic.

Ast::LookAround(ref x) => {

regex-syntax/src/hir/literal.rs:2458

• [nitpick] Verify that the tests for lookaround in literal handling accurately reflect the intended behavior, and consider adding tests for lookarounds with non-empty sub-patterns.

fn lookaround() { assert_eq!(exact(["ab"]), e(r"a(?<=qwe)b")); ... }

[Multimodcrafter]

LGTM