Add support for lookarounds in regex filtering (fancy-regex integration)

[sabbellasri]

Background

The recently added regex filtering feature works well using Rust's regex crate. However, it does not support advanced regex features like lookarounds ((?=...), (?!...), etc.), which can be useful for more complex filtering scenarios.

This limitation was observed during usage and also discussed in #554.

Problem

Rust's regex crate intentionally does not support lookarounds due to performance guarantees (linear-time matching using NFA).

As a result, users cannot write certain expressive patterns that are commonly used in tools like grep.

Proposed Solution

Integrate fancy-regex as an optional or fallback engine:

• Use regex crate for standard/simple patterns (fast path)
• Fallback to fancy-regex when advanced features (like lookarounds) are detected

This approach aligns with how fancy-regex works internally — delegating simple cases to the fast NFA engine and handling complex patterns separately.

Benefits

• Enables support for lookarounds and other advanced regex features
• Maintains performance for common/simple patterns
• Improves usability for users familiar with grep-style filtering

Considerations

• Performance impact for complex patterns
• Need for pattern detection logic (to decide which engine to use)
• Additional dependency (fancy-regex)

Next Steps

I will start implementing this and open a PR shortly.

cc: @beelze