Add regex command implementation

specs/006-add-regex-command/plan.md

@@ -0,0 +1,96 @@
+# Implementation Plan: Regex Command for Pattern-Based Renaming
+
+**Branch**: `006-add-regex-command` | **Date**: 2025-10-30 | **Spec**: `specs/006-add-regex-command/spec.md`
+**Input**: Feature specification from `/specs/006-add-regex-command/spec.md`
+
+**Note**: This template is filled in by the `/speckit.plan` command. See `.specify/templates/commands/plan.md` for the execution workflow.
+
+## Summary
+
+Deliver a `renamer regex` subcommand that compiles a user-supplied pattern, substitutes numbered capture groups into a replacement template, surfaces deterministic previews, and records ledger metadata so undo and automation workflows remain safe and auditable.
+
+## Technical Context
+
+**Language/Version**: Go 1.24  
+**Primary Dependencies**: `spf13/cobra`, `spf13/pflag`, Go `regexp` (RE2 engine), internal traversal/history/output packages  
+**Storage**: Local filesystem and `.renamer` ledger files  
+**Testing**: `go test ./...`, contract suites under `tests/contract`, integration flows under `tests/integration`, targeted smoke script  
+**Target Platform**: Cross-platform CLI (Linux, macOS, Windows shells)  
+**Project Type**: Single CLI project (`cmd/`, `internal/`, `tests/`, `scripts/`)  
+**Performance Goals**: Preview + apply 500 regex-driven renames in <2 minutes end-to-end  
+**Constraints**: Preview-first confirmation, reversible ledger entries, Unicode-safe regex evaluation, conflict detection before apply  
+**Scale/Scope**: Expected to operate on thousands of entries per invocation within local directories
+
+## Constitution Check
+
+*GATE: Must pass before Phase 0 research. Re-check after Phase 1 design.*
+
+- Preview flow MUST show deterministic rename mappings and require explicit confirmation (Preview-First Safety). ✅ Use shared preview renderer to list original → proposed names plus skipped/conflict indicators prior to apply.
+- Undo strategy MUST describe how the `.renamer` ledger entry is written and reversed (Persistent Undo Ledger). ✅ Append ledger entries containing pattern, template, captured groups per file, enabling `renamer undo` to restore originals.
+- Planned rename rules MUST document their inputs, validations, and composing order (Composable Rule Engine). ✅ Build a dedicated regex rule that compiles patterns, validates templates, and plugs into traversal pipeline without altering shared state.
+- Scope handling MUST cover files vs directories (`-d`), recursion (`-r`), and extension filtering via `-e` without escaping the requested path (Scope-Aware Traversal). ✅ Reuse traversal filters so regex respects directory, recursion, hidden, and extension flags identically to other commands.
+- CLI UX plan MUST confirm Cobra usage, flag naming, help text, and automated tests for preview/undo flows (Ergonomic CLI Stewardship). ✅ Add Cobra subcommand with documented flags, examples, help output, and contract/integration coverage for preview/apply/undo flows.
+
+*Post-Design Verification (2025-10-30): Research, data model, contracts, and quickstart documents confirm preview coverage, ledger metadata, regex template validation, and CLI UX updates — no gate violations detected.*
+
+## Project Structure
+
+### Documentation (this feature)
+
+```text
+specs/006-add-regex-command/
+├── plan.md
+├── research.md
+├── data-model.md
+├── quickstart.md
+├── contracts/
+└── tasks.md   # Generated via /speckit.tasks
+```
+
+### Source Code (repository root)
+
+```text
+cmd/
+├── root.go
+├── list.go
+├── replace.go
+├── remove.go
+├── extension.go
+├── insert.go
+├── regex.go          # NEW
+└── undo.go
+
+internal/
+├── filters/
+├── history/
+├── listing/
+├── output/
+├── remove/
+├── replace/
+├── extension/
+├── insert/
+└── regex/            # NEW: pattern compilation, template evaluation, engine, ledger metadata
+
+tests/
+├── contract/
+├── integration/
+├── fixtures/
+└── unit/
+
+scripts/
+├── smoke-test-list.sh
+├── smoke-test-replace.sh
+├── smoke-test-remove.sh
+├── smoke-test-extension.sh
+├── smoke-test-insert.sh
+└── smoke-test-regex.sh   # NEW
+```
+
+**Structure Decision**: Extend the single CLI project by introducing `cmd/regex.go`, a new `internal/regex` package for rule evaluation, and corresponding contract/integration tests plus a smoke script under existing directories.
+
+## Complexity Tracking
+
+> **Fill ONLY if Constitution Check has violations that must be justified**
+
+| Violation | Why Needed | Simpler Alternative Rejected Because |
+|-----------|------------|-------------------------------------|