Directory Pattern Matching: Beware Substring Matches

Date: 2025-11-04 Context: .git/ pattern excluded .gitconfig, breaking git configuration

The Problem

Substring matching for directory patterns causes false positives:

# BROKEN: ".git" substring matches ".gitconfig"
if pattern.endswith("/") and pattern.rstrip("/") in path_str:
    return True  # BUG!

Impact: .gitconfig was never symlinked to home directory.

The Solution

Check for complete path components:

if pattern.endswith("/"):
    dir_name = pattern.rstrip("/")
    if f"/{dir_name}/" in path_str or path_str.startswith(f"{dir_name}/"):
        return True

Results:

• /.git/ matches foo/.git/bar ✓
• .git/ matches .git/config ✓
• .git does NOT match .gitconfig ✓

Key Learnings

• Check complete path components - substring matching is almost never correct for paths
• Test similar filenames - .git/ vs .gitconfig, node_modules/ vs node_modules.txt
• Write regression tests - prevent fixed bugs from returning

Testing

Always test edge cases:

# Directory should be excluded
assert should_exclude(Path(".git/config"))
assert should_exclude(Path("foo/.git/hooks"))

# Similar-named files should NOT be excluded
assert not should_exclude(Path(".gitconfig"))
assert not should_exclude(Path(".gitignore"))
assert not should_exclude(Path(".github/workflows/ci.yml"))