Lesson 16 · Anchored patterns - Build a Glob and Gitignore Matcher

Anchored patterns

A pattern with a slash in it is anchored - it matches from the base directory, not at any depth. Today Compile marks anchored rules and Ignored matches them against the whole path with your path-aware Match.

The goal

Anchor a pattern that has a leading or interior slash to the base directory.

Start here - the target

TO DO

Scenario: A slash in the pattern anchors it to the base

Givenpatterns with a leading or interior slash

WhenIgnored is asked about a path

Thena leading slash anchors to the base: Compile("/foo").Ignored("foo", false) is true and Compile("/foo").Ignored("a/foo", false) is false

Andan interior slash also anchors: Compile("a/b").Ignored("a/b", false) is true and Compile("a/b").Ignored("x/a/b", false) is false, while a slash-free pattern still floats - Compile("foo").Ignored("a/foo", false) is true

Background

The slash decides where a pattern applies. A pattern that contains a slash - either a leading one like /foo or an interior one like a/b - is anchored to the directory the .gitignore lives in. It matches only from the base, not at arbitrary depth: /foo ignores foo but not a/foo, and a/b ignores a/b but not x/a/b. A pattern with no slash still floats and matches the basename anywhere, as in the last lesson.

Compile decides this once: if the line starts with /, mark it anchored and drop that leading slash (it was only a signal); if the line contains a slash anywhere, mark it anchored too. Then Ignored branches on the flag - an anchored rule is matched against the whole path with the path-aware Match you built, a floating rule against the basename with matchSegment. This is exactly why Match splits on the slash: an anchored gitignore pattern is just a path glob rooted at the base, and everything it does - segments, the double-star - now works for gitignore for free.

Make it work

// in Compile: decide anchoring, strip a leading slash
anchored := false
if strings.HasPrefix(line, "/") { anchored = true; line = line[1:] }
if strings.Contains(line, "/") { anchored = true }
// in Ignored, per rule:
if r.Anchored {
  ok = Match(r.Pattern, path)      // whole path, path-aware
} else {
  ok = matchSegment(r.Pattern, base)
}

CheckpointDONE

A pattern with a slash anchors to the base directory. Commit and stop here.