Lesson 05 · A character class - Build a Glob and Gitignore Matcher

A character class

A bracket expression matches one character from a set, like [aeiou] for any vowel. Today matchOne learns to scan a class and test membership - the first token that spans more than one pattern character.

The goal

Make a bracket class match one character drawn from the set inside it.

Start here - the target

TO DO

Scenario: A class matches exactly one character from its set

Givenpatterns containing a bracket class

WhenMatch is called against various names

ThenMatch("[abc]", "b") is true and Match("[abc]", "d") is false

AndMatch("h[aeiou]t", "hat") is true, Match("h[aeiou]t", "hit") is true, and Match("h[aeiou]t", "hct") is false

Background

A bracket expression - [abc] - matches exactly one character, drawn from the set listed between the brackets. It is a ? with a restricted alphabet: h[aeiou]t matches hat, hit, hot, but not hct, because c is not one of the listed characters. Like ?, it consumes exactly one character of the name.

This is the first token that spans several pattern characters, which is precisely why lesson 4 pulled matchOne out into its own helper: it scans from the opening [ to the closing ], checks whether the name character is one of the members, and returns the index past the ] so the main scan continues after the whole class. Today handles a plain list of characters; ranges, negation, and the awkward edge cases each extend this same scanner in the next few lessons.

Make it work

// inside matchOne, when the token starts with '['
if pat[p] == '[' {
  j := p + 1
  matched := false
  for j < len(pat) && pat[j] != ']' {
    if pat[j] == c { matched = true }
    j++
  }
  return matched, j + 1   // j is at ']'; the token ends after it
}

CheckpointDONE

A bracket class matches one character from its set. Commit and stop here.