Lesson 02 · The single-character wildcard - Build a Glob and Gitignore Matcher

The single-character wildcard

The question mark is the simplest wildcard - it matches exactly one character, no more and no fewer. Adding it turns Match from a string comparison into a real position-by-position matcher, the shape every other wildcard will slot into.

The goal

Make the question mark match exactly one character.

Start here - the target

TO DO

Scenario: A question mark matches any single character but never zero

Giventhe pattern 'f?o'

WhenMatch is called against various names

ThenMatch("f?o", "foo") is true and Match("f?o", "fao") is true - the question mark stands for any one character

AndMatch("f?o", "fo") is false (the question mark needs one character) and Match("f?o", "fooo") is false, and Match("?", "") is false

Background

The literal comparison from lesson 1 can no longer be a single ==: a ? in the pattern must line up against some character in the name, so the matcher has to walk the two strings together, one position at a time. That walk is the skeleton every wildcard plugs into.

The rule for ? is that it matches any one character, but never zero - so f?o matches foo and fao, but not fo (nothing for the ? to consume) and not fooo (a leftover o). The recursion makes this precise: at each step, if the pattern character is ? or equals the name character, consume one from each and continue; when the pattern runs out, the name must be exactly used up too. Keep this recursive shape - the star in the next lesson is one more case inside it.

Make it work

// walk pattern and name together, one character at a time
func Match(pattern, name string) bool {
  if pattern == "" {
    return name == ""
  }
  if name != "" && (pattern[0] == '?' || pattern[0] == name[0]) {
    return Match(pattern[1:], name[1:])   // consume one of each
  }
  return false
}

CheckpointDONE

The question mark matches exactly one character. Commit and stop here.