Lesson 06 · Ranges in a class - Build a Glob and Gitignore Matcher

Ranges in a class

A class can name a range with a dash, so [a-z] means every lowercase letter and [0-9] every digit. Today the class scanner learns to read a low-dash-high triple and test whether the character falls in the span.

The goal

Make a dash inside a class denote an inclusive character range.

Start here - the target

TO DO

Scenario: A dash denotes an inclusive range of characters

Givenclasses that contain a range

WhenMatch is called against various names

ThenMatch("[a-z]", "m") is true, Match("[a-z]", "M") is false, and Match("[0-9]", "5") is true

Anda class can mix ranges and singles: Match("[a-cx-z]", "y") is true and Match("[a-cx-z]", "m") is false

Background

Listing every character gets old fast, so a class lets you write a range with a dash: [a-z] is every character from a through z, [0-9] every digit. The comparison is just on the character’s code, so [a-z] matches m but not M - uppercase sits in a different part of the table. A class can mix ranges and plain members freely: [a-cx-z] accepts a, b, c, x, y, z, so it matches y but not m.

The scanner change is small: at each position, peek ahead - if the next character is a - and the one after it is not the closing ], read the three characters as a low-dash-high range and test whether the name character falls inside it; otherwise treat the character as a single member as before. Guarding that the character after the dash is not ] is what keeps a trailing dash (as in [a-]) from being mistaken for an unfinished range - that literal-dash case is a lesson of its own shortly.

Make it work

// inside the class scan, look for a low-dash-high triple
if j+2 < len(pat) && pat[j+1] == '-' && pat[j+2] != ']' {
  if pat[j] <= c && c <= pat[j+2] { matched = true }
  j += 3            // consumed low, '-', high
} else {
  if pat[j] == c { matched = true }
  j++
}

CheckpointDONE

A dash inside a class matches an inclusive range. Commit and stop here.