fix code snippet in 'Collection lookups with spans (What's new in .NET libraries for .NET 9)' documentation

[wipiano]

Summary

Fixes #43701

[gewarren]

Thanks @wipiano, and sorry for the delay. You're right that the original code doesn't do what it's supposed to.

I wonder why ValueMatchEnumerator.Current isn't a Range like ValueSplitEnumerator.Current is. That would make your suggested code even simpler because you wouldn't have to instantiate the Range instance.

Since EnumerateSplits was introduced in .NET 9, I wonder if this code used it intentionally over EnumerateMatches. If so, we could still use EnumerateSplits with code like the following, where the "match" is actually the characters between the words. But it does require a check for empty ranges.

static Dictionary<string, int> CountWords(ReadOnlySpan<char> input)
{
    Dictionary<string, int> wordCounts = new(StringComparer.OrdinalIgnoreCase);
    Dictionary<string, int>.AlternateLookup<ReadOnlySpan<char>> spanLookup =
        wordCounts.GetAlternateLookup<ReadOnlySpan<char>>();

    foreach (Range wordRange in Regex.EnumerateSplits(input, @"\b\W+"))
    {
        if (wordRange.Start.Value == wordRange.End.Value)
        {
            continue; // Skip empty ranges.
        }
        ReadOnlySpan<char> word = input[wordRange];
        spanLookup[word] = spanLookup.TryGetValue(word, out int count) ? count + 1 : 1;
    }

    return wordCounts;
}

@stephentoub Do you have any preference as to whether the code that demonstrates GetAlternateLookup uses EnumerateMatches or EnumerateSplits?

[stephentoub]

I wonder why ValueMatchEnumerator.Current isn't a Range like ValueSplitEnumerator.Current is.

Leaving the door open to add additional state as part of enabling EnumerateMatches to also gather capture group information.

Do you have any preference as to whether the code that demonstrates GetAlternateLookup uses EnumerateMatches or EnumerateSplits?

No preference.

[wipiano]

@gewarren Thanks for your review. I think your suggested code is simple and I prefer it. I'll check it and incorporate it into this pull request.