Adding Regex.EnumerateMatches

[joperezr]

Fixes #65011
Fixes #23602

Adding EnumerateMatches method which returns an enumerator that can iterate over the matches in a passed-in span. The operation is performed amortized allocation free.

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Issue Details

---

Fixes #65011
Fixes #23602

Adding EnumerateMatches method which returns an enumerator that can iterate over the matches in a passed-in span. The operation is performed amortized allocation free.

Author:	joperezr
Assignees:	-
Labels:	area-System.Text.RegularExpressions, new-api-needs-documentation
Milestone:	-

[joperezr]

Here is a quick benchmark I wrote to see how this compares with the existing way to iterate over a MatchCollection using Regex.Matches:

    // regex pattern used is "\b\w+\b" and the input is loremIpsum 5 paragraph string.

    [Benchmark(Baseline = true)]
    public int MatchCollection()
    {
        int x = 0;
        for (int i = 0; i < 1000; i++)
        {
            foreach (Match match in regex.Matches(loremIpsum))
            {
                if (match.ValueSpan[0] >= 'a' && match.ValueSpan[0] <= 'z')
                    x++;
            }
        }

        return x;
    }

    [Benchmark]
    public int MatchEnuemrator()
    {
        int x = 0;
        ReadOnlySpan<char> span = loremIpsum.AsSpan();
        for (int i = 0; i < 1000; i++)
        {
            foreach (ValueMatch word in regex.EnumerateMatches(span))
            {
                if (span.Slice(word.Index, word.Length)[0] >= 'a' && span.Slice(word.Index, word.Length)[0] <= 'z')
                    x++;
            }
        }

        return x;
    }

And the results are:

Method	Mean	Error	StdDev	Ratio	RatioSD	Gen 0	Gen 1	Allocated
MatchCollection	208.5 ms	3.97 ms	4.72 ms	1.00	0.00	43000.0000	1000.0000	273,544,480 B
MatchEnuemrator	146.0 ms	2.71 ms	2.67 ms	0.70	0.02	-	-	-

[stephentoub]

@olsaarik, in the current NonBacktracking code, it would benefit from knowing that indexes are needed but not captures. Will that still be the case after your upcoming fixes?