Stripping Non-AlphaNumerics in C# Benchmark

Program.cs

 [MemoryDiagnoser]
    public class Benchmark1
    {
        [Params("555-55-5555", "555-123-fsdfjksdf-xxx_fdslfs fkslfd_fkdlsf")]
        public string Input { get; set; }

        [Benchmark]
        public string StringBuilderForeach()
        {
            var sb = new StringBuilder();
            foreach (var c in Input)
            {
                if (char.IsLetterOrDigit(c))
                    sb.Append(c);
            }

            return sb.ToString();
        }

        [Benchmark]
        public string StringBuilderFor()
        {
            var sb = new StringBuilder();
            for(int i = 0; i < Input.Length; i++)
            {
                var c = Input[i];

                if (char.IsLetterOrDigit(c))
                    sb.Append(c);
            }

            return sb.ToString();
        }

        static readonly Regex _r = new Regex(@"[^a-zA-Z0-9]");
        static readonly Regex _rCompiled = new Regex(@"[^a-zA-Z0-9]", RegexOptions.Compiled);

        [Benchmark]
        public string RegularExpression()
        {
            return _r.Replace(Input, "");
        }

        [Benchmark]
        public string RegularExpressionCompiled()
        {
            return _rCompiled.Replace(Input, "");
        }
    }

Results.md

BenchmarkDotNet=v0.10.14, OS=Windows 10.0.17134
Intel Core i7-6650U CPU 2.20GHz (Skylake), 1 CPU, 4 logical and 2 physical cores
Frequency=2156250 Hz, Resolution=463.7681 ns, Timer=TSC
  [Host]     : .NET Framework 4.7.1 (CLR 4.0.30319.42000), 32bit LegacyJIT-v4.7.3101.0
  DefaultJob : .NET Framework 4.7.1 (CLR 4.0.30319.42000), 32bit LegacyJIT-v4.7.3101.0

Method	Input	Mean	Error	StdDev	Median	Gen 0	Allocated
StringBuilderForeach	555-123-fsdfjksdf-xxx_fdslfs fkslfd_fkdlsf	590.4 ns	11.853 ns	34.008 ns	579.6 ns	0.1593	336 B
StringBuilderFor	555-123-fsdfjksdf-xxx_fdslfs fkslfd_fkdlsf	557.2 ns	7.810 ns	6.924 ns	557.4 ns	0.1593	336 B
RegularExpression	555-123-fsdfjksdf-xxx_fdslfs fkslfd_fkdlsf	2,937.3 ns	54.250 ns	50.745 ns	2,926.6 ns	0.5112	1080 B
RegularExpressionCompiled	555-123-fsdfjksdf-xxx_fdslfs fkslfd_fkdlsf	2,556.4 ns	49.963 ns	61.359 ns	2,556.6 ns	0.5112	1080 B
StringBuilderForeach	555-55-5555	138.1 ns	2.780 ns	3.415 ns	137.6 ns	0.0494	104 B
StringBuilderFor	555-55-5555	147.9 ns	3.997 ns	11.468 ns	144.4 ns	0.0494	104 B
RegularExpression	555-55-5555	1,123.5 ns	53.205 ns	154.356 ns	1,061.6 ns	0.1659	352 B
RegularExpressionCompiled	555-55-5555	955.9 ns	49.846 ns	146.190 ns	880.4 ns	0.1659	352 B