C# helper to remove all characters that are not digits/numbers 0-9 from the string.

/// <summary>
/// Removes all characters that are not digits 0-9 from the string.
/// </summary>
/// <param name="val">The value to replace.</param>
/// <returns>The <paramref name="val"/> with non-digits removed.</returns>
public static string DigitsOnly(string val)
{
    if (string.IsNullOrEmpty(val))
    {
        return val;
    }

    var result = new StringBuilder(val.Length);
    foreach (var c in val)
    {
        if (c >= '0' && c <= '9')
        {
            result.Append(c);
        }
    }

    return result.ToString();
}

//
// SQL CLR....

using System.Data.SqlTypes;
using Microsoft.SqlServer.Server;

// ReSharper disable CheckNamespace
// ReSharper disable UnusedMember.Global

#pragma warning disable S1118 // Utility classes should not have public constructors
#pragma warning disable S3903 // Types should be defined in named namespaces

public partial class SqlFunctions
{
    [SqlFunction(
        Name = "DigitsOnly",
        DataAccess = DataAccessKind.None,
        SystemDataAccess = SystemDataAccessKind.None
    )]
    public static SqlString DigitsOnly([SqlFacet(MaxSize = 128, IsNullable = true)] SqlString val)
    {
        return val.IsNull ?
            SqlString.Null :
            Pods.Common.Utils.StringUtils.DigitsOnly(val.Value);
    }
}

#pragma warning restore S1118 // Utility classes should not have public constructors
#pragma warning restore S3903 // Types should be defined in named namespaces