Sample Regular Expression

[TestMethod]
public void ParseVideoFileForDateTime2Test()
{
    var filePath = @"D:\Temp\Nokia 6\Photos\OpenCamera\VID_20190623_085427.mp4";
    var fileNameSansExt = Path.GetFileNameWithoutExtension(filePath);

    var dateTimeFormat = "yyyyMMdd_HHmmss";
    var result = string.Empty;
    Regex regExp = null;
    CalcRegExpForDateTimeFormat(dateTimeFormat, out regExp);
    if (regExp != null)
    {
        result = regExp.MatchFirstRegExp(fileNameSansExt);
    }
    Assert.IsNotNull(regExp);
    Assert.IsTrue(result.Length == 15);
    Assert.IsTrue(result == "20190623_085427");
}

private static bool CalcRegExpForDateTimeFormat(
    string dateTimeFormat, out Regex regExp)
{
    var res = false;
    regExp = null;
    var newRegex = new StringBuilder(dateTimeFormat.Length);
    foreach (var ch in dateTimeFormat)
    {
        if ((ch == 'y') || (ch == 'M') || (ch == 'd') ||
            (ch == 'H') || (ch == 'm') || (ch == 's'))
        {
            newRegex.Append(@"\d");
        }
        else
        {
            newRegex.Append(ch);
        }
    }
    var regexStr = newRegex.ToString();
    res = (regexStr.Length > 0);
    if (res)
    {
        regExp = new Regex(regexStr, RegexOptions.Compiled);
    }
    return res;
}

public static class RegularExpressionExtensions
{
/// <summary>
/// Match the first occurance of a regular expression in a target string.
/// </summary>
/// <param name="target">string to search</param>
/// <param name="regexp">regular expression object to use for the
/// search</param>
/// <returns>first match of the regular expression otherwise an 
/// empty string if there is no match</returns>
  public static string MatchFirstRegExp(this Regex reg, string target)
  {
    var result = "";
    MatchCollection mc = reg.Matches(target);
    if (mc.Count > 0)
    {
        result = mc[0].Value;
    }
    return result;
  }
}

Regex Matches Tester

Can find and list matching regular expressions in a string

internal class TestRegexMatches
{

    public void Test()
    {
        string typeName = typeof(double).ToString().Replace('.', '_');
        string name = "Fudge Factor".Replace(" ", "_");
        string plugin = "Smb.Orca.Theo.Samba.Delta, Version=4.20111.99.0, Culture=neutral, PublicKeyToken=19ae4a476ca5c63x";
        string[] splitStr = plugin.Split(',');
        plugin = splitStr[0].Replace('.', '_');

        string id = typeName + "\t" + name + "\t" + plugin;
        int ix=1;
        MatchCollection matches = Regex.Matches(id, @"[^\t]+");
        foreach (Match match in matches)
        {
            Console.WriteLine("Match " + ix++.ToString() + ": " + match.Value);
        }
    }
}

produces the output

Match 1: System_Double
Match 2: Fudge_Factor
Match 3: Smb_Orca_Theo_Samba_Delta

Filtering a Multiline string Line by Line using a Regular Expression

For other regular expression stuff see also this blog
This routine filters a string containing multiple lines using the supplied regular expression filter. Set the invert parameter to true if you want the results inverted

private string Filter(string reportText, string regExFilter, bool invert)
{
    if (regExFilter.Length > 0)
    {
        StringBuilder output = new StringBuilder();
        string[] lines = reportText.Split(
          new string[] { Environment.NewLine }, 
          StringSplitOptions.None);
        foreach (string line in lines)
        {
            if ((line.Length > 0))
            {
                bool matches = Regex.IsMatch(line, regExFilter,
                                             RegexOptions.None);
                if ((matches && !invert) || (!matches && invert))
                    output.AppendLine(line);
            }
        }
        return output.ToString();
    }
    else
    {
        return reportText;
    }
}

Regular Expressions

• For a great step by step tutorial with a test at the end of each step try this web-site: https://regexone.com/
• Regular Expressions Quickstart - This is good for a quick lookup
• Regular Expressions Detailed
• Brief Regex Syntax Guide
• Regular expression tester

For matching purposes:
? = 0 or 1 match
* = 0..n matches
+ = 1..n matches
{m,n} = from 'm' to 'n' matches where 'm' and 'n' are digits
{n} = exactly 'n' matches where 'n' is a digit
\d = numeric digit
\D = anything but a numeric digit
\s = any whitespace character
\S = any non-whitespace character
\w = Any alphabetical character and '_'
\W = Any non-alphabetical character or '_'

This example matches a United Kingdom postcode:

using System.Text.RegularExpressions;
...
const string PostCodeRegEx = @"(GIR0AA|[A-PR-UWYZ]([0-9]{1,2}|" + 
  "([A-HK-Y][0-9]|[A-HK-Y][0-9]([0-9]|[ABEHMNPRV-Y]))|" +
  "[0-9][A-HJKS-UW])[0-9][ABD-HJLNP-UW-Z]{2})";
public bool IsPostCode(string postCode)
{
    postCode = postCode.Trim().Replace(" ", string.Empty);
    bool res = Regex.IsMatch(postCode, PostCodeRegEx);
    return res;
}

This example Matches a SID:

using System.Text.RegularExpressions;
...
result = string.Empty;
const string SID_PATTERN = @"(\w-\d-\d-\d{2}-\d+-\d+-\d+)";
Regex reg = new Regex(SID_PATTERN);
MatchCollection mc = reg.Matches(sid);
if (mc.Count > 0)
{
    result = mc[0].Value;
}