How Can I Write a Regex to Capture a SIP URI and Optional Tag in C#?

Below is one way to write a regex that meets your requirements. In this version we:

• Match the literal "To:" followed by any whitespace
• Optionally match a leading "<" (with possible spaces)
• Capture the entire sip URI in a named group “from”
• Optionally match a trailing ">"
• Optionally capture the tag (if any) in a second named group “tag”

Try this regex:

CSHARPCopied
@"To:\s*(?:<\s*)?(?<from>sip:[^>;\s]+)(?:\s*>\s*)?(?:;tag=(?<tag>[^;\s]+))?"

Explanation

• To:\s*
  Matches the literal "To:" followed by any whitespace.

• (?:<\s)?*
  Optionally matches a "<" (with possible trailing spaces).

• (?sip:[^>;\s]+)
  Captures the SIP URI starting with "sip:" in group "from". The character class ([^>;\s]+) stops at any whitespace, '>' or ';', which makes the group work with both forms.

• (?:\s>\s)?**
  Optionally matches a closing ">" (with optional spaces around it).

• (?:;tag=(?[^;\s]+))?
  Optionally matches a tag specification beginning with ";tag=" and captures the tag value in group "tag". Again the character class stops at whitespace or a semicolon.

Testing

Below is an example program that shows how these inputs get processed:

CSHARPCopied
using System;
using System.Text.RegularExpressions;

class Program
{
    static void Main()
    {
        string pattern = @"To:\s*(?:<\s*)?(?<from>sip:[^>;\s]+)(?:\s*>\s*)?(?:;tag=(?<tag>[^;\s]+))?";
        string[] inputs = {
            "To: <sip:alice@wonderland.com:5080>;tag=myTag",
            "To: <sip:alice@wonderland.com:5080>",
            "To: sip:alice@wonderland.com:5080",
            "To: sip:alice@wonderland.com:5080;tag=myTag"
        };

        foreach (string input in inputs)
        {
            var match = Regex.Match(input, pattern);
            if (match.Success)
            {
                Console.WriteLine($"Input: {input}");
                Console.WriteLine($" Group 'from': {match.Groups["from"].Value}");
                Console.WriteLine($" Group 'tag': {match.Groups["tag"].Value}");
                Console.WriteLine("---");
            }
            else
            {
                Console.WriteLine($"No match for: {input}");
            }
        }
    }
}

What to Expect

For each input the regex returns:

• Group "from":
  sip:alice@wonderland.com:5080

• Group "tag":
  "myTag" when present, or an empty string when no tag is provided

This should work well in C# and meet your requirements for both formats that include angle brackets and those that don’t.