Correctly locating a string in string array - asp.net

Question

I have an array of strings that contains usernames that I'd like to extract and add it to a list. So for example, I have this string => "UserName: JohnSmith" and I want to extract JohnSmith. The below is the code I'm attempting to get working:
var documentSource = content.Replace("\n" , "").Trim();
string[] contents = documentSource.Split(',');
List<Model> modelList = new List<Model>();
Model model = new Model();
foreach (var item in contents)
{
//this is where I'd like to locate the username or usernames
if(item.Any())
{
model.userName = //username only;
modelList.Add(model);
}
}
return modelList;

Answer 1

This is the kind of problem that can be easily solved with regular expressions:
using System.Text.RegularExpressions;
// ...
var regex = new Regex("UserName: ([^,]*)");
// ..
foreach(var item in contents)
{
var match = regex.Match(item);
if(match.Success)
{
modelList.Add(
new Model
{
userName = match.Groups[1].Value
}
);
}
}

Answer 2

In it's simplest form, since you know that each username start after "UserName: " part that is fixed, it would be something like this:
var documentSource = content.Replace("\n" , "").Trim();
string[] contents = documentSource.Split(',', StringSplitOptions.RemoveEmptyEntries);
var modelList = new List<Model>();
foreach (var item in contents)
{
var username = item.Substring("UserName: ".Length).Trim();
if (!string.IsNullOrEmpty(username))
{
Model model = new Model();
model.userName = username;
modelList.Add(model);
}
}
return modelList;

Answer 3

Since you know that you have one : inside of the username string and the second part of the : is your actual username that you want, so, you can split the string in two parts and get the second one. Something like below:
foreach (var item in contents)
{
var username = item.Split(':').Last()?.Trim();
if(!string.IsNullOrEmpty(username))
{
modelList.Add(new Model{ userName = username });
}
}