[Solved] C# Regex for Username

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Try this code it will help you: (^[A-Z][a-z]{2,}_[A-Z][a-z]{2,})

using System;
using System.Text.RegularExpressions;

public class Example
{
    public static void Main()
    {
        string pattern = @"(^[A-Z][a-z]{2,}_[A-Z][a-z]{2,})";
        string input = @"Firstname_Lastname
fi_na
Fi_na
fi_Na
Fir_Name
Firs_name
firs_Name";
        RegexOptions options = RegexOptions.Multiline;
        
        foreach (Match m in Regex.Matches(input, pattern, options))
        {
            Console.WriteLine("'{0}' found at index {1}.", m.Value, m.Index);
        }
    }
}

Here is the demo: https://regex101.com/r/8RUcaG/1

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solved C# Regex for Username