I want to write more simple [if] statement at [//Add a string all matching] comments

SearchStringWithRegularExpressions

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.IO;
using System.Text.RegularExpressions;

namespace SearchStringWithRegularExpressions
{
	class Program
	{
		/// <summary>
		/// **Setting** Change The Encod Text from Input File.
		/// </summary>
		private const string _FileEncoding = "Shift_JIS";

		/// <summary>
		/// **Setting** specify the matching pattern of RegularExpressions
		/// </summary>
		private const string _MatchingPattern = "\"Core_E[0-9]{5,}";

		/// <summary>
		/// **Setting** when set to false , Duplicate results are output to ignore
		/// </summary>
		private const bool _AllowDuplicateChars = false;


		/// <summary>
		/// Entry Point
		/// </summary>
		/// <param name="args"></param>
		static void Main( string[] args )
		{
			// argument check
			string inputFilePath;
			if( ! IsArgumentValid( args, out inputFilePath ) )
			{
				return;
			}

			// input file
			//StringBuilder sb = new StringBuilder();
			List<string> sL = new List<string>();

			using( StreamReader sr = new StreamReader(
						inputFilePath, Encoding.GetEncoding( _FileEncoding ) ) )
			{
				string line;
				while( ( line = sr.ReadLine() ) != null )
				{
					// saving string by RegularExpressions 
					Regex r = new System.Text.RegularExpressions.Regex(
							 _MatchingPattern, System.Text.RegularExpressions.RegexOptions.IgnoreCase );
					System.Text.RegularExpressions.Match m = r.Match( line );
					while( m.Success )
					{
						//Add a string all matching
						if( ! _AllowDuplicateChars )
						{
							if( !sL.Contains( m.Value ) )
							{
								sL.Add( m.Value );
							}
						}
						else
						{
							//[sL.Add( m.Value );] also includes two ... Unattractive...
							sL.Add( m.Value );
						}
						m = m.NextMatch();
					}
				}
			}
			// output file
			System.Text.Encoding enc = System.Text.Encoding.GetEncoding( _FileEncoding );
			string outputFilePath = Path.Combine(
				Path.GetDirectoryName( inputFilePath ) //Directory
				, Path.GetFileNameWithoutExtension( inputFilePath ) + "_Reviewed" //Filename
				+ Path.GetExtension( inputFilePath ) ); // Extension

			//always overwrite
			string outputLine = String.Join( System.Environment.NewLine, sL.ToArray() );
			System.IO.File.WriteAllText( outputFilePath, outputLine, enc );
			Console.WriteLine( "Complete File Review: \nOutput File Name Is " + outputFilePath );
		}

		/// <summary>
		/// check the validity of the argument.
		/// </summary>
		/// <param name="args"></param>
		/// <returns>args Length == 0 : false , File is Not exist : false </returns>
		private static bool IsArgumentValid( string[] args, out string filePath )
		{
			filePath = "";
			// argument check
			if( args.Length == 0 )
			{
				Console.WriteLine( "Error No Argument" );
				return false;
			}

			// File Exists
			foreach( string s in args )
			{
				if( ! File.Exists( s ) )
				{
					Console.WriteLine( "Error No Argument" );
					return false;
				}
				filePath = s;
				break;
			}
			return true;
		}
	}
}