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

public class Test
{
	public static void Main()
	{
		var text = "This is a /Sample Text: in a sentence";
		var oldValue = "/Sample Text:";
		var newValue = "sample text";
		var matchCase = RegexOptions.IgnoreCase;
		var result = Regex.Replace(text, $@"(?!\B\w){Regex.Escape(oldValue)}(?<!\w\B)", newValue, matchCase);
       	Console.WriteLine(result);
	}
}