C# Online Compiler | .NET Fiddle

<no name> by Anonymous

using System;
using System.Collections.Generic;
using System.Text.RegularExpressions;
					
public class Program
{
	public static void Main()
	{
		string string1 = "<body style=\"HEIGHT: 218px; margin: 0px; background-color: #ffffff;\" jQuery111105496473080628138=\"10\">";
            string string2 = "<body jQuery111105496473080628138=\"10\" style=\"HEIGHT: 218px; margin: 0px; background-color: #ffffff;\" >";
            string string3 = "<test style=\"HEIGHT: 218px; margin: 0px; background-color: #ffffff;\" jQuery111105496473080628138=\"10\">";
            List<string> theList = new List<string> { string1, string2, string3 };

            Regex heightMatchingRegex = new Regex("(HEIGHT:\\s*\\d{1,}[^;]*;)(?<=<body.*style=\"[^\"]*)(?=[^\"].*\"\\s*>)");

            foreach (string item in theList)
            {
                if (heightMatchingRegex.IsMatch(item))
                {
                    Console.WriteLine("The match: " + heightMatchingRegex.Match(item));
                    Console.WriteLine("Original: " + item);
                    Console.WriteLine("Modified: " + heightMatchingRegex.Replace(item, ""));
                }
            }

            Console.ReadLine();
	}
}

​x
 
using System;
using System.Collections.Generic;
using System.Text.RegularExpressions;
                    
public class Program
{
    public static void Main()
    {
        string string1 = "<body style=\"HEIGHT: 218px; margin: 0px; background-color: #ffffff;\" jQuery111105496473080628138=\"10\">";
            string string2 = "<body jQuery111105496473080628138=\"10\" style=\"HEIGHT: 218px; margin: 0px; background-color: #ffffff;\" >";
            string string3 = "<test style=\"HEIGHT: 218px; margin: 0px; background-color: #ffffff;\" jQuery111105496473080628138=\"10\">";
            List<string> theList = new List<string> { string1, string2, string3 };
​
            Regex heightMatchingRegex = new Regex("(HEIGHT:\\s*\\d{1,}[^;]*;)(?<=<body.*style=\"[^\"]*)(?=[^\"].*\"\\s*>)");
​
            foreach (string item in theList)
            {
                if (heightMatchingRegex.IsMatch(item))
                {
                    Console.WriteLine("The match: " + heightMatchingRegex.Match(item));
                    Console.WriteLine("Original: " + item);
                    Console.WriteLine("Modified: " + heightMatchingRegex.Replace(item, ""));
                }
            }
​
            Console.ReadLine();
    }
}

View IL Code