Instead of using regex you can use substring method as well,
below is the syntax to extract the first value after 'From : ’ and attaching screenshot as well for your reference
strInput.Substring(strInput.IndexOf("From : ")+"From : ".Length).Trim.Split(Environment.NewLine.ToCharArray)(0).
if you want in regex format you can use below syntax in assign activity,
System.Text.RegularExpressions.Regex.Match(strInput,“(?<=From\s*:\s*)(.*)[^\r\n]”).ToString
