Regex.IsMatch vs string.Contains

Question

Is there any difference in speed/memory usage for these two equivalent expressions:

Regex.IsMatch(Message, "1000") 

Vs

Message.Contains("1000") 

Any situations where one is better than other ?

The context of this question is as follows: I was making some changes to legacy code which contained the Regex expression to find whether a string is contained within another string. Being legacy code I did not make any changes to that and in the code review somebody suggested that Regex.IsMatch should be replaced by string.Contains. So I was wondering whether the change was worth making.

Answer 1

For simple cases String.Contains will give you better performance but String.Contains will not allow you to do complex pattern matching. Use String.Contains for non-pattern matching scenarios (like the one in your example) and use regular expressions for scenarios in which you need to do more complex pattern matching.

A regular expression has a certain amount of overhead associated with it (expression parsing, compilation, execution, etc.) that a simple method like String.Contains simply does not have which is why String.Contains will outperform a regular expression in examples like yours.

Answer 2

String.Contains is slower when you compare it to a compiled regular expression. Considerably slower, even!

You can test it running this benchmark:

class Program {   public static int FoundString;   public static int FoundRegex;    static void DoLoop(bool show)   {     const string path = "C:\\file.txt";     const int iterations = 1000000;     var content = File.ReadAllText(path);      const string searchString = "this exists in file";     var searchRegex = new Regex("this exists in file");      var containsTimer = Stopwatch.StartNew();     for (var i = 0; i < iterations; i++)     {       if (content.Contains(searchString))       {         FoundString++;       }     }     containsTimer.Stop();      var regexTimer = Stopwatch.StartNew();     for (var i = 0; i < iterations; i++)     {       if (searchRegex.IsMatch(content))       {         FoundRegex++;       }     }     regexTimer.Stop();      if (!show) return;      Console.WriteLine("FoundString: {0}", FoundString);     Console.WriteLine("FoundRegex: {0}", FoundRegex);     Console.WriteLine("containsTimer: {0}", containsTimer.ElapsedMilliseconds);     Console.WriteLine("regexTimer: {0}", regexTimer.ElapsedMilliseconds);      Console.ReadLine();   }    static void Main(string[] args)   {     DoLoop(false);     DoLoop(true);     return;   } }