How would you count occurrences of a string (actually a char) within a string?

If you want to be able to search for whole strings, and not just characters:

src.Select((c, i) => src.Substring(i))
    .Count(sub => sub.StartsWith(target))

Read as "for each character in the string, take the rest of the string starting from that character as a substring; count it if it starts with the target string."

int count = new Regex(Regex.Escape(needle)).Matches(haystack).Count;

string source = "/once/upon/a/time/";
int count = 0;
foreach (char c in source) 
  if (c == '/') count++;

Has to be faster than the source.Replace() by itself.

If you're using .NET 3.5 you can do this in a one-liner with LINQ:

int count = source.Count(f => f == '/');

If you don't want to use LINQ you can do it with:

int count = source.Split('/').Length - 1;

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You might be surprised to learn that your original technique seems to be about 30% faster than either of these! I've just done a quick benchmark with "/once/upon/a/time/" and the results are as follows:

Your original = 12s
source.Count = 19s
source.Split = 17s
foreach (from bobwienholt's answer) = 10s

(The times are for 50,000,000 iterations so you're unlikely to notice much difference in the real world.)