Is there a better way to count string format placeholders in a string in C#?

Merging Damovisa's and Joe's answers. I've updated answer afer Aydsman's nad activa's comments.

int count = Regex.Matches(templateString, @"(?<!\{)\{([0-9]+).*?\}(?!})")  //select all placeholders - placeholder ID as separate group
                 .Cast<Match>() // cast MatchCollection to IEnumerable<Match>, so we can use Linq
                 .Max(m => int.Parse(m.Groups[1].Value)) + 1; // select maximum value of first group (it's a placegolder ID) converted to int

This approach will work for templates like:

"{0} aa {2} bb {1}" => count = 3

"{4} aa {0} bb {0}, {0}" => count = 5

"{0} {3} , {{7}}" => count = 4

Counting the placeholders doesn't help - consider the following cases:

"{0} ... {1} ... {0}" - needs 2 values

"{1} {3}" - needs 4 values of which two are ignored

The second example isn't farfetched.

For example, you may have something like this in US English:

String.Format("{0} {1} {2} has a {3}", firstName, middleName, lastName, animal);

In some cultures, the middle name may not be used and you may have:

String.Format("{0} {2} ... {3}", firstName, middleName, lastName, animal);

If you want to do this, you need to look for the format specifiers {index[,length][:formatString]} with the maximum index, ignoring repeated braces (e.g. {{n}}). Repeated braces are used to insert braces as literals in the output string. I'll leave the coding as an exercise :) - but I don't think it can or should be done with Regex in the most general case (i.e. with length and/or formatString).

And even if you aren't using length or formatString today, a future developer may think it's an innocuous change to add one - it would be a shame for this to break your code.

I would try to mimic the code in StringBuilder.AppendFormat (which is called by String.Format) even though it's a bit ugly - use Lutz Reflector to get this code. Basically iterate through the string looking for format specifiers, and get the value of the index for each specifier.