How to write C# regular expression pattern to match basic printf format-strings like "%5.2f"?

Question

It should match the following criteria (conditional parts are in square brackets:

%[some numbers][.some numbers]d|f|s

notation d|f|s means that one of them has to be there.

Thanks & BR -Matti

Answer 1

This should do it:

string input = "Bloke %s drank %5.2f litres of water and ate %d bananas";
string pattern = @"%(\d+(\.\d+)?)?(d|f|s)";

foreach (Match m in Regex.Matches(input, pattern))
{
    Console.WriteLine(m.Value);
}

I didn't use [dfs] in my pattern since I planned to update it to use named groups. This is based on your earlier question about figuring out a replacement strategy for C-style format strings.

Here's an idea:

string input = "Bloke %s drank %5.2f litres of water and ate %d bananas";
string pattern = @"%(?<Number>\d+(\.\d+)?)?(?<Type>d|f|s)";

int count = 0;
string result = Regex.Replace(input, pattern, m =>
{
    var number = m.Groups["Number"].Value;
    var type = m.Groups["Type"].Value;
    // now you can have custom logic to check the type appropriately
    // check the types, format with the count for the current parameter
    return String.Concat("{", count++, "}");
});

---

C#/.NET 2.0 approach:

private int formatCount { get; set; }

void Main()
{
    string input = "Bloke %s drank %5.2f litres of water and ate %d bananas";
    Console.WriteLine(FormatCStyleStrings(input));  
}

private string FormatCStyleStrings(string input)
{
    formatCount = 0; // reset count
    string pattern = @"%(?<Number>\d+(\.\d+)?)?(?<Type>d|f|s)";
    string result = Regex.Replace(input, pattern, FormatReplacement);
    return result;
}

private string FormatReplacement(Match m)
{
    string number = m.Groups["Number"].Value;
    string type = m.Groups["Type"].Value;
    // custom logic here, format as needed
    return String.Concat("{", formatCount++, "}");
}