Variable expansion with Regex .NET style

Question

I'm developing a small application with needs to replace .NET style "variables" in strings with their values. An example is given below:

Replace:

"{D}/{MM}/{YYYY}"

With: "31/12/2009"

I have decided to use Regex to find the occurrences of "{var}" but I'm not too sure how to properly go about it.

Answer 1

Here's some code I use in a logging utility that does almost exactly what you want, although with a slightly different format:

 Regex rgx = new Regex("{([Dd]:([^{]+))}");

 private string GenerateTrueFilename()
 {
  // Figure out the new file name
  string lfn = LogFileName;
  while (rgx.IsMatch(lfn))
  {
   Match m = rgx.Matches(lfn)[0];
   string tm = m.Groups[1].Value;
   if (tm.StartsWith("D:"))
   {
    string sm = m.Groups[2].Value;
    string dts = DateTime.Now.ToString(sm);
    lfn = lfn.Replace(m.Groups[0].Value, dts);
   }
  }

  return lfn;
 }

The rgx defines the format, which currently really only has one format:

D:{datestring}

LogFileName is a variable defined elsewhere in code, but has a default:

LogFileName = "LogFile.{D:yyyy-MM-dd}.log"

so hopefully you can see from this how to proceed with other variables if needed.

Answer 2

Regular Expressions are really powerful, but also take a lot of overhead. It would probably be simpler to use something like inputString.Replace("{var}", value) instead:

void Foo()
{
    string s = "{D}/{MM}/{YYYY}";
    s = ReplaceTokenInString(s, "{D}", "31");
    s = ReplaceTokenInString(s, "{MM}", "12");
    s = ReplaceTokenInString(s, "{YYYY}", "2009");
}

string ReplaceTokenInString(string input, string token, string replacement)
{
    input.Replace(token, replacement);
}

Answer 3

If you're specifically doing dates, you should look into using a format string. Obviously if you're doing more than that this won't help you.

Answer 4

If you're going to use Regex, and you have a known pattern, it doesn't get much simpler than using a match evaluator method instead of calling replace a bunch of times:

void Main() // run this in LinqPad
{
 string text = "I was here on {D}/{MMM}/{YYYY}.";
 string result = Regex.Replace(text, "{[a-zA-Z]+}", ReplaceMatched);

 Console.WriteLine(result);
}

private string ReplaceMatched(Match match)
{
 if( match.Success )
 {
  switch( match.Value )
  {
   case "{D}":
    return DateTime.Now.Day.ToString();
   case "{YYYY}":
    return DateTime.Now.Year.ToString();
   default:
    break;
  }
 }
 // note, early return for matched items.

 Console.WriteLine("Warning: Unrecognized token: '" + match.Value + "'");
 return match.Value;
}

gives the result

Warning: Unrecognized token: '{MMM}'
I was here on 2/{MMM}/2009.

It's not entirely implemented obviously.

Check out LinqPad and a Regex Tool like Expresso

Answer 5

I've done it with Regex and using a StringBuilder. I'm finding the matches in the source string and appending blocks of the string and evaluated matches.

StringBuilder text = new StringBuilder(snippet.Length * 2);
Regex pattern = new Regex("\\{[A-Za-z]+\\}");
MatchCollection matches = pattern.Matches(snippet);
if (matches.Count == 0) return snippet;

int from = 0;
foreach (Match match in matches)
{
    text.Append(snippet.Substring(from, match.Index - from));
    string variable = snippet.Substring(match.Index + 1, match.Length - 2);
    text.Append(EvaluateVariable(variable));
    from = match.Index + match.Length;
}
text.Append(snippet.Substring(from));
return text.ToString();