Parsing formatted string

Question

I am trying to create a generic formatter/parser combination.

Example scenario:

• I have a string for string.Format(), e.g. var format = \"{0}-{1}\

Answer 1

While the comments about lost information are valid, sometimes you just want to get the string values of of a string with known formatting.

One method is this blog post written by a friend of mine. He implemented an extension method called string[] ParseExact(), akin to DateTime.ParseExact(). Data is returned as an array of strings, but if you can live with that, it is terribly handy.

public static class StringExtensions
{
    public static string[] ParseExact(
        this string data, 
        string format)
    {
        return ParseExact(data, format, false);
    }

    public static string[] ParseExact(
        this string data, 
        string format, 
        bool ignoreCase)
    {
        string[] values;

        if (TryParseExact(data, format, out values, ignoreCase))
            return values;
        else
            throw new ArgumentException("Format not compatible with value.");
    }

    public static bool TryExtract(
        this string data, 
        string format, 
        out string[] values)
    {
        return TryParseExact(data, format, out values, false);
    }

    public static bool TryParseExact(
        this string data, 
        string format, 
        out string[] values, 
        bool ignoreCase)
    {
        int tokenCount = 0;
        format = Regex.Escape(format).Replace("\\{", "{");

        for (tokenCount = 0; ; tokenCount++)
        {
            string token = string.Format("{{{0}}}", tokenCount);
            if (!format.Contains(token)) break;
            format = format.Replace(token,
                string.Format("(?'group{0}'.*)", tokenCount));
        }

        RegexOptions options = 
            ignoreCase ? RegexOptions.IgnoreCase : RegexOptions.None;

        Match match = new Regex(format, options).Match(data);

        if (tokenCount != (match.Groups.Count - 1))
        {
            values = new string[] { };
            return false;
        }
        else
        {
            values = new string[tokenCount];
            for (int index = 0; index < tokenCount; index++)
                values[index] = 
                    match.Groups[string.Format("group{0}", index)].Value;
            return true;
        }
    }
}

Answer 2

A simple solution might be to

• replace all format tokens with (.*)
• escape all other special charaters in format
• make the regex match non-greedy

This would resolve the ambiguities to the shortest possible match.

(I'm not good at RegEx, so please correct me, folks :))

Answer 3

Assuming "-" is not in the original strings, can you not just use Split?

var arr2 = formattedString.Split('-');

Note that this only applies to the presented example with an assumption. Any reverse algorithm is dependent on the kind of formatting employed; an inverse operation may not even be possible, as noted by the other answers.

Answer 4

It's simply not possible in the generic case. Some information will be "lost" (string boundaries) in the Format method. Assume:

String.Format("{0}-{1}", "hello-world", "stack-overflow");

How would you "Unformat" it?

Answer 5

You can't unformat because information is lost. String.Format is a "destructive" algorithm, which means you can't (always) go back.

Create a new class inheriting from string, where you add a member that keeps track of the "{0}-{1}" and the { "asdf", "qwer" }, override ToString(), and modify a little your code.

If it becomes too tricky, just create the same class, but not inheriting from string and modify a little more your code.

IMO, that's the best way to do this.

Answer 6

After formatting, you can put the resulting string and the array of objects into a dictionary with the string as key:

Dictionary<string,string []> unFormatLookup = new Dictionary<string,string []>
...
var arr = new string [] {"asdf", "qwer" };
var res = string.Format(format, arr);
unFormatLookup.Add(res,arr);

and in Unformat method, you can simply pass a string and look up that string and return the array used:

string [] Unformat(string res)
{
  string [] arr;
  unFormatLoopup.TryGetValue(res,out arr); //you can also check the return value of TryGetValue and throw an exception if the input string is not in.
  return arr; 
}