Question

Given the following scenario, I am wondering if a better solution could be written with Regular Expressions for which I am not very familiar with yet. I am seeing holes in my basic c# string manipulation even though it somewhat works. Your thoughts and ideas are most appreciated.

Thanks much,

Craig

Given the string “story” below, write a script to do the following:

1. Variable text is enclosed by { }.
2. If the variable text is blank, remove any other text enclosed in [ ].
3. Text to be removed can be nested deep with [ ].

Format:

    XYZ Company [- Phone: [({404}) ]{321-4321} [Ext: {6789}]]

Examples:

1. All variable text filled in.

   XYZ Company - Phone: [phone] Ext: 6789
   

2. No Extension entered, remove “Ext:”.

   XYZ Company - Phone: [phone]
   

3. No Extension and no area code entered, remove “Ext:” and “( ) “.

   XYZ Company - Phone: [phone]
   

4. No extension, no phone number, and no area code, remove “Ext:” and “( ) ” and “- Phone: “.

   XYZ Company
   

Here is my solution with plain string manipulation.

private string StoryManipulation(string theStory)
    {
        // Loop through story while there are still curly brackets
        while (theStory.IndexOf("{") > 0)
        {
            // Extract the first curly text area
            string lcCurlyText = StringUtils.ExtractString(theStory, "{", "}");                

            // Look for surrounding brackets and blank all text between
            if (String.IsNullOrWhiteSpace(lcCurlyText))
            {
                for (int lnCounter = theStory.IndexOf("{"); lnCounter >= 0; lnCounter--)
                {
                    if (theStory.Substring(lnCounter - 1, 1) == "[")
                    {
                        string lcSquareText = StringUtils.ExtractString(theStory.Substring(lnCounter - 1), "[", "]");
                        theStory = StringUtils.ReplaceString(theStory, ("[" + lcSquareText + "]"), "", false);
                        break;
                    }
                }
            }
            else
            {
                // Replace current curly brackets surrounding the text
                theStory = StringUtils.ReplaceString(theStory, ("{" + lcCurlyText + "}"), lcCurlyText, false);
            }
        }
        // Replace all brackets with blank (-1 all instances)
        theStory = StringUtils.ReplaceStringInstance(theStory, "[", "", -1, false);
        theStory = StringUtils.ReplaceStringInstance(theStory, "]", "", -1, false);
        return theStory.Trim();
    }

Answer 1

Dealing with nested structures is generally beyond the scope of regular expressions. But I think there is a solution, if you run the regex replacement in a loop, starting from the inside out. You will need a callback-function though (a MatchEvaluator):

string ReplaceCallback(Match match)
{
    if(String.IsNullOrWhiteSpace(match.Groups[2])
        return "";
    else
        return match.Groups[1]+match.Groups[2]+match.Groups[3];
}

Then you can create the evaluator:

MatchEvaluator evaluator = new MatchEvaluator(ReplaceCallback);

And then you can call this in a loop until the replacement does not change anything any more:

newString = Regex.Replace(
    oldString,
    @"
    \[    # a literal [
    (     # start a capturing group. this is what we access with "match.Groups[1]"
        [^{}[\]]
          # a negated character class, that matches anything except {, }, [ and ]
        * # arbitrarily many of those
    )     # end of the capturing group
    \{    # a literal {
    ([^{}[\]]*)
          # the same thing as before, we will access this with "match.Groups[2]"
    }     # a literal }
    ([^{}[\]]*)
          # "match.Groups[3]"
    ]     # a literal ]
    ",
    evaluator,
    RegexOptions.IgnorePatternWhitespace
);

Here is the whitespace-free version of the regex:

\[([^{}[\]]*)\{([^{}[\]]*)}([^{}[\]]*)]