String Manipulation Using RegEx

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:

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

Format:

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

Examples:

All variable text filled in.

XYZ Company - Phone: (404) 321-4321 Ext: 6789

No Extension entered, remove "Ext:".
```
XYZ Company - Phone: (404) 321-4321
```
No Extension and no area code entered, remove "Ext:" and "( ) ".
```
XYZ Company - Phone: 321-4321
```
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();
    }

Do I get that right... you search for {...} if it's empty or contains only whitespace, you want to remove the "parent-level" [...] enclosing the matched {...}? Also, if area code and extension are entered, but the main phone number is omitted, you still want to remove everything, I suppose? — Martin Ender
– Martin Ender, Commented Nov 7, 2012 at 22:38
Yes, that is correct. Any text is outside of [...] it is left alone. — Craig_Tucker
– Craig_Tucker, Commented Nov 7, 2012 at 22:46

Martin Ender · Accepted Answer · 2012-11-10 00:34:58Z

2

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:

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

edited Nov 10, 2012 at 0:34

answered Nov 7, 2012 at 22:49

Martin Ender

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4 Comments

Craig_Tucker Over a year ago

Wow that was fast and I think is what I was looking for!

ridgerunner Over a year ago

Although the closing literal brackets/braces don't need to be escaped (and you haven't), for clarity it is best practice to go ahead and escape them anyway (IMHO). Text is cheap and readability should always be a top priority. Also, why not just write the regex out once in free-spacing mode with comments built-in to start off with and be done with it-> self-documentation!. Once again, nice explanation. +1

Martin Ender Over a year ago

@ridgerunner see, I leave the escaping out where possible exactly for the reason of readability :D. I guess it's a matter of taste then. the freespacing thing... yup, that is something I need to get used to for these answers (when using or writing regexes for myself, I prefer them to be oneliners, because I can usually grasp their meaning faster than by reading long explanations)

ridgerunner Over a year ago

Yeah - no need to comment the short ones. But for anything non-trivial, commenting is essential. After many years writing assembly language I developed the habit of commenting every instruction (regex is low level too). But there is also an art to commenting - no need to state the obvious and one must keep things concise. This page shows the commenting style I've developed for complex regexes. Indentation allows visually seeing bracket matching and alternatives for deeply nested structures (YMMV). Cheers.

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