Question

I have a regular expression which uses GroupCollections in it’s capture to capture a group of Item Id’s (which can be comma separated, also accounting for the final one to have the word ‘and’):

(\bItem #(?<ITEMID>\d+))|(,\s?(?<ITEMID>\d+))|(,?\sand\s(?<ITEMID>\d+))

Is there an easy way using C#’s Regex class to replace the ITEMID numbers with a url? Right now, I have the following:

foreach (Match match in matches)
{
    var group = match.Groups["ITEMID"];
    var address = String.Format(UnformattedAddress, group.Value);

    CustomReplace(ref myString, group.Value, address,
        group.Index, (group.Index + group.Length));
}

public static int CustomReplace(ref string source, string org, string replace,
    int start, int max)
{
    if (start < 0) throw new System.ArgumentOutOfRangeException("start");
    if (max <= 0) return 0;

    start = source.IndexOf(org, start);

    if (start < 0) return 0;

    var sb = new StringBuilder(source, 0, start, source.Length);

    var found = 0;
    while (max-- > 0)
    {
        var index = source.IndexOf(org, start);

        if (index < 0) break;

        sb.Append(source, start, index - start).Append(replace);
        start = index + org.Length;
        found++;
    }

    sb.Append(source, start, source.Length - start);
    source = sb.ToString();

    return found;
}

The CustomReplace method I found online as an easy way to replace one string with another inside of a string source. The problem is I’m sure that there is probably an easier way, probably using the Regex class to replace the GroupCollections as necessary. I just can’t figure out what that is. Thanks!

Example text:

Hello the items you are looking for are Item #25, 38, and 45. They total 100 dollars.

25, 38, and 45 should be replaced with the URL strings I am creating (this is an HTML string).

Answer 1

Your pattern works for your input, but it does have a bug. Specifically, it will match any number in your input that appears after a comma or the word ” and “.

I went ahead and rewrote your pattern to avoid this issue. To achieve this I am actually using two regex patterns. It’s possible to pull this off using one pattern, but it’s fairly complicated and less readable than the approach I opted to share.

The main pattern is: \bItem #\d+(?:,? \d+)*(?:,? and \d+)?
No capturing groups are used here since I am only interested in matching the items. The (?: ... ) bit is a non-capturing group. The usage of (?:,? \d+)* is to match more than one comma separated value in the middle portion of the string.

Once items are matched, I use Regex.Replace to format the items, then reconstruct the string to swap out the original items with the formatted items.

Here’s an example with a couple of different inputs:

string[] inputs =
{
    "Hello the items you are looking for are Item #25, 38, 22, and 45. They total 100 dollars.",
    "... Item #25, 38 and 45. Other numbers 100, 20, and 30 untouched.",
    "Item #25, and 45",
    "Item #25 and 45",
    "Item #25"
};

string pattern = @"\bItem #\d+(?:,? \d+)*(?:,? and \d+)?";
string digitPattern = @"(\d+)";
// $1 refers to the first (and only) group in digitPattern
string replacement = @"<a href=""http://url/$1.html"">$1</a>";

foreach (var input in inputs)
{
    Match m = Regex.Match(input, pattern);
    string formatted = Regex.Replace(m.Value, digitPattern, replacement);
    var builder = new StringBuilder(input)
                        .Remove(m.Index, m.Length)
                        .Insert(m.Index, formatted);
    Console.WriteLine(builder.ToString());
}

In case you need to use an existing method to format the URL, instead of using a regex replacement pattern, you could use the Regex.Replace overload that accepts a MatchEvaluator. This can be achieved using a lambda and is nicer than the tedious approach shown in the MSDN documentation.

For example, let’s assume you have a FormatItem method that accepts a string and returns a formatted string:

public string FormatItem(string item)
{
    return String.Format("-- {0} --", item);
}

To use FormatItem you would change the Regex.Replace method used in the earlier code sample with the following:

string formatted = Regex.Replace(m.Value, digitPattern,
                       d => FormatItem(d.Value));