Replace all matches of regex with manipulation on specific capturing group

Question

I have different Xml strings that can contain one or more parts in the following format:

47862656

The valu

Answer 1

You shouldn't use regex to manipulate XML, they're not the appropriate tool for that and won't always work. For instance, the XML file could use a namespace prefix other than ns1, mapped to the same namespace, and it would be semantically equivalent, but your regex wouldn't work anymore.

You should use an XML parser instead; the easiest to use is Linq to XML:

var doc = XDocument.Parse(Xml);
var ns1 = XNamespace.Get("http://TheNamespaceMappedToTheNs1Prefix");
var elements = doc.Descendants(ns1 + "AcctId");
foreach (var e in elements)
{
    e.Value = IBANHelper.ConvertBBANToIBAN(e.Value); 
}
Xml = doc.ToString();

Answer 2

Apart from the usual don't use regex to manipulate XML.

string regex = "(?<=<ns1:AcctId>).*?(?=</ns1:AcctId>)";
Xml = Regex.Replace(Xml, regex, delegate(Match m) {
                           return IBANHelper.ConvertBBANToIBAN(m.Value);
                         });

This uses positive look ahead and look behind so that the match is just the account number and then the overload to Regex.Replace the takes a match evaluator.