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Encoding Special Characters Before Processing Invalid XML

I have some invalid XML from a vendor that I need to process. Here is an example:
<a>foo</a>
<b>bar</b>
<c>foobar is < $15</c>
So, we have a few problems. First, there is no root document. I overcome that by adding a root document. No problem. The second, and more difficult problem, is the less than symbol. I can just encode the whole thing but it will encode the XML tags. Is there a library or simple method out there somewhere for handling this? I really don't want to reinvent the wheel as I'm sure hundreds of people have dealt with "quasi-XML" like this. Appreciate any help.

I would read the file line by line and use a regex to get the values between the nodes. Your example doesn't have nested elements so this is pretty easy. While reading line by line you can replace encode the inner values. The named capture group (?.*?) will get everything between the nodes into the group named xml.
var regex = "<.*?>(?<xml>.*?)</.*?>"
var badXML = Regex.Match(line, regex , RegexOptions.IgnoreCase).Groups["xml"].Value;

Regex using Multiline and Groups

Hi guyes just had a quick question about using multi-line in regex:
The Regex:
string content = Regex.Match(onix.Substring(startIndex,endIndex - startIndex), #">(.+)<", RegexOptions.Multiline).Groups[1].Value;
Here is the string of text I am reading:
<Title>
<TitleType>01</TitleType>
<TitleText textcase="02">18th Century Embroidery Techniques</TitleText>
</Title>
Here is what I am getting:
01
What I want is everything between the
<Title> and </Title>.
This works perfectly when everything is on one line but since starts on another line it seems to be skipping it or not including it into the pattern.
Any assistance is much appreciated.

You must also use the Singleline option, along with Multiline:
string content = Regex.Match(onix.Substring(startIndex,endIndex - startIndex), #">(.+)<", RegexOptions.Multiline | RegexOptions.Singleline).Groups[1].Value;
But do yourself a favor and stop parsing XML using Regular Expressions! Use an XML parser instead!
You can parse the XML text using the XmlDocument class, and use XPath selectors to get to the element you're interested in:
XmlDocument doc = new XmlDocument();
doc.LoadXml(...); // your load the Xml text
XmlNode root = doc.SelectSingleNode("Title"); // this selects the <Title>..</Title> element
// modify the selector depending on your outer XML
Console.WriteLine(root.InnerXml); // displays the contents of the selected node

RegexOptions.Multiline will just change the meaning of ^ and $ to beginning/end of lines instead of beginning/end of the entire string.
You want to use RegexOptions.Singleline instead, which will result in . match line breaks (as well as everything else).

You might want to parse what is probably XML instead. If possible this is the preferred way of working instead of parsing it by employing regular expressions. Please disregard if not applicable.

Find and replace text inside xml document using regular expression

I am using c# console app to get xml document. Now once xmldocument is loaded i want to search for specific href tag:
href="/abc/def
inside the xml document.
once that node is found i want to strip tag completly and just show Hello.
Hello
I think i can simply get the tag using regex. But can anyone please tell me how can i remove the href tag completly using regex?

xml & html same difference: tagged content. xml is stricter in it's formatting.
for this use case I would use transformations and xpath queries rebuild the document. As #Yahia stated, regex on tagged documents is typically a bad idea. the regex for parsing is far to complex to be affective as a generic solution.

The most popular technology for similar tasks is called XPath. (It is also a key component of XQuery and XSLT.) Would the following perhaps solve your task, too?
root.SelectSingleNode("//a[#href='/abc/def']").InnerText = "Hello";

You could try
string x = #"<?xml version='1.0'?>
<EXAMPLE>
<a href='/abc/def'>Hello</a>
</EXAMPLE>";
System.Xml.XmlDocument doc = new XmlDocument();
doc.LoadXml(x);
XmlNode n = doc.SelectSingleNode("//a[#href='/abc/def']");
XmlNode p = n.ParentNode;
p.RemoveChild(n);
System.Xml.XmlNode newNode = doc.CreateNode("element", "a", "");
newNode.InnerXml = "Hello";
p.AppendChild(newNode);
Not really sure if this is what you are trying to do but it should be enough to get you headed in right direction.

extract a value from a string using regex

I'm trying to extract a value from a string using regex. The string looks like this:
<faultcode><![CDATA[900015The new password is not long enough. PasswordMinimumLength is 6.]]></faultcode>
I am trying to diplay only the error message to end user.

Since you probably want everything <![CDATA[ and ]]> this should fit:
<!\[CDATA\[(.+?)\]\]>

The only sensible thing is to load it into an XElement (or XDocument, XmlDocument) and extract the Value from the CDATA element.
XElement e = XElement.Parse(xmlSnippet);
string rawMsg = (e.FirstNode as XCData).Value;
string msg = rawMsg.Substring("900015".Length);

First, and foremost, using regex to parse XML / HTML is bad.
Now, by error message I assume you mean the text, not including the numbers. An expression like so would probably do the trick:
\<([^>]+)\><!\[CDATA\[\d*(.*)\]\]>\</\1\>
The error message will be in the second group. This will work with the sample that you have given, but I'd sooner use XDocument or XmlDocument to parse it. If you are using C#, there really isn't a good reason to not use either of those classes.

Updated to correspond with the question edit:
var xml = XElement.Parse(yourString);
var allText = xml.Value;
var stripLeadingNumbers = Regex.Match(xml.Value, #"^\d*(.*)").Groups[1].Value;

replacing substring inside attributes of XmlDocument

I'm using C# with .net 3.5 and have a few cases where I want to replace some substrings in the XML attributes of an XmlDocument with something else.
One case is to replace the single quote character with ' and the other is to clean up some files that contain valid XML but the attributes' values are no longer appropriate (say replace anything attribute which starts with "myMachine" with "newMachine").
Is there a simple way to do this, or do I need to go through each attribute of every node (recursively)?

One way to approach it is to select a list of the correct elements using Linq to XML, and then iterate over that list. Here's an example one-liner:
XDocument doc = XDocument.Load(path);
doc.XPathSelectElements("//element[#attribute-name = 'myMachine']").ToList().ForEach(x => x.SetAttributeValue("attribute-name", "newMachine"));
You could also do a more traditional iteration.

I suggest taking a look at LINQ to XML. There's a collection of code snippets that can help you get started here - LINQ To XML Tutorials with Examples
LINQ to XML should allow you to do what you're looking to do, and you'll probably find it easy once you've played with it a bit.