I've recently been working on an application that will allow members of our content management teams to search within a chosen folder in Tridion for specific content. You might think that's well enough covered by the built-in search functionality, but we're heading towards a search and replace feature, so we pretty much have to process the content ourselves. In the end users' view of the world, a Rich Text field in a component has... well...  a rich text view, and, for the power-users, a Source tab where you can see the underlying HTML. That's all fine, but once you get to the technical implementation, it's a bit more complicated, and we'll end up replicating some of Tridion's own smoke and mirrors to present a view to the users that's consistent with what they are used to. This means not only that we need to be able to translate from text to HTML, but also from "XML in the XHTML namespace" to HTML. One of the bulding blocks we need to do this is the ability to take XML with namespace declarations, and get rid of them so that the result isn't in a namespace. 

A purist (such as myself) might say that the only correct way to parse XML is with an XML parser, and just in case you've never ended up there, I heartily recommend that you read this answer on Stack Exchange before proceding further. Still - in this case, what I want to do is amenable to RegExes, and yes, I know: now I have two problems. Anyway - FWIW - I started this at the office, thinking I'd just quickly Google for a namespace-stripping regex and I'd be on my way. Suffice it to say that the Internet is rubbish at this. I ended up with a page of links to rubbish regexes that just weren't going to float my boat. So I mailed the problem to myself at home, and today, in the quiet of a Sunday morning, it didn't seem quite so daunting. Actually, I'm still considering whether an XML-parser approach, or an XSLT might not be better, and I may end up there if my needs turn out to be more complex, but for now, here's the namespace stripper. 

static Regex namespaceRegex = new Regex(@"    
             xmlns             # literal
             (:[^\s=]+)?       # : followed by one or more non-whitespace, non-equals chars
             \s*               # optional whitespace
             =                 # literal
             \s*               # optional whitespace
             (?<quote>['""])   # Either a single or double quote - giving it the name 'quote' for back-reference
             .+?               # Non-greedily match anything
             \k<quote>         # The end-quote to match the one we found earlier
             ", RegexOptions.Singleline | RegexOptions.IgnoreCase | RegexOptions.IgnorePatternWhitespace);

public static string RemoveNamespacesFromDocument(string xml)
{            
      return namespaceRegex.Replace(xml, string.Empty);
}

Of course, this is written in C#, and I'm taking advantage of the IgnorePatternWhitespace feature in .NET regexes, which allows for the copious comments that might well be necessary if I ever have to actually read this code instead of just writing it. 

But just in case you are hardcore, and all that named matches and commenting fuss is for wusses, here's the TL;DR...

@"(?is)xmlns(:[^\s=]+)?\s*=\s*(['""]).+?\2"

What's not to like? :-)