Regex is a small programming language. The language's intent is to define patterns of text. To that end, it has several ways to define "character classes", which is basically "more than one character that matches a certain category".

\b stands for "word boundary". What it actually means is pretty complex, but for the most part it matches "the space in between words".

So a pattern for the word "Hello" that would match "Hello World" but not "HelloWorld" would look like:
Code:
Hello\b
If you use that pattern with Replace(), you will also replace the boundary. That is to say, "foo Hello World" would become "foo World" instead of "foo World" with two spaces. Maybe you want that, maybe you don't, it'll be more complicated if that's not what you want. My hunch is you want it.