I'm working on a web-based help system that will auto-insert links into the explanatory text, taking users to other topics in help. I have hundreds of terms that should be linked, i.e.

"Manuals and labels" (describes these concepts in general)
"Delete Manuals and Labels" (describes this specific action)
"Learn more about adding manuals and labels" (again, more specific action)

I have a RegEx to find / replace whole words (good ol' \b), which works great, except for linked terms found inside other linked terms. Instead of:

```
<a href="#">Learn more about manuals and labels</a>

```

I end up with 

```
<a href="#">Learn more about <a href="#">manuals and labels</a></a>

```

Which makes everyone cry a little. Changing the order in which the terms are replaced (going shortest to longest) means that I''d get:

```
Learn more about <a href="#">manuals and labels</a>

```

Without the outer link I really need.

The further complication is that the capitalization of the search terms can vary, and I need to retain the original capitalization. If I could do something like this, I'd be all set:

```
Regex _regex = new Regex("\\b" + termToFind + "(|s)" + "\\b", RegexOptions.IgnoreCase);

string resultingText = _regex.Replace(textThatNeedsLinksInserted, "<a>" + "$&".Replace(" ", "_") + "</a>));

```

And then after all the terms are done, remove the "\_", that would be perfect. "Learn\_more\_about\_manuals\_and\_labels" wouldn't match "manuals and labels," and all is well.

It would be hard to have the help authors delimit the terms that need to be replaced when writing the text -- they're not used to coding. Also, this would limit the flexibility to add new terms later, since we'd have to go back and add delimiters to all the previously written text.

Is there a RegEx that would let me replace whitespace with "\_" in the original match? Or is there a different solution that's eluding me?