regex syntax clarification

The following search returns results: "context"

But this one does not: regex "context"

And neither does this: regex _raw="context"

+Why not?+

I am using Splunk 4.3.3 and according to http://docs.splunk.com/Documentation/Splunk/4.3.3/SearchReference/Regex the previous 2 are valid and should return results.

I know that the word "context" is mentioned in my "_raw" field many times because I've exported a number of log events and found it there.