This is not currently supported, though it has been requested before. If it is important to you, you can vote for the issue here:
Incidentally, if you put the user's @-mention name as part of the search (e.g. "@SteveSweales some search terms") it will limit the results to either 1) messages that actually contain the string "@SteveSweales" or messages from that user. It's not the perfect solution, but does provide a measure of flexibility that might help you out for now.
...from the beginning. We have built up a lot of content in HipChat, with it being a core tool in our distributed company model. While it is true that we didn’t need to move to Slack immediately, we felt it...
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