To search files in a stream right-click the stream or stream folder and select 'Find In Files'. This will launch the 'Find In Files' window.
From here you can enter the text to search and select whether this is case sensitive. If you select 'Use Regular Expressions' then you can enter a regular expression instead of plain text - but this will only be matched for each individual line.
If necessary you can specify a subfolder of the stream you selected to narrow the search as well as the file extensions to consider. The file extensions drop-down will give you some examples.
On pressing Find All a list of files matching the search will appear and you can stop the search at any time with the Stop Search button.
Double clicking a file will open the editor for this file with the first match selected. You can expand a file to reveal the text for the first 3 lines which match. Double-clicking on these lines will open the editor with the match in that line selected.