Not sure what to tell you then. I tested a similar regex against a config on one of my fortinet boxes. The actual regex I used was:
^\s*set admintimeout 480
Worked just fine as shown above. If I tacked a $ at the end it didn't work, but putting a ".*$" at the end did, so there might be odd characters at the end of the config line.
I tested it both for "String is found" and "String is NOT found", and it worked properly with each variation... Maybe you have this set wrong, have you checked these settings?
As I expected, simple "^set admintimeout 480" did not work.
So, if you're doing something similar and its not working, I'd suggest opening a case with support...
mrxinu comments on turning off regex search and just simply searching for the string worked for me also BTW...