Unless I am mistaken, this /@font-face\s*{[}]+}/g regular expression will capture all @fontface declaration.
We capture all string beginning with @font-face, followed with : zero or more () space or tab...(\s), one opening curly brace (which is to escape with a backslash), one or more (the + sign after the closing square bracket) non () closing curly brace characters and one closing curly brace. The g for general will make all capture and not only the first.
A \s at the end of the regular expression (just before the backslash) could capture all spaces, tabulations, carriage returns or line feeds at the end of the pattern.