This will find a “b” that is not followed by “e”:
$ echo "one be two bring brought" | egrep 'b[^e]'
Or if perl is available but egrep is not:
$ echo "one be two bring brought" | perl -ne 'print if /b[^e]/;'
And if you want to find lines with “b” not followed by “e” but no words that contain “be” (using the \w perl metacharacter to catch another character after the b), and avoiding any words that end with b:
$ echo "lab bribe two bring brought" | perl -ne 'print if /b\w/ && ! /be/'
So the final call would:
$ perl -ne 'print if /b\w/ && ! /be/' f3.txt