Use pipe of commands as argument for diff

If you’re using bash:

diff file <(grep -E '^[0-9]+$' file)

The <(COMMAND) sequence expands to the name of a pseudo-file (such as /dev/fd/63) from which you can read the output of the command.

But for this particular case, ruakh’s solution is simpler. It takes advantage of the fact that - as an argument to diff causes it to read its standard input. The <(COMMAND) syntax becomes more useful when both arguments to diff are command output, such as:

diff <(this_command) <(that_command)