Q: grep reg expression

I'm trying to "grep" the 2.21 MiB value from the 3g-3G interface.
Anyone here who might know how a reg expression for this looks like?

Code:

  rx  /  tx  /  total  /  estimated
 3g-3G:
  Oct '15  1.85 MiB  /  369 KiB  /  1.35 MiB  /  0 KiB
  today  1.85 MiB  /  369 KiB  /  2.21 MiB  /  --
 eth0:
  Oct '15  7.51 MiB  /  508 KiB  /  8.01 MiB  /  0 KiB
  today  7.51 MiB  /  508 KiB  /  8.01 MiB  /  --
 br-lan:
  Oct '15  913 KiB  /  9.62 MiB  /  10.51 MiB  /  0 KiB
  today  913 KiB  /  9.62 MiB  /  10.51 MiB  /  --

 

grep -Poz '3g-3G:[^:/]+/[^:/]+/[^:/]+/[^:/]+/[^:/]+/\s*\K[^:/]+'

 

Thank you..
I've really got to learn this regular expression stuff!