In what follows, file "conjectures" is a 6 billion bytes file in which each
line contains at most one letter P, and few (see output) have a digit
following a P. "rusage" is just a home-brew resource usage summary command.

  rusage egrep 'P[0-9]' conjectures > xxx
695.55 real 688.33 user 2.40 sys 0 pf 186 pr 0 sw 0 rb 8 wb 1 vcx 19206 icx
2488 mx 0 ix 0 id 0 is

  cat xxx
A[21]=11{11}:22<LP3

  rusage egrep 'P[[:digit:]]' conjectures > xxx
14.88 real 13.36 user 1.43 sys 0 pf 186 pr 0 sw 0 rb 8 wb 0 vcx 516 icx
2500 mx 0 ix 0 id 0 is

  cat xxx
A[21]=11{11}:22<LP3

Using what is to me the more obvious [0-9] pattern takes almost 50 times as
long as using the [[:digit:]] pattern. Seems very strange.

  grep --version
grep (GNU grep) 2.25
Copyright (C) 2016 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html
>.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Written by Mike Haertel and others, see <http://git.sv.gnu.org/cgit/
grep.git/tree/AUTHORS>.

  uname -a
Linux jpl 4.4.0-66-generic #87-Ubuntu SMP Fri Mar 3 15:29:05 UTC 2017
x86_64 x86_64 x86_64 GNU/Linux