grep -E -> grep -P

[cepal]

this change makes ptrap work on older distros like RHEL5 where "grep -E" works differently and doesn't provide intended result.

[mvitale1989]

Hi cepal. Thanks for your contribution!
I am not confident in defaulting to -P, as the grep manpage on my Ubuntu 18.04 reports the following:

Perl-compatible regular expressions give additional functionality, and are documented in pcresyntax(3) and pcrepattern(3), but work only if PCRE is available in the system.

A nicer alternative would be making it configurable instead, like having a variable GREP_MATCHER that defaults to -E but can be set to -P on older systems, .e.g. letting you run GREP_MATCHER="-P" ptrap ... instead.