PCRE is installed on pretty much all server and desktop Linux systems, but you can't necessarily expect it on lightweight systems or embedded systems (phones, routers, TVs, and other IoT), as they often have very trimmed versions of the standard userland (basically, anything with a busybox base is all but guaranteed to lack PCRE).
Debian has a Popularity Contest feature that measures installation metrics on various packages.
grep (25th most common, 176k installs) depends (not optionally) on
libpcre3 (94th most common, 174k installs). I cannot explain the discrepancy, but I also wouldn't worry about it.
It should be safe to assume that modern desktop and servers running full Linux distributions will have versions of
grep compiled with PCRE support.
Still, if you want PCRE with a better assurance of portability, don't rely on
grep -P or
pcregrep (9363th at 1k installs) or
ack (21728th at 180 installs), use
perl (88th at 175k installs) directly:
perl -ne 'print if /regexp/'
Note, there are servers that intentionally lack perl, python, and php for "security purposes," namely that many rogue scripts (e.g. rootkits) depend on these and therefore cannot run. This is very rare (and kind of silly since there are plenty of potent rogue POSIX shell scripts).
Note 2: Perl is slow (as is python). LibPCRE is much faster, but the simpler your regexps, the better performance you'll get. If possible, use
grep alone (BRE, basic regexps) or else try
grep -E (ERE, extended regexps) rather than diving deeper into PCRE land.