I think, you could have a look at the official perldoc to get information about Perl's regular expressions:

perlretut - Tutorial

perlrequick - Quick Reference

perlre - Regex documentation

And not to forget the pcre man pages:

http://www.pcre.org/pcre.txt

If you want/have to stick to pcregrep, I suggest something like:

pcregrep -v  "^[^:]*:[^:]*:-?(1?\d?\d|20[0-4]):" /etc/passwd
[download]

It (simplified) assumes/uses:

• the pattern is anchored at beginning of line (^)
• "login name" field must not contain a colon
• "password" field must not contain a colon
• "UID" field:
  • leading optional "-"
  • grouped alternatives:
    • number between 0 and 199 OR
    • number between 200 and 204
• don't care about the rest of the line