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:
If you want/have to stick to pcregrep, I suggest something like:
pcregrep -v "^[^:]*:[^:]*:-?(1?\d?\d|20[0-4]):" /etc/passwd
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
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