Originally Posted by fanoflq
I looked at man page for grep:
grep [OPTIONS] PATTERN [FILE...]
Then I looks for description of permissible PATTERN.
But there is not such discussion in man grep!
This seems like a recurring problem in man pages.
Where can I get information within Linux?
I am aware I can look online.
But I like to learn how to get such information locally.
Sure there is - look down the page for REGULAR EXPRESSONS, character classes, anchoring, repetition, concatenation, alteration, precedence, etc...
It actually has quite a bit of information on pattern matching (i.e. regular expressions), but in reality that is a much larger subject in its own right.
Admittedly it can take a little time to learn your way around the man pages, but they are surprisingly complete for the most part, although they can require a different reading style than most other things.
One quick way to scan through them is to repetitively search for a word of interest, such as "pattern", by typing /pattern <ENTER>, then /<ENTER> for next, ?<ENTER> for previous. That would lead you to regular expressions if you did not know the term.
Also, at the bottom of the man page is a SEE ALSO section which will list related man pages. In the case of grep there is a see-also for awk, sed, pcre, pcresyntax and regex, all of which have valuable information on regular expression pattern matching.
But, extra points to you for trying to find your way around - time well spent!