grep - Meta Characters and Character Classes

Meta Characters and the grep Command

The fundamental building blocks are the regular expressions that match a single character. Most characters, including all letters and digits, are regular expressions that match themselves. Any meta-character with special meaning may be quoted by preceding it with a backslash.
A regular expression may be followed by one of several repetition operators, as shown here:
" . " matches any single character.
" ? " indicates that the preceding item is optional and will be matched at most once: Z? matches Z or ZZ.
" * " indicates that the preceding item will be matched zero or more times: Z* matches Z, ZZ, ZZZ, and so forth.

" + " indicates that the preceding item will be matched one or more times: Z+ matches ZZ, ZZZ, and so forth.

" {n} " indicates that the preceding item is matched exactly n times: Z{3} matches ZZZ.
" {n,} " indicates that the preceding item is matched n or more times: Z{3,} matches ZZZ, ZZZZ, and so forth.
" {,m} " indicates that the preceding item is matched at most m times: Z{,3} matches Z, ZZ, and ZZZ.
" {n,m} " indicates that the preceding item is matched at least n times, but not more than m times: Z{2,4} matches ZZ, ZZZ, and ZZZZ.
The empty regular expression matches the empty string (i.e., a line in the input stream with no data). Two regular expressions may be joined by the infix operator (|). When used in this manner, the infix operator behaves exactly like a logical “OR” statement, which directs the grep command to return any line that matches either regular expression.