$ grep hello

$ grep hello world
grep: world: No such file or directory

$ grep 'hello world'

$ grep .

$ grep [0123456789]

$ grep [0-9]

$ grep [5-9]

$ grep [9-5]

$ grep [9-9]

$ grep [.]

$ grep [aeiou]

$ grep h[ae]llo

$ grep h[^ae]llo

$ grep h[0-9a-z]llo

$ grep h[^0-9a-z]llo

$ grep [:punct:]

$ grep [[:alpha:]]

$ grep [[:upper:]]

$ grep [[:punct:]]

$ grep [[:space:]]

$ grep '\w'