According to the grep manual:  * The preceding item will be matched zero or more times.  + The preceding item will be matched one or more times.Let's test itecho 'agb' | grep 'a.*b' # returns agbecho 'agb' | grep 'a.+b' # returns nothingWhy + did not match three gs? According to my knowledge 3 is more than 1.
grep + operator
grep;regular expression
Whichever grep manual you're reading should also have a section explaining the different kinds of regular expressions (basic and extended). the + operator is not available in basic, only extended. To use extended regular expressions, you need the -E option.