grep - Character Classes
$ echo 'abc' | grep '[[:alpha:]]'
abc
$ echo '123' | grep '[[:alpha:]]'
$ echo 'abc123' | grep '[[:alpha:]]'
abc123
$ echo 'abc' | grep '[[:alnum:]]'
abc
$ echo '123' | grep '[[:alnum:]]'
123
$ echo 'abc123' | grep '[[:alnum:]]'
abc123
$ echo 'abc' | grep '[0-9]'
$ echo '123' | grep '[0-9]'
123
$ echo 'abc123' | grep '[0-9]'
abc123
$ echo 'abc123' | grep -w '[0-9]'
$ echo '123' | grep '[0-9]\{2\}'
123
$ echo '123' | grep -w '[0-9]\{2\}'
$ echo '123' | grep '[0-9]\{3\}'
123
$ echo '123' | grep -w '[0-9]\{3\}'
123