REGEX for n characters or at least m characters in Unix

{m}	Match the prceeding characters exactly m times
Examples	Correct	Incorrect
A{3}	AAA	A
		AA
		AAAA
		AAAAA
{m,}	Match the prceeding characters atleast m times
Examples	Correct	Incorrect
A{3,}	AAA	A
	AAAA	AA
	AAAAA
{min,max}	Match the prceeding characters between min and max times
Examples	Correct	Incorrect
A{3,5}	AAA	A
	AAAA	AA
	AAAAA	AAAAAA
		AAAAAAA

Examples :

File Name : numbers.dat

12
12345
123456
12345678
10.1
.1

grep '^[0-9]\{6\} numbers.dat # line should be start between number 0 to 9 and length of the digit is exactly 6

grep '^[0-9]\{4,6\} numbers.dat# line should be start between number 0 to 9 and length of the digit is between 4 and 6

grep '^[0-9]\{3,\} numbers.dat # line should be start between number 0 to 9 and length of the digit is minimum 3

grep '^\.[0-9]' numbers.dat # line should be start with .(dot) and followed by digits