{n,m} Bounded repeats

Whereas the '+' or '*' characters will repeat the previous section as much, or as little, as required it is possible to specify a specific number of required repeats using the '{' and '}' characters.

 a{n}        matches 'a' repeated exactly n times

 a{n,}        matches 'a' repeated n or more times

 a{n,m}        matches 'a' repeated between n and m times inclusive.

For example,

 [0-9]{4,6}        matches any number between 1000 and 999999