reg exp question

[Stuart Clark]

How do I match a pattern that starts with a 4 and has 16 numbers in it.
 
I try /^4\d{16}/;
 
 
Also  how do I match a 16 digit number that starts with either 6565 or
starts with a number in the range of  555000-555100
 
 
/(^6565(\d{16})|^{555000-555100}(\d{16}))/