javascript regex to replace space with a hyphen

I need a regex which should read a string :

1. with a 1 to unlimited numbers (say upto 10)


2. read a space in between


3. with a 1 to unliimited numbers (say upto 10)

then replace the space in the string with a '-'

for ex: 123 456 should be replaced with 123-456.

there are no any other characters apart from numbers in the string.