...
[0-9] = a char in range 0-9 (inclusive)
{5} = repeat the prior criteria 5 times
Example #3: Validating that a kit ID is either 6 digits or 6 digits with a space in the middle
(e.g., 012 293 or 012293):
[0-9]{3} ?[0-9]{3}
[0-9] = a char in range 0-9 (inclusive)
{3} = repeat the prior criteria 3 times
“ ?” = 0 or 1 spaces (? = match 0 or 1 of the preceding character, which in this case is a blank space)