regular expressions: match x times OR y times

^(\d{3}|\d{6})$

You have to have some sort of terminator otherwise \d{3} will match 1234. That's why I put ^ and $ above. One alternative is to use lookarounds:

(?<!\d)(\d{3}|\d{6})(?!\d)

to make sure it's not preceded by or followed by a digit (in this case). More in Lookahead and Lookbehind Zero-Width Assertions.

How about:

(\d\d\d){1,2}

although you'll also need guards at either end which depend on your RE engine, something like:

[^\d](\d\d\d){1,2}[^\d]

or:

^(\d\d\d){1,2}$

For this case we can get away with this crafty method:

Clean Implementation

/(\d{3}){1,2}/
/(?:\d{3}){1,2}/

How?!

This works because we're looking for multiples of three that are consecutive in this case.

Note: There's no reason to capture the group for this case so I add the ?: non capture group flag to the capture group.

This is similar to paxdiablo implementation, but slightly cleaner.

Matching Hex

I was doing something similar for matching on basic hex colors since they could be 3 or 6 in length. This allowed me to keep my hex color checker's matching DRY'd up ie:

/^0x(?:[\da-f]{3}){1,2}$/i