Macro interval 

macro_rules! interval {
    ($n: expr) => { ... };
    ($a: expr, $($b: expr),*) => { ... };
    ($n: expr => $adv: expr) => { ... };
    ($a: expr, $($b: expr),* => $adv: expr) => { ... };
}

Defines a repeating pattern Interval

interval!(4) means every 4 chars

interval!(4, 1) means every 4 chars, then every 1 char

interval!(4 => 3) means every 4 chars, starting from 3.

interval!(4 => -10) means match 10 chars, then every 4 char.

interval!(4, 2 => 1) means every 4 chars, then every 2 chars, starting from 1

Note that unlike most other patterns, this uses [Sep::Yield] by default.