Given an input string containing only alphanumeric ASCII characters and starting with a letter, swap each letter run with the digit run which follows.

A *run* is a sequence of consecutive letters or digits. Note that in the case where the input string ends with a run of letters, this run is left untouched.

**Walk-through example**

For instance, given the input string `uV5Pt3I0`:

 1. Separate runs of letters and runs of digits: `uV 5 Pt 3 I 0`
 2. Identify pairs of runs: `(uV 5) (Pt 3) (I 0)`
 3. Swap pairs of runs: `(5 uV) (3 Pt) (0 I)`
 4. Concatenate: `5uV3Pt0I`

**Examples**

    uV5Pt3I0 -> 5uV3Pt0I
    J0i0m8 -> 0J0i8m
    abc256 -> 256abc
    Hennebont56Fr -> 56HennebontFr
    Em5sA55Ve777Rien -> 5Em55sA777VeRien
    nOoP -> nOoP

This is [tag:code-golf] so the shortest answer in bytes wins. Explanations are encouraged.