Regular expression 0 - number range.

[Scott Sauyet]

The idea is to work recursively with ranges that have both a start and
an end, with a tree structure for, say 267 - 4301, of

    267-4301
    +- 267-999
    |  +- 267-299
    |  |  +- 267-269: 26[7-9]
    |  |  `- 270-299: 2[7-9]\d
    |  `- 300-999: [3-9]\d\d
    `- 1000-4301
       +- 1000-3999: [1-3]\d\d\d
       `- 4000-4301
          +- 4000-4299: 4[0-2]\d\d
          `- 4300-4301: 43[0-1]

That last line should read 4300-4301: 430[0-1].

-- Scott