grizzley:

I'm guessing he used [0-9] for visual symmetry with [0-2], [0-3], et. al. I was going to suggest that it would be easier to read, but when I converted a little bit from this:

&&  $part !~ m{    # Time or partial time (or period):
        ^(?:|P)T [012][0-9] (?:| :?[0-5][0-9] (?:| :?[0-5][0-9] ) )
        (?:| [.,][0-9]+ )$
    }x
[download]

to this:

&&  $part !~ m{    # Time or partial time (or period):
        ^(?:|P)T [012]\d (?:| :?[0-5]\d (?:| :?[0-5]\d ) )
        (?:| [.,]\d+ )$
    }x
[download]

I found that the better 'visual balance' of [0-9] was counterbalanced by the square brackets, which are a little too similar to vertical bars for my eyes. After looking at them both, I don't really have a preference--Perhaps if I had a better font...

...roboticus

When your only tool is a hammer, all problems look like your thumb.