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Neo wrote: Considering that the 'world' contains a valid hex digit ('d') would that explain anything?

It matches that line because of the 'd' alright, but not because it's a hexdigit, but because it is contained in [digtx]...

AR wrote: I tried "egrep -o [:xdigit:] a.txt" and got:

x
x
d

So your assumption is probably correct

Hmm... so a hexdigit means anything with an 'x'?

No. What was said is that [:xdigit:] is broken, and grep doesn't understand that it has a special meaning, and therefore it works like a normal character group, which containts :, x, d, i, g, t and a duplicate of i and : which are ignored.

Btw... now that I tried it on localhost, I notice that even GNU grep seems not to understand those, at least not without some magick switch (which I'm too lazy to figure out right now for no good purpose).