There is an experimental feature (that I haven't used yet), Extended Bracketed Character Classes, that should allow you to define your own classes (or more specifically, compose a character class out of others, which you can also interpolate into a regex).

However, /\b(?:[[:digit:]]{1,3}\.){3}[[:digit:]]{1,3}\b/ is not a character class (emphasis mine):

A character class is a way of denoting a set of characters in such a way that one character of the set is matched. It's important to remember that: matching a character class consumes exactly one character in the source string.

What you've got is a whole regex, and I think that LanX is right that simply compiling that into a variable with qr and interpolating that into another regex is the best way to go. Later in that thread you asked if you could isolate the variable, and the usual ${} can be applied here too, as well as the /x modifier:

my $x = qr/foo/;
my $y = qr/${x}bar/;    # (?^:(?^:foo)bar)
my $z = qr/ $x bar /x;  # (?^x: (?^:foo) bar )

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Regarding the example you've shown, note there is $RE{net}{IPv4} from Regexp::Common::net.

Minor edits for clarification.

In reply to Re: Defining your own regex character class by haukex
in thread Defining your own regex character class by igoryonya

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