# Perl 5
my $quad = qr/(?: 25[0-5] | 2[0-4]\d | [0-1]??\d{1,2} )/x;

$str =~ m/ $quad \. $quad \. $quad \. $quad /x;

# Perl 6
rule quad {  (\d<1,3>) :: { fail unless $1 < 256 }  }

$str =~ m/ <quad> <dot> <quad> <dot> <quad> <dot> <quad> /x;

# Perl 6 (same great approach, now less syntax)
rule quad {  (\d<1,3>) :: <($1 < 256)>  }

$str =~ m/ <quad> <dot> <quad> <dot> <quad> <dot> <quad> /x;