#!/usr/bin/perl

use strict;
use warnings;

$/ = undef;

my $string = <DATA>;

while ($string =~ /(            # Capture into $1
                     ^U\d+\.\d  # A 'U' at the start of a line, and 2
                                # dot-separated numbers
                   )
                   \n\n         # 2 newlines
                   (            # Capture into $2
                     (?:
                       [^\n]    # Match anything but a newline
                       |        # or
                       \n(?!\n) # A newline, but only if not followed
                                # by another newline
                     )+
                   )
                   /mgx) {
    print "\$1: <$1>:\n\$2: \n<$2>\n\n";
}

__DATA__
U448.1

Several walls of the imperial city of Constantinople which had been freshly
rebuilt with masonry, and fifty-seven towers collapsed as a result of a
violent earthquake which prevailed in various places.

U483.2

Or, this year the battle of Ocha according to others, won by Muirchertach
Mac Erca and by Fergus Cerball son of Conall of Cremthann, and by Fiachra
Lon son of the king of Dal Araide.