How to use sed to find the line and its following line - sed

Question

I would like to get the lines which begin with "Xboy" and its following lines which begins with "+". How can I do this by using sed?
The input looks like below:
Xapple
+apple1
+apple2
.ends
Xboy
+boy1
+boy2
V2
Xcat
+cat1
+cat2
Xcat
The output should look like below:
Xboy
+boy1
+boy2

Answer 1

This will do the job in sed, but really this problem is more complicated than sed is intended for. You'd be better off using perl or python.
$ cat foo.txt
Xapple
+apple1
+apple2
.ends
Xboy
+boy1
+boy2
V2
Xcat
+cat1
+cat2
Xcat
$ sed ':section;/Xboy/!d;:plusline;n;/^+/b plusline;b section' foo.txt
Xboy
+boy1
+boy2
In a proper programming language, the nested loop structure of the data becomes clearer, and we can be more confident there are no edge cases we've forgotten about.
In Perl:
my $line = <>;
while (defined($line)) {
chomp($line);
if ($line eq "Xboy") {
print $line, "\n";
$line = <>;
while (defined($line) && $line =~ /^\+/) {
print $line;
$line = <>;
}
}
else {
$line = <>;
}
}
In Python:
import fileinput
lines = fileinput.input()
line = lines.readline()
while line != '':
line = line.rstrip('\n')
if line == 'Xboy':
print(line)
line = lines.readline()
while line != '' and line.startswith('+'):
print(line, end='')
line = lines.readline()
else:
line = lines.readline()

Answer 2

An awk version
awk '/Xboy/ {f=1;print;next} {/^+/?a=1:f=0} a&&f' file
Xboy
+boy1
+boy2

Answer 3

This might work for you (GNU sed):
sed -n ':a;/Xboy/{:b;p;n;/^+/bb;ba}' file
If a line contains Xboy, print it and any following lines that begin + otherwise be silent.
I guessed this is what you intended, however you may have meant that other lines beginning with non-word-like characters should also be ignored, use:
sed -n ':a;/Xboy/{:b;p;:c;n;/^+/bb;/^\W/bc;ba}' file
or perhaps you meant this:
sed -n ':a;/Xboy/{:b;p;:c;n;/^+/bb;/^[^[:upper:]]/bc;ba}' file
It may be that you only want to print Xboy if there is a line following that begins +, then use:
sed -n ':a;/Xboy/{$d;h;:b;n;/^+/{H;$!bb};x;/\n/p;x;ba}' file