#!/usr/local/bin/perl  

use strict;
use warnings;

###########################################################
#In this program there is a text always ontaining a title #followed by       #
#a sentence.I wish to check if there is a regexp B in the #actual sentence.  #
#If so, I wish to print the title of this sentence and the #sentence itself  #
#If the element B does not appear in the sentence, I want #both the sentence #
#and its title to be #ignored.                                               #
#############################################################################


my $text = "A This is a title of the first sentence
some text here containing the element B I need and more text here
A This is a title of the second sentence
C some text here but no element I need so it should be ignored
A This is a title for the third sentence
last sentence with the element  B that I'm looking for";

my $expa = "A"; #title element which appears before every #sentence
my $expb = "B"; #the regexp I'm loooking for
my $line;       # the line
my $s;

while($line = $text){
        if($line =~m/$expa/){  #if A is found
        $s = $line;        #then, read the following line
        $line = $text;

         if($line =~m/$expb/){     #if B is found
         print "$s\n\t$line\n\n\n";   #print both lines
}
}
}