parse multiple lines in perl regular expression and extract value - regex

Question

I am a beginner in perl. I have a text file with text similar to as below. i need to extract VALUE="<NEEDED VALUE>". Say for SPINACH, i should be getting SALAD alone.
How to use perl regex to get the value. i need to parse multiple lines to get it. ie between each #ifonly --- #endifonly
$cat check.txt
while (<$file>)
{
if (m/#ifonly .+ SPINACH .+ VALUE=(")([\w]*)(") .+ #endifonly/g)
{
my $chosen = $2;
}
}
#ifonly APPLE CARROT SPINACH
VALUE="SALAD" REQUIRED="yes"
QW RETEWRT OIOUR
#endifonly
#ifonly APPLE MANGO ORANGE CARROT
VALUE="JUICE" REQUIRED="yes"
as df fg
#endifonly

Answer 1

use strict;
use warnings;
use 5.010;
while (<DATA>) {
my $rc = /#ifonly .+ SPINACH/ .. (my ($value) = /VALUE="([^"]*)"/);
next unless $rc =~ /E0$/;
say $value;
}
__DATA__
#ifonly APPLE CARROT SPINACH
VALUE="SALAD" REQUIRED="yes"
QW RETEWRT OIOUR
#endifonly
#ifonly APPLE MANGO ORANGE CARROT
VALUE="JUICE" REQUIRED="yes"
as df fg
#endifonly
This uses a small trick described by brian d foy here. As the link describes, it uses the scalar range operator / flipflop.

Answer 2

In case your file is very big (or you want to read it line by line for some other reason) you could do it as follows:
#!/usr/bin/perl
use strict;
use warnings;
use Getopt::Long;
my ($file, $keyword);
# now get command line options (see Usage note below)
GetOptions(
"f=s" => \$file,
"k=s" => \$keyword,
);
# if either the file or the keyword has not been provided, display a
# help text and exit
if (! $file || ! $keyword) {
print STDERR<<EOF;
Usage: script.pl -f filename -k keyword
EOF
exit(1);
}
my $found; # indicator that the keyword has been found
my $returned_word; # will store the word you want to retrieve
open FILE, "<$file" or die "Cannot open file '$file': $!";
while (<FILE>) {
if (/$keyword/) {
$found = 1;
}
# the following condition will be true between all lines that
# start with '#ifonly' or '#endifonly' - but only if the keyword
# has been found!
if (/^#ifonly/ .. /^#endifonly/ && $found) {
if (/VALUE="(\w+)"/) {
$returned_word = $1;
print "looking for $keyword --> found $returned_word\n";
last; # if you want to get ALL values after the keyword
# remove the 'last' statement, as it makes the script
# exit the while loop
}
}
}
close FILE;

Answer 3

You can read the file contents in a string and then search for the pattern in the string:
my $file;
$file.=$_ while(<>);
if($file =~ /#ifonly.+?\bSPINACH\b.+?VALUE="(\w*)".+?#endifonly/s) {
print $1;
}
Your original regex needs some tweaking:
You need to make your quantifiers
non-greedy.
Use the s modifier to make .
match newline as-well.
Ideone Link

Answer 4

Here's another answer based on the flip-flop operator:
use strict;
use warnings;
use 5.010;
while (<$file>)
{
if ( (/^#ifonly.*\bSPINACH\b/ .. /^#endifonly/) &&
(my ($chosen) = /^VALUE="(\w+)"/) )
{
say $chosen;
}
}
This solution applies the second test to all of the lines in the range. The trick #Hugmeir used to exclude the start and end lines isn't needed because the "inner" regex, /^VALUE="(\w+)"/, can never match them anyway (I added the ^ anchor to all regexes to make doubly sure of that).

Answer 5

These two lines in one answer given two days ago
my $file;
$file.=$_ while(<>);
are not very efficient. Perl will likely read the file in big chunks, break those chunks into lines of text for the <> and then the .= will join those lines back to make a big string. It would be more efficient to slurp the file. The basic style is to alter \$ the input record separator.
undef $/;
$file = <>;
The module File::Slurp; (see perldoc File::Slurp) may be even better.