Grep using perl - perl

Question

I'm trying to grep multiple patterns from a log file using perl. For the first pattern i'm getting the desired matching pattern via read only variable($1,$2..). But for the next pattern the read only variable is returning the previous value but not the value matching the second pattern.
here is the code:
$tmp = `grep "solo_video_channel_.*(0): queueing" $log`;
chomp($tmp);
$tmp =~ m/(.*):.*solo_video_channel_write(.*): queueing page (.*).*/;
$chnl = $2;
$page = $3;
$timestamp = $1;
$tmp1 = `grep "(0): DUMP GO" $log`;
chomp($tmp1);
$tmp1 =~ m/(.*): solo_video_channel_write(0): DUMP GO/;
$dmp = $1;
print "dump go time = $1\n";
tmp1's value after grep is coming as expected. but $1 value remains same as the previous one.
Any suggestions?

Answer 1

Always make sure that you verify that a regex matched before using a captured variable.
Additionally, there is no reason to shell out to grep. Use Perl's file processing instead:
use strict;
use warnings;
local #ARGV = $log;
while (<>) {
chomp;
if (/solo_video_channel_.*\(0\): queueing/) {
if ( my ( $timestamp, $chnl, $page ) = m/(.*):.*solo_video_channel_write(.*): queueing page (.*).*/ ) {
print "$. - $timestamp, $chnl, $page\n";
}
}
if ( my ($dmp) = m/(.*): solo_video_channel_write\(0\): DUMP GO/ ) {
print "dump go time = $dmp\n";
}
}
Note, your first set of if's could almost certainly be combined into a single if statement, but I left it as is for now.

Answer 2

Why not use Pure Perl? It's faster than running external greps. Plus, you can grep both regular expressions at once. Faster than looping through the file twice.
Always check the value of your rexp match. Here I'm using if statements to do this. Note too that I am printing all lines that don't match with UNMATCHED LINES. You can remove the else when you see that everything is working, or simply redirect 2> /dev/null.
use strict;
use warnings;
use autodie;
use feature qw(say);
my $log = "log.txt";
open my $log_fh, "<", $log;
while ( my $line = <$log_fh> ) {
my $timestamp;
my $channel;
my $page;
my $gotime;
if ( $line =~ /(.*):.*solo_video_channel_(.*):\s+queueing page (.*)/ ) {
$timestamp = $1;
$channel = $2;
$page = $3;
say qq(Timestamp = "$timestamp" Channel = "$channel" Page = "$page");
}
elsif ( $line =~ /(.*): solo_video_channel_write(0): DUMP GO/ ) {
$gotime = $1;
say "Dump Go Time = $1";
}
else {
say STDERR qq(UNMATCHED LINES: "$line");
}
}
close $log_fh;

Answer 3

In the second regexp you need to escape the literal brackets
$tmp1 =~ m/(.*): solo_video_channel_write\(0\): DUMP GO/
This is because the expression \(0\) matches the exact pattern (0)
In the example given in this answer this would include strings such as
37: solo_video_channel_write(0): DUMP GO
In contrast, the expression (0) matches the exact pattern 0 and sets a capture group.
With the regexp given in your original question
$tmp1 =~ m/(.*): solo_video_channel_write(0): DUMP GO/;
matching would occur on strings such as
37: solo_video_channel_write0: DUMP GO
Of course in the original program the strings are not in this format, so they do not match and $1 is not set
The regular expression syntax for the shell program grep is (confusingly) different
To use round brackets for setting a capture group they must be escaped with a backslash, which is the opposite to the syntax in perl