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I'm just learning perl and I'm trying to learn regular expressions at the same time. Basically I'm trying to open a log file and print out any lines that match user input to a new file. Using the following code I get no output at all if I type in the word "Clinton". But if I replace

print MYFILE if /\$string\;

with

print MYFILE if /\Clinton\;

it runs as expected. Any ideas? I know it is something simple that I am missing.

print "Enter a word to look up: ";
$string = <>;
print "You put $string";
open(LOG,"u_ex121011.log") or die "Unable to open logfile:$!\n";
open (MYFILE, '>>data2.txt');
while(<LOG>){   
   print MYFILE if /\Q($string)\E/;
}
close (MYFILE); 
close(LOG);
print "Check data2.txt";
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3 Answers 3

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4

In Perl, unlike in some languages, the input operator doesn't silently remove a trailing newline. So your $string is actually "Clinton\n" rather than than "Clinton". To fix it, use the chomp function:

$string = <>;
chomp $string;
print "You put $string\n";
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6 Comments

Ah perfect. I combined all 3 answers and it works now. Thanks guys!
You're welcome! But for the record, squiguy's answer is wrong: the $_ =~ is optional. (I mean, (s)he wrote "should", which I guess makes it a matter of opinion, but personally I disagree.) And I think you should keep the \Q and \E: as (s)he says, you only need it if there's a chance the user will input special characters -- which is always. So you should keep it.
Exactly why is the $_ optional? Sorry I'm still new to perl.
I have the following now and it's working pretty well. Any revisions that I should make before I move forwards? print "Enter a word to look up: "; $string = <>; chomp $string; print "You put $string"; open(LOG,"u_ex121011.log") or die "Unable to open logfile:$!\n"; open (MYFILE, '>>data_excluding_json.txt'); while(<LOG>){ my @lines = split( ' ', $_ ); if (/\Q$string\E/ && !/\json\b/i){ print MYFILE "$lines[0] $lines[1] $lines[4] $lines[7] \n"; } } close (MYFILE); close(LOG); print "Check data_excluding_json.txt";
Re: "Exactly why is the $_ optional?": It's because $_ is the "default variable"; various operators and functions, such as m// and s/// and chomp and split, operate on it unless they're specifically given something else to operate on instead.
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You should also use the 3 argument version of open.

open( my $LOG, '<', 'u_ex121011.log' ) or die "Unable to open file:$!\n";

open http://perldoc.perl.org/functions/open.html

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-1

In addition to what ruakh said, you should check if the string is on the line by using the $_ variable and the =~ operator.

print MYFILE "$_\n" if $_ =~ /\Q$string\E/;

Going off of your comment, you can split the line up surprisingly enough using split. Here is an example of what you could do:

my @lines = split( ' ', $_ );
print MYFILE "$lines[0] $lines[1] $lines[2] $lines[3]\n";

Here is documentation of split: http://perldoc.perl.org/functions/split.html

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3 Comments

I have another question building on this. My log files have columns separated by a space. I know that I only want everything before the 4th space that occurs and don't want anything else. How would I do that?
-1 for the bad advice to remove the \Q\E modifiers. Since $string contains user input, you should use the \Q\E modifiers to escape any metacharacters unless you purposefully want the user input to be interpreted as a regular expression instead of a string literal.
@SamChoukri Well it's all preference based on what he needs.

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