How to use REGEX Match Variable from one 'While' String in Another 'While' String

GoldfishOfDoom has asked for the wisdom of the Perl Monks concerning the following question:

Dear PerlMonks,

I am completely new (apologies in advance for any politically incorrect terminology) to PERL and am trying to write a script that searches a file for a pattern then generates a new file based on a specific match from each pattern.

Using the match from the first file, I need to search a second file for a pattern that contains that match; however, since there are two files and two while statements, the variable that I generated using the match in the first while statement must stay in its own loop. Is there anyway to use the first REGEX match to generate the second REGEX statement?

Here's a sample of the code. I'm sure there are a ton of ways to clean this up, but at the moment, my biggest concern is finding a way to use the match from the first while's REGEX in the second while and REGEX. Any help is greatly appreciated. I've only been working with PERL for two days (with no previous programming experience) so this is all relatively new to me.

use warnings;
use strict;


# Variables
my $pkt_file = $ARGV[0]; 
my $dat_file = $ARGV[1]; 

# Read data file
open (IN_FILE, "<", $pkt_file) or die "ERROR Opening $pkt_file (Error 
+= $!)\n";
 print "Opening: $pkt_file for read...\n";

# Search IN_FILE for Packet Headers
while (<IN_FILE>) {
    my $line_in = $_;
      if (($line_in =~ /(\w+) (\w+)/)) {
          open (OUT_FILE, ">>", "$2_tlmval.txt") or die "ERROR Opening
+ file (ERROR = $!)\n";
          print "Opening $2_tlmval.txt\n";
          print OUT_FILE " \n";
          print OUT_FILE "$2:\n"; #prints CSTOL Label
          print OUT_FILE " \n";
          my $pkt = "$2";
          my $out_file = "$2_tlmval.txt";
          
          # Close OUT_FILE
          close(OUT_FILE);
          print "Closing: OUT_FILE\n";
          
          #Open data_list file for read
          open (DAT_FILE, "<", $dat_file) or die "ERROR Opening $dat_f
+ile (Error = $!)\n";
          print "Opening: $dat_file for read...\n";
          
          # Reopen OUT_FILE for write
          open (OUT_FILE,, ">>", $out_file) or die "ERROR Opening $out
+_file (Error = $!)\n";
          print "Re-opening: $out_file for write...\n";
          
          #Search DATA_LIST file
          while (<DAT_FILE>) {
              if (($line_in =~ /($pkt) (\w+) = (\d+)/)) {
                  print OUT_FILE "check $pkt $2 vs $3\n";
                  }
              }
          }

      

# Close IN_FILE
close(IN_FILE);
 print "Closing: IN_FILE\n";

#Close OUT_FILE
close (OUT_FILE);
print "Closing: OUT_FILE\n";

# Close DAT_FILE
close(DAT_FILE);
print "Closing: DAT_FILE\n";
[download]

Comment on How to use REGEX Match Variable from one 'While' String in Another 'While' String Download Code

Replies are listed 'Best First'.
Re: How to use REGEX Match Variable from one 'While' String in Another 'While' String by AnomalousMonk (Archbishop) on May 07, 2014 at 02:53 UTC
In addition to the changes suggested by `AnonyMonk` above, I find it is wise to capture regex 'capture' variables and get them nailed down as soon as possible. This is because the evaluation of any further regex will probably cause the contents of these variables to... well, vary. (You say, "But I'm not using any other regex and never will." Famous Last Words. Nail them down. And, in fact, you do have another regex, the one in the second, nested `while`-loop. If you were ever to add a statement after the second `while`-loop that made reference to the `$1 $2 $3 $`n capture variables expecting that they would retain the values they had after execution of the first regex, you would be unpleasantly surprised. Nail them down.) Maybe something like this. (Since you don't seem to use `$1` anywhere, I leave it out of account.) (Note the `/g` modifier on the `m//xmsg` regexes.) (Untested) my $pkt_file = ...; my $dat_file = ...; ... open (my $fh_pkt, '<', $pkt_file) or die "ERROR Opening $pkt_file (Err +or = $!)\n"; ... while (defined(my $line_in = <$fh_pkt>)) { if (my ($pkt) = $line_in =~ m{ \w+ \s+ (\w+) }xmsg) { my $out_file = "${pkt}_tlmval.txt"; open (my $fh_out, '>>', $out_file) or die "ERROR Opening '$out +_file' (ERROR = $!)\n"; ... open (my $fh_data, '<', $dat_file) or die "..."; while (defined(my $line_in = <$fh_data>)) { if (my ($foo, $bar) = $line_in =~ m{ \Q$pkt\E \s+ (\w+) \s ++ = \s+ (\d+) }xmsg) { print $fh_out "check '$pkt' '$foo' vs '$bar'\n"; } } } } [download] Note that the `$line_in` lexical variable in the nested `while`-loop is guaranteed to be completely separate and isolated from the lexical of the same name in the outer loop. Even so, I think I would prefer to use different names for these two variables just for the sake of self-documentation and maintainability.	[reply] [d/l] [select]
Re^2: How to use REGEX Match Variable from one 'While' String in Another 'While' String by GoldfishOfDoom (Initiate) on May 07, 2014 at 18:44 UTC
AnomalousMonk, Thank you for the additional comments. I'm always looking on ways to improve and make things flexible for any 'unexpected' changes or additions. This was extremely helpful in cleaning up my script. I appreciate you taking the time to respond.	[reply]
Re: How to use REGEX Match Variable from one 'While' String in Another 'While' String by Anonymous Monk on May 06, 2014 at 22:27 UTC
Is there anyway to use the first REGEX match to generate the second REGEX statement? Yes, there is, and I see you're already doing this in your regular expression `/($pkt) (\w+) = (\d+)/`. If, for example, `$pkt` is `"abc.def"`, the pattern will be the equivalent of `/(abc.def) (\w+) = (\d+)/`. If you don't want the dot and any other characters to be interpreted as special regex characters, you can write your regex as `/(\Q$pkt\E) (\w+) = (\d+)/`, and the previous example regex becomes `/(abc\.def) (\w+) = (\d+)/` (see quotemeta). If this isn't working, after a first glance I will venture a guess as to why: your inner while loop is matching against `$line_in` instead of the current line from `DAT_FILE`, is that correct? If you want to match against the current line from `DAT_FILE`, it's enough to change that line to `if (/($pkt) (\w+) = (\d+)/) {` If that's not the problem, it would help if you could provide more information on your question: some sample input, the desired program output, any error messages you might be getting, and a description of what the program is supposed to be doing and how that's different from what it's actually doing. As you said there are other ways your code could be cleaned up (for example, it looks like you're opening and closing the file `"$2_tlmval.txt"` as `OUT_FILE` twice in a row, when you could just keep the file open), but first things first `:-)`	[reply] [d/l] [select]
Re^2: How to use REGEX Match Variable from one 'While' String in Another 'While' String by GoldfishOfDoom (Initiate) on May 07, 2014 at 18:40 UTC
Ah! Much easier than I was expecting. Thank you very much for the help! Seems like removing the $line_in did the trick. The regex match only had alphanumeric characters and underscores, so special characters wasn't an issue. I also removed the extra open and close of the file. Thank you for the suggestion! I really appreciate you taking the time to respond.	[reply]
Re^3: How to use REGEX Match Variable from one 'While' String in Another 'While' String by Anonymous Monk on May 07, 2014 at 22:39 UTC
The regex match only had alphanumeric characters and underscores... If your `$pkt_file` doesn't contain regular expressions, it's probably better to use `\Q$pkt\E`, because that'll prevent your program from doing unexpected things if `$pkt` does someday for whatever reason contain special characters. Similar to what AnomalousMonk wrote below about `$1 $2 $3` etc. it's a "future-proofing" measure.	[reply] [d/l] [select]