Gossamer Forum: General: Perl Programming: Understanding a regular Expression

Jul 13, 2010, 10:46 PM

gpta_varun

Novice (24 posts)

Jul 13, 2010, 10:46 PM

Post #1 of 18

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Understanding a regular Expression

Hi All ,

I have a small query. I have written a perl script that takes input from a configuration file.

The configuration file is like

MACHINE NAME = X
CONFIG = Y

I need to read this file and enter these values in a hash , as in ...

hash{'MACHINE NAME'}=X

I have seen a regular expression in one of the scripts , to identify the A=B pattern

It is :

if (($name,$value)= /^\s*([^=\s]+)\s*=\s*(.*?)\s*$/ ){

Just need to understand , how does this expression , preserves the value in $name and $ value variables

I understand some part though ...

/^\s* -> for starting whitespaces
([^=\s]+) --> ?? whats this ??
\s* -> white spaces before '='
=\s* -> white spaces after '='
(.*?) -> $value
\s* --> =\s* -> white spaces after $value
$/

Thanks

Jul 14, 2010, 12:15 AM

Andy

Veteran / Moderator (17364 posts)

Jul 14, 2010, 12:15 AM

Post #2 of 18

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Re: [gpta_varun] Understanding a regular Expression In reply to

The ([^=\s]+) bit is telling the regex to "break" the rule wehen it comes up with a = sign, and capture the result into $1

Cheers

Andy (mod)
andy@ultranerds.co.uk

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Jul 14, 2010, 12:38 AM

gpta_varun

Novice (24 posts)

Jul 14, 2010, 12:38 AM

Post #3 of 18

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Re: [Andy] Understanding a regular Expression In reply to

Thanks Andy ! You come to my rescue time and again !

I am stuck with another point now.

Below is the function that takes the config file as a parameter.

CONFIG file is something like :

[MACHINE NAMES]
A=1,2
B=3,4

** where 1,2,3,4 are the partitions on a machine 'A' or 'B'

I am creating a hash table (hash) , which stores data in the format

hash{MACHINE NAMES}{A}=1;

In the code , while $hash{$key_value}{$name} works , $hash{"MACHINE NAME"}{"A"} does not !!!
I would return the hash so formed to the main function and use it !

Code:

sub read_conf_file($) 
{ 
  my $i=0; 
  my $config_file=shift(); 
  # store the attributes within [] in the key_table hash 
  my %hash=(); 
  my $key_value; 

  print "******************\n"; 
  open(CONFIG, "<$config_file") || die "I/O Success - Cannot process $config_file file"; 

  while (<CONFIG>) 
  { 
     chomp; 
     if(/^\s*(#.*)$/){next;} 

     if(/^\s*\[(.*)\]$/) 
     { 
      $key_value=$1; 
      print $key_value; 
      #$key_table[$i++]=$1; 
     } 
     #elsif (/^\s*(?.*)    \s*=\s*(?.*)\s*$/ ) 
     else {  
          if(($name,$value)= /^\s*([^=\s]+)\s*=\s*(.*?)\s*$/ ) 
          { 
        print "\n *** $name=$value ****\n"; 
        $hash{$key_value}{$name}=$value;            
print "\n *** $hash{$key_value}{$name}****\n"; --> WORKS
                  } 
          } 


  } 
      print "** ** ** IN FUNC ** ** ** \n"; 
print $hash{"MACHINE NAME"}{"A"}; --> not working 
  print "** ** ** IN FUNC ** ** ** \n"; 

  return(\%hash); 


}

Jul 14, 2010, 12:43 AM

gpta_varun

Novice (24 posts)

Jul 14, 2010, 12:43 AM

Post #4 of 18

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Re: [gpta_varun] Understanding a regular Expression In reply to

Also ,

With multiple machines A , B , C ..... etc ,

How would i dynamically grep values from the hash .... Need something like ...

hash {MACHINE NAME} { ....loop through all values }

Jul 14, 2010, 12:44 AM

Andy

Veteran / Moderator (17364 posts)

Jul 14, 2010, 12:44 AM

Post #5 of 18

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Re: [gpta_varun] Understanding a regular Expression In reply to

I hope this isn't coursework I'm helping you with Tongue

After:

Code:
print $hash{"MACHINE NAME"}{"A"};

try doing:

Code:
use Data::Dumper; 
print Dumper($hash);

...then post the results (so I can see what kinda format the data is in)

Cheers

Andy (mod)
andy@ultranerds.co.uk

Jul 14, 2010, 12:47 AM

Andy

Veteran / Moderator (17364 posts)

Jul 14, 2010, 12:47 AM

Post #6 of 18

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Re: [gpta_varun] Understanding a regular Expression In reply to

gpta_varun wrote:

Also ,

With multiple machines A , B , C ..... etc ,

How would i dynamically grep values from the hash .... Need something like ...

hash {MACHINE NAME} { ....loop through all values }

you would do something like:

Code:
if ($value =~ /,/) { 
   foreach (split /,/, $value) { 
     $hash{$key_value}{$name}=$_;  
   } 
} else { 
   $hash{$key_value}{$name}=$value;  
}

Cheers

Andy (mod)
andy@ultranerds.co.uk

Jul 14, 2010, 1:00 AM

gpta_varun

Novice (24 posts)

Jul 14, 2010, 1:00 AM

Post #7 of 18

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Re: [Andy] Understanding a regular Expression In reply to

Result of Execution :

Quote:

*** A=1,2 ****

*** 1,2**** --> valid
Dump $VAR1 = '';
$VAR2 = {
'A' => '1,2'
};
** ** ** IN FUNC ** ** ** --> NO VALUES PRINTED HERE
** ** ** IN FUNC ** ** **

For the second part , I guess I have not been able to clarify things

I would maintain a hash , something like :

hash {MACHINE NAME}{A}='1,2'
hash {MACHINE NAME}{B}='3,4'

Once the hash is complete , I would return this to the main function

In the main function , for all machines A, B ..... I need to process all the partitions

However , the problem is I don't know the number of entries under the section [MACHINE NAMES]

I need to loop through all the machines A , B , C . Within a loop I'll try to split $value by ',' and process ....

Jul 14, 2010, 1:07 AM

Andy

Veteran / Moderator (17364 posts)

Jul 14, 2010, 1:07 AM

Post #8 of 18

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Re: [gpta_varun] Understanding a regular Expression In reply to

What is the value of your config file? I think you need to really do this a different way :) (you seem to be overcomplicating things :))

Cheers

Andy (mod)
andy@ultranerds.co.uk

Jul 14, 2010, 1:09 AM

gpta_varun

Novice (24 posts)

Jul 14, 2010, 1:09 AM

Post #9 of 18

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Re: [Andy] Understanding a regular Expression In reply to

#MACHINE_NAME=<RESPECTIVE PARTITIONS NUMBERS>

[MACHINE NAME]
A=1,2
B=3,4

[GENERAL CONFIGURATIONS]
IP=1.2.3.4

Jul 14, 2010, 1:13 AM

Andy

Veteran / Moderator (17364 posts)

Jul 14, 2010, 1:13 AM

Post #10 of 18

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Re: [gpta_varun] Understanding a regular Expression In reply to

Untested, but try something like:

Code:
my $section; 
my $vals; 
open (READIT, "./filename.cfg") || die "Cant read filename.cfg. Reason: $!"; 

	while (<READIT>) { 
		if ($_ =~ /^#/ || $_ =~ /^\s+/) { next; } 

		if ($_ =~ /\[(.*)\]/) { 
			$section = $1; 
		} else { 
			my ($col,$value) = split /=/, $_; 
			$vals->{$section}->{$col} = $value; 
		} 
	} 
close (READIT);

..then call with something like:

Code:
$vals->{'MACHINE NAME'}->{"A"}

Cheers

Andy (mod)
andy@ultranerds.co.uk

Jul 14, 2010, 1:27 AM

gpta_varun

Novice (24 posts)

Jul 14, 2010, 1:27 AM

Post #11 of 18

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Re: [Andy] Understanding a regular Expression In reply to

Thanks Andy !

Two questions :

1) 'vals' should be a hash right ?
2) Would I be able to graze through all the entries defined in $section = MACHINE NAMES ?

Jul 14, 2010, 1:29 AM

Andy

Veteran / Moderator (17364 posts)

Jul 14, 2010, 1:29 AM

Post #12 of 18

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Re: [gpta_varun] Understanding a regular Expression In reply to

$vals is a hashref :)

Quote:

2) Would I be able to graze through all the entries defined in $section = MACHINE NAMES ?

You could do it with something like:

Code:
my $tmp = $vals->{"MACHINE NAMES"}; 
map { 
    print qq|Column: $_ ----> VAL: $tmp->{$_}\n| 
 } keys %$tmp;

Cheers

Andy (mod)
andy@ultranerds.co.uk

Jul 14, 2010, 1:33 AM

gpta_varun

Novice (24 posts)

Jul 14, 2010, 1:33 AM

Post #13 of 18

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Re: [Andy] Understanding a regular Expression In reply to

Its not working :(

Interesting part is that while print $hash{$key_value}{$name} works , (that means atleast the population of hash is OK , the $hash{'MACHINE NAME'}{'A'} does not !

Jul 14, 2010, 1:39 AM

Andy

Veteran / Moderator (17364 posts)

Jul 14, 2010, 1:39 AM

Post #14 of 18

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Re: [gpta_varun] Understanding a regular Expression In reply to

I'll test it locally later on - bit busy atm ;)

Cheers

Andy (mod)
andy@ultranerds.co.uk

Jul 14, 2010, 1:41 AM

gpta_varun

Novice (24 posts)

Jul 14, 2010, 1:41 AM

Post #15 of 18

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Re: [Andy] Understanding a regular Expression In reply to

oks !

Thanks Andy.

Jul 14, 2010, 2:09 AM

Andy

Veteran / Moderator (17364 posts)

Jul 14, 2010, 2:09 AM

Post #16 of 18

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Re: [gpta_varun] Understanding a regular Expression In reply to

Just tested it, and it works fine for me???

Code:
#!/usr/bin/perl 

use Data::Dumper; 

my $section; 
my $vals; 
open (READIT, "./test.cfg") || die "Cant read filename.cfg. Reason: $!"; 

	while (<READIT>) { 
		if ($_ =~ /^#/ || $_ =~ /^\s+/) { next; } 

		if ($_ =~ /\[(.*)\]/) { 
			$section = $1; 
		} else { 
			my ($col,$value) = split /=/, $_; 
			$value =~ s/\s+$//g; 
			$vals->{$section}->{$col} = $value; 
		} 
	} 
close (READIT); 

print Dumper($vals); 

print "FOO: " . $vals->{'MACHINE NAME'}->{'A'};

Gives:

Code:
 perl test.cgi 
$VAR1 = { 
          'GENERAL CONFIGURATIONS' => { 
                                        'IP' => '1.2.3.4' 
                                      }, 
          'MACHINE NAME' => { 
                              'A' => '1,2', 
                              'B' => '3,4' 
                            } 
        }; 
FOO: 1,2

Cheers

Andy (mod)
andy@ultranerds.co.uk

Jul 14, 2010, 4:55 AM

gpta_varun

Novice (24 posts)

Jul 14, 2010, 4:55 AM

Post #17 of 18

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Re: [Andy] Understanding a regular Expression In reply to

Thanks Andy !

I compared the code that you had posted with the one that I created .

You won't believe , it was all due a small error in the regular expression used.

used

Code:
if(/^\s*\[(.*)\]/) 
     { 
      $key_value=$1; 
      chomp($key_value); 
     }

instead

Code:

if(/^\s*\[(.*)\]$/) 
     { 
      $key_value=$1; 
      chomp($key_value); 
     }

That created all the problem . Now working fine.

Here is how I cycled through the multidimensional hash

while(($machine,$partition_list) = each(%{$conf_hash->{"MACHINE NAME"}}))

Thanks again.

Regards
Varun

Jul 14, 2010, 4:57 AM

Andy

Veteran / Moderator (17364 posts)

Jul 14, 2010, 4:57 AM

Post #18 of 18

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Re: [gpta_varun] Understanding a regular Expression In reply to

Cool glad its working. That "little error" is one of the joys you get when working with any programming language <G> (although Perl is actually very good at returning helpful debugging error messages, compared to stuff like PHP, which I never really like the debugging options that gave you :()

Cheers

Andy (mod)
andy@ultranerds.co.uk