understanding regex

rookie696 has asked for the wisdom of the Perl Monks concerning the following question:

Hi monks,

I have started to learn perl now only. I was trying to list and download files in my app which matches the perl regex which i give. Let's say we have 3 files

1) asdf 
2) efgh 
3) qwer

The problem is i want to download all files except asdf. For this i got two ideas. one way is [^(asdf)]and the other way is ^(?!asdf) I want to know the difference between both. Could some one please explain this ?
Thanks in advance,
Rookie696.

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Re: understanding regex
by Athanasius (Archbishop) on Sep 08, 2013 at 06:19 UTC

Hello rookie696, and welcome to the Monastery!

In a regex, [^(asdf)] is a character class which says: match one character as long as that character is not a parenthesis or a lower-case ‘a’, ‘s’, ‘d’, or ‘f’. This is not what you want.

^(?!asdf) says: match the beginning of the line not followed by ‘asdf’. This will work, but it uses a negative look-ahead assertion which is overkill for your purpose.

What you want is simply to negate a normal match. For example:

my @files = ...;

for my $file (@files)
{
    download($file) unless $file =~ /^asdf/;
}
[download]

Or, equivalently,

for my $file (@files)
{
    download($file) if $file !~ /^asdf/;
}
[download]

You should read up on regexes. Here are some good references:

perlretut
perlrequick
perlre
Chapter 6, “Regular Expressions and Matching” of Modern Perl by chromatic (free online).

Hope that helps,

Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,

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Re: understanding regex
by rjt (Curate) on Sep 08, 2013 at 17:37 UTC

To add to the excellent advice by Athanasius, now would also be a good time to learn about grep:

my @downloads = grep { !/^asdf$/ } @file_list;

print "Downloading: @downloads\n";
[download]

It's also worth noting that if you truly want to exclude a literal string, you don't need a regex at all, so you could also do:

my @downloads = grep { $_ ne 'asdf' } @file_list;

This works just as well if asdf is in a variable.

my $exclude   = 'asdf'; # Maybe user input or other program logic
my @downloads =  grep { $_ ne $exclude } @file_list;
[download]

use strict; use warnings; omitted for brevity.

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