Rather than being daunted by the problem, try breaking it down into simple steps.

• Field starts with ? or &
• Continues with either
  • username=
  • alphameric string
  or
  • userid=
  • numeric string
• Field terminates with & or EOL

I think his problem isn't the "simple steps", but going from "simple steps" to a "complicated total".

Given that the username may, or may not, preceed the userid, how would you proceed? How would you proceed if there could be 10 fields, in any order?

I wouldn't use one regular expression. I'd use two -- one for the User ID, one for the name. State information tells you if they're both from the same line.

I realize brevity is the soul of wit, but I occasionally prefer verbosity if it results in ease of support. (Translation: I'm not as good a programmer as the rest of you...)

When you're dealing with something in a known/spec'd format, it's usually best to reach for a parser.

use URI;
use URI::QueryParam;

while ( <DATA> )
{
    chomp;
    my $uri = URI->new($_);
    print $uri, "\n";
    for my $param ( qw( id username ) )
    {
        printf("   %s --> %s\n", $param,
               join(", ", $uri->query_param($param))
               );
    }
}

__DATA__
http://beta.websitename.com/tool/view/mb/file?id=3323213&username=anir
+udh007
http://beta.websitename.com/tool/view/mb/file?username=anirudh007&id=4
+3242342
[download]

See recent: url get variables regex

use 5.010;
while (<DATA>) {
    chomp;
    /(?=.*username=(?<username>[^&]*))(?=.*id=(?<id>[^&]*))/ or next;
    say "username = $+{username}; id = $+{id}";
}

__DATA__
http://beta.websitename.com/tool/view/mb/file?username=anirudh007&id=4
+3242342
http://beta.websitename.com/tool/view/mb/file?id=3323213&username=anir
+udh007


username = anirudh007; id = 43242342
username = anirudh007; id = 3323213
[download]

Another possibility using split's:

use 5.010;
use strict;
use warnings;

while (<DATA>){
  chomp;
  my %ps = map {my @k = split /=/; $k[0] => $k[1]} split /&/,(split /\
+?/)[1];
  say "username = $ps{username}; id=$ps{id}"
}

__DATA__
http://beta.websitename.com/tool/view/mb/file?username=anirudh007&id=4
+3242342
http://beta.websitename.com/tool/view/mb/file?id=3323213&username=anir
+udh007
[download]

username = anirudh007; id=43242342
username = anirudh007; id=3323213
[download]