• you say "word" but you mean string, right?
• you don't anchor to the end of the string with $ but to the start with ^
• you use an all quantifier * for the negated class, which also means 0-times, so r doesn't need to be rejected and the if-clause is entered.

Actually I don't know any more what your real goal is, so no recommendation other than

  DB<114> $in_str = 'aa<c><d>r';
 => "aa<c><d>r"

  DB<115> if ($in_str =~ /aa((<[^>]*>)*)$/) {print "$1"}
 => ""

  DB<116> $in_str = 'aa<c><d>';
 => "aa<c><d>"

  DB<117> if ($in_str =~ /aa((<[^>]*>)*)$/) {print "$1"}
 => 1
<c><d>
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Thanks for the replies. Let me explain my goal using a few examples:

Case 1:

$in_str = ‘somewordaa<metadata><moremetadata>r another wo<metadata>rd’; => aa should not be replaced by a, since aa is immediately followed by another alphabet 'r' once the metadata is ignored.

Case 2:

$in_str = ‘aword<metadata>aa<metadata> another wo<metadata>rd’; => aa should be replaced by a because it is at the end of a word. It is followed by space.

Case 3:

$in_str = 'wor<metadata>dsandmor<metadata>ewordsaa<metadata>;' => aa should be replaced by a because aa is at the end of the word (also end of string) once metadata is ignored.

I now understand that the quantifier * on a negation of character class won't work because of the "0 times" match. I need to check if aa is at the end of a word. I plan to do this by checking if the following character (again, after ignoring metadata) is either a non-alphabet character or end of string. How would I go about doing this?

$in_str =~ s/aa((<[^>]*>)*)([^a-zA-Z])/a$1$3/g;

I think you can do this with a single substitution regex (s/pattern/replacement/;), but it's going to need a lookahead assertion at the end. Also, this is only possible if you can write a regex segment that will consistently match the metadata. (Making it match metadata if there is any and not block matching if there isn't any is then trivial: precede the metadata-match portion in (?: and follow it up with )*.

Perhaps something along these lines...

s/(a(?:[<][^>]+[>])*)a((?:[<][^>]+[>])*(?:\s+|[;,.])+)/$1$2/;

That appears to work with the actual sample data you provided, but I don't know if it will work with your real data.

I would approach the problem this way:

c:\@Work\Perl>perl -wMstrict -le
"use Test::More 'no_plan';
 use Test::NoWarnings;
 ;;
 my $metadata = qr{ < [^>]* > }xms;
 ;;
 for my $ar_vector (
   [ 'somewordaa<metadata><moremetadata>r another wo<metadata>rd',
     'somewordaa<metadata><moremetadata>r another wo<metadata>rd',
     ],
   [ 'aword<metadata>aa<metadata> another wo<metadata>rd',
     'aword<metadata>a<metadata> another wo<metadata>rd',
     ],
   [ 'wor<metadata>dsandmor<metadata>ewordsaa<metadata>',
     'wor<metadata>dsandmor<metadata>ewordsa<metadata>',
     ],
   ) {
   my ($s, $expected) = @$ar_vector;
   $s =~ s{ aa (?! $metadata* r) }{a}xmsg;
   is $s, $expected, qq{'$expected'};
   }
"
ok 1 - 'somewordaa<metadata><moremetadata>r another wo<metadata>rd'
ok 2 - 'aword<metadata>a<metadata> another wo<metadata>rd'
ok 3 - 'wor<metadata>dsandmor<metadata>ewordsa<metadata>'
ok 4 - no warnings
1..4
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Note that making the character class match 'atomic' or possessive may speed things up a bit for large data (for some definition of 'large') by suppressing backtracking on subsequent match failure:
    my $metadata = qr{ < (?> [^>]*) > }xms;
and for Perl version 5.10 and above:
    my $metadata = qr{ < [^>]*+ > }xms;
(both variations tested). Your data will determine if possessiveness actually makes a difference. The regex compiler may be smart enough to make this optimization on its own (I haven't checked), but giving the compiler a hint never hurts.

And of course, you will want to add many more test cases!

Hello perluser4102, and welcome to the Monastery!

I would think the 'r' would fail a match at([^a-zA-Z]*) and the if block won't be entered.

The * quantifier specifies zero or more matches, so in this case it successfully matches zero times and the if condition is true. But if you terminate the regex with an end-of-line metacharacter $, the match fails:

if ($in_str =~ /^aa((<[^>]*>)*)([^a-zA-Z]*)$/)
#                                          ^
[download]

Hope that helps,