This does what I want 
my $line = q{one <two>};
$line =~ /
    ([^<]+)
    (?:
        <
        (two|three)
        >
    )?
/x;

print $1, $2;

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one two

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The <two> is optional and that works too. But I need 
my $line = q{one <four>};

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to return one <four> in $1 and for $2 to be undef. Only capture to $2 if a pattern is matched otherwise capture the whole lot to $1.

Is this something that a look around assertion could handle? I've yet to conquer the blighters :-(

It will be used like 

while ($line =~ /pattern/g){
  # do something with $1 and $2
}

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and a typical string 
one [link|file.html] two [email|me@home.com] three [not a link]

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What I have at the moment is fine but I'm not picking up three [not a link] in $1 

  while (
    $line =~ 
      /
        (
          [^\[]*
        )
        (?:
          \[
          (
            (?:link|email|pdf)
            [^\]]*
          )
          \]
        )?
      /xg
  )
  {
    my $txt = $1;
    my $link = $2;
    if ($link){
      my ($type, $address, $txt) = split(/\|/, $link);
      # more stuff
    }
  }

[download]
tia

In reply to Regex: Optional/alternative catpures by wfsp

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