Right fix davorg, but wrong explanation :). Here is the relevant code section again:

if ( $contents =~ /line3/g ) {
  if ( $contents =~ /(line2-)(\w*)/g ) {
    print $2;
  }
}

[download]

In $contents is the slurped file:
line1-11
line2-12
line3-13
The first if matches 'line3' and returns true. The second match picks up at the position where the first match left (because it has also the /g modifier) and fails (!) because line2 is before line3. So removing the /g modifier on the second 'if' solves the problem as the match is now done from the start of $contents. As a matter of fact, the /g modifier can be left out for both matches.

Some further optimisations I would suggest for this regex:

This then leads to the following code:

if ( $contents =~ /^line3/mo ) {
  if ( $contents =~ /^line2-(\w*)/mo ) {
    print $1; # has to be changed as well
  }
}

[download]

To clarify the /g modifier a little bit further let's take a look at this code (of course see also perlre and perlop):

my $string = "abcde abcde adcde";
while ($string =~ /cd/og) {
  print "pos = ", pos $string, "\n";
    if ($string =~ /a(.)/ocg) {
      print "a$1 matched at ",pos $string, "\n";
    }
}

[download]

Here the first match happens in a while loop, but important still in scalar context. The inner matching starts at the position where the first left off as it also has the /g modifier. Then the outer match takes its turn again starting where the inner left off. The position in the string is only reset when a match fails. This does not happen when the /c modifier is given. This is necessary for the second match in this case - otherwise there is an infinite loop.

Taking this code and playing a bit with the modifiers and the string helps a lot in understanding these (not so easy) things. And I haven't even started talking about m//g in list context yet ...

-- Hofmator

In reply to Re: Re: weird regex problem by Hofmator
in thread weird regex problem by toadi

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