Well there are quite a few typos in your code which mean that it doesn't compile, but I assume they are just transcription errors.

Fixing them and running your code, I see that nothing is output as you describe. The problem is in the line

The /g is unnecessary here and is causing the expression to evaluate as false. Removing the /g makes the code work as expected.

The /g option matches the regex as often as it can against your string. The final time it tries to match, the match fails and the operator returns false. Without the /g the match only takes place the one time it needs to succeed and the operator returns true.

Update: Yeah. As others in this thread point out, I had the right fix, but the wrong explanation. Should drink more coffee before posting :)

Perl Training in the UK <http://www.iterative-software.com>

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:


• Leave out the capturing brackets for (line2-)
• Use multiline matching (/m)
• Compile the patterns only once (/o)
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

$_ = "c a";
if (/a/g/ and /c/g) { ... }
[download]

The right solution has been offered. . . but I'm a little puzzled:


#THIS LINE BELOW!
  if ( $contents =~ /^line2-(\w*)/mo ) {
    print $1; # has to be changed as well
  }
}
[download]

Why do you use \w*? First, if all you have is 2-11, 2-12, etc., then a \d is by far better. Second, by having the * you allow this line to be matched: line2-
I assume you do not wish that to happen, so add \d+ (unless you really need the \w).

Also, as to Hofmator's optimization, I disagree. The /o will have NO effect, since the variable is not in the regex, but is bound to it. If it were $contents =~ /$line/ then it would be an optimization, but now it has no effect, and only muddies up the waters, so to say. Also, I would speculate that the /m is useless in this situation. The format is:
line1: /stuff here/
line2-2: /more stuff/
.
.
.

Therefore, I assume by what he gave us, that the information appears only at the beginning of the lines, and would not make sense to spread multiple lines; therefore taking away the /m would be better(not to mention /m doesn't optimize it, but slightly detracts from it).

UPDATE: By the way, to optimize it further, you should add the caret to the regex; /^line1/ if I am correct in that line1 is at the beginning of the line.