Re: weird regex problem

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

if ( $contents =~ /(line2-)(\w*)/g ) {

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 :)

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Re: Re: weird regex problem by Hofmator (Curate) on Jun 14, 2001 at 15:57 UTC
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	[reply] [d/l] [select]

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Re: Re: weird regex problem
by Hofmator (Curate) on Jun 14, 2001 at 15:57 UTC

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)

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

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[d/l]
[select]