kort has asked for the wisdom of the Perl Monks concerning the following question:

Hello Monks!

I'm an Perl initiate, and script I'm trying to write is far above what I've read and searched all over the place, hence the registration and question.

I would like to write a script that goes through all the folders of my C project and catches all calls to a C function: printf and assign every single catch to an array as a single element. And all that above is pretty easy, I did something like this (files array is filled with File::Find::Rule):

foreach my $file (@files) {
    open my $fh, '<', $file or warn "Coudn't open $file \n";

    while (<$fh>) {
        if (/(printf.*\;)/sm) {
            my $buffer = $1;
            push @printfs, $buffer;
        }
    }
    close $fh;
}

[download]

Above catches only one line printfs. Problem is, the printf calls sometimes span through multiple lines (from 2 up to 5), and that makes it hard to catch for me.

I've already tried things like last if (m/\;$/), yet I failed horribly. Can you help me with that?

Replies are listed 'Best First'.
Re: Catching multiple lines and assigning them as a single array element
by Eily (Monsignor) on Oct 30, 2015 at 00:57 UTC

    while(<$fh>) reads the file line by line, so even with /sm the string that is tested for a match is either the line that contains the printf, or the line that contains the end ';' (if it spans over multiple lines). You can read the documentation on the input record separator: $/ which defines how much perl reads from a file on each iteration. By default the value of $/ is "\n" which will make perl stop everytime it sees an end of line. But you can also set it to undef to read (slurp) the whole file at once. Slurping will give wrong results though as the string: 

    printf("");
sleep();
print("");

    [download]
    will match only once because .* (with /s) will read the whole thing up to the last ;. Even line by line "printf(""); "printf(""); will only match once. One way to avoid that is to replace .* by .*? which means the shortest match possible (each time perl matches a new character it will try to stop the regex by looking for a ; before "eating" anything else with the dot)

    In the best case scenario you can do something like: 

    {
  local $/ = ";"; # local means perl will restore the value of $/ at t
+he end of the block
                  # and the default behaviour when reading a file
  for my $file (@files)
  {
     # Note the $!, which contains the reason for the opening failure
     open my $fh, '<', $file or warn "Coudn't open $file: $!\n";
     while (<$fh>) # will stop at each ";", so read instruction by ins
+truction
     {
       push @printfs, $1 if /(printf.*);/;
     }
  }
}

    [download]

    Note that a case like printf(";"); will make your regex yield an unexpected result either with .*? or $/ = ";". This comes from the fact that C code is actually not a regular language, which means it can't be parsed by regular expressions. Luckily, perl regexes are far more powerful than simple regular expressions, even if the name seems to say otherwise. Making a foolproof C parser is still a really difficult (and kind of foolish) thing to do with regexes. So if you're quite sure of your input data and know it fits a certain pattern, you're probably safe with regexes, but if you notice you keep getting harder and harder to include exceptions, give up that path (what you would need then is a grammar parser).

[reply]
[d/l]
[select]

      Thanks for the tips. I found that in my case a simple regex would do (of course I'm also finding idiotic comments in the multiline printfs sometimes, but that's something I was aware of from the beginning ;)). I have used: printf[^;]+; and found it does the job I wanted it to do ;)

[reply]
[d/l]
Re: Catching multiple lines and assigning them as a single array element
by Anonymous Monk on Oct 29, 2015 at 23:35 UTC
    Its easy, slurp the file ( Path::Tiny ), then you're working with all the lines, then you can match across multiple lines

    Use Regexp::Common , maybe $RE{balanced}{-parens=>'()'}

[reply]

      Thanks a lot - worked like a charm ;)

[reply]

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