Capture not working

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

I have a problem with missing captured strings in a script supposed to parse the output of the mysqlbinlog tool:

my $comment = '^(\/\*.*\*\/\;)(.*)';
[...]
while ($statement !~ m{$comment}s) { $statement .= <$file> or die "Pre
+mature EOF.\n"; }
print "Found type 1 comment: $1\n";
[download]

Executing this code always produces:

Use of uninitialized value in concatenation (.) or string at ./test.pl
+ line 16, <$file> line 1.
Found type 1 comment:
[download]

For the line containing the print statement, meaning that $1 is not defined. But I don't understand how that could be. If the regular expression matched there should be something captured, and if not the line wouldn't even be executed...

I tried to reformulate this as while (!($statement =~ m{$comment}s)) but that didn't help either. Could anyone tell me, why nothing's captured here?

Comment on Capture not working Select or Download Code

Replies are listed 'Best First'.
Re: Capture not working by ikegami (Patriarch) on Jan 08, 2009 at 13:26 UTC
`$1` and family are implicitly ~~declared~~localized in the scope in which they reside. In this case, the scope created by `while`. `'a'=~/(.)/; print("$1\n"); # a while ( 'b'=~/(.)/ ) { print("$1\n"), # b last; } print("$1\n"); # a` [download] So don't use `$1`. `my $comment_re = qr{/\.?\/;}s; my ($comment, $rest); while (!( ($comment, $rest) = $statement =~ m{^($comment_re)(.)}s )) +{ defined( my $line = <$file> ) or die "Premature EOF.\n"; $statement .= $line; } print("Found type 1 comment: $comment\n");` [download] Also fixed `.` ⇒ `.?` Also fixed buggy premature EOF test.	[reply] [d/l] [select]
Re: Capture not working by cdarke (Prior) on Jan 08, 2009 at 13:35 UTC
I don't think this has anything to do with captures. The error message seems to report an unitialised value in the string concatenation, which means that <$file> is returning undef. So why is your die not working? Try changing the low precedence operator `or` to `\|\|`. update: Yep, I'm wrong again.	[reply] [d/l] [select]
Re^2: Capture not working by moritz (Cardinal) on Jan 08, 2009 at 13:51 UTC
If you're unsure where the warning comes from, just run it through perl-5.10: `mlenz@wvbh074:~$ cat test.pl use strict; use warnings; my $comment = ''; my $statement = ''; while ($statement !~ m{^(/\.\/;)(.)}s) { $statement .= <DATA> or d +ie "Premature EOF\n"; } print "Found type 1 comment: $1\n"; __DATA__ /* foo */; stuff; mlenz@wvbh074:~$ perl test.pl Use of uninitialized value $1 in concatenation (.) or string at test.p +l line 6, <DATA> line 1. Found type 1 comment:` [download] Here it tells you `uninitialized value $1` explicitly.	[reply] [d/l] [select]
Re^2: Capture not working by ikegami (Patriarch) on Jan 08, 2009 at 13:43 UTC
I beg to differ ...uh agree with the OP `>perl -wle"while ('a'=~/(.)/, 0) {} print(qq{Found $1})" Use of uninitialized value in concatenation (.) or string at -e line 1 +. Found` [download] Turns out `$x .= <>` doesn't even cause a warning at EOF?! `>perl -we"our $x; $x .= <>" <nul >` [download]	[reply] [d/l] [select]
Re^3: Capture not working by Anonymous Monk on Jan 08, 2009 at 14:03 UTC
Thank you, that's very helpful. I didn't know you can assign the results of a pattern operation to a list like this. I don't quite see in how far the EOF test was buggy though, since the data can neither contain blank lines nor lines containing nothing but a "0"...	[reply]
Re^4: Capture not working by ikegami (Patriarch) on Jan 08, 2009 at 14:32 UTC
Re^2: Capture not working by Corion (Patriarch) on Jan 08, 2009 at 13:39 UTC
~~Indeed, the `or die` statement is not working, as it checks that `$statement .= <$file>` is not a false value, which it will be in most cases.~~. Worng answer - see the answers of people actually testing their responses :)	[reply] [d/l] [select]