String Match Use of $1 Uninitialized

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

Why does this work:

my $text = "mississippi";
$text =~ m/(i.*s)/;
print $1 . "\n";
[download]

And this gives me the error: 'Use of uninitialized value $1 in print'?

my $line = "PID: AIR-LAP1142N-A-K9   ,  VID: V01,  SN: FTX1332SC15";
$line =~ m/AIR-LAP1142N-A-K9/;
print $1 . "\n";
[download]

Please understand that before anyone tells me to go RTFM, I already have, and I wouldn't be asking questions if I didn't need some help. Thanks!

Comment on String Match Use of $1 Uninitialized Select or Download Code

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Re: String Match Use of $1 Uninitialized by jettero (Monsignor) on Dec 16, 2009 at 19:35 UTC
You second regex doesn't have any ()s in it. The $1 is populated from the ()s. Rather than saying RTFM, I'll say R this part of TFM: extracting matches. -Paul	[reply]
Re: String Match Use of $1 Uninitialized by kennethk (Abbot) on Dec 16, 2009 at 19:36 UTC
The issue is that you are not capturing any text from your regular expression in the second case, so that sets all the captured groups to `undef`. Which strings are captured is controlled by parentheses in your regular expression - see perlre, perlretut. For example, of you wanted to capture the letters 'AIR' from your string into the `$1` buffer, you might use: `my $line = "PID: AIR-LAP1142N-A-K9 , VID: V01, SN: FTX1332SC15"; $line =~ m/(AIR)-LAP1142N-A-K9/; print $1 . "\n";` [download]	[reply] [d/l] [select]
Re^2: String Match Use of $1 Uninitialized by spickles (Scribe) on Dec 16, 2009 at 19:58 UTC
Gentlemen: Makes a lot of sense now. Thanks for that. The docs I was reading weren't from perlretut and didn't make the function of grouping apparent. Regards, Scott	[reply]
Re^3: String Match Use of $1 Uninitialized by ikegami (Patriarch) on Dec 16, 2009 at 21:47 UTC
Putting stuff in $1 is called capturing. While parens also do grouping, it's usually done using `(?: ... )`. For example, `/abc(?:def\|ghi)/`.	[reply] [d/l] [select]