reset of match variables

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

in scripts I usually match like this:

my ($match) = $string =~ /$regex/;
[download]

but sometimes I do one-liners like this:

perl -ne '/$regex/, $SUM += $1; END { print $SUM }' file.txt
[download]

In this context I was just bitten by the (documented) behaviour that a failed match does not reset the match variables which in my case lead to a wrong sum as on empty lines where the regex did not match the previous match was (again) added to the sum.

For curiosity's sake now:

Assuming I did not want to introduce new variables but use $1 etc: How would I reset these variables to avoid the wrong sum as both "undef $1" as "$1=undef" does not work?

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Re: reset of match variables by Corion (Patriarch) on Oct 03, 2011 at 14:40 UTC
You can't reset them without doing a new match, but you can easily check whether there was a match: `perl -ne '/$regex/ and $SUM += $1; END { print $SUM }' file.txt` [download]	[reply] [d/l]
Re: reset of match variables by choroba (Cardinal) on Oct 03, 2011 at 14:38 UTC
Just use `/$regex/ and $SUM += $1`	[reply] [d/l]
Re: reset of match variables by JavaFan (Canon) on Oct 03, 2011 at 15:09 UTC
Untested: `$SUM += /$regex/ && $1; # Don't turn on warnings.` [download]	[reply] [d/l]
Re^2: reset of match variables by AnomalousMonk (Archbishop) on Oct 03, 2011 at 22:25 UTC
With warnings and strictures: `>perl -wMstrict -lne "$::SUM += /(\d+)/ && $1; END { print $::SUM } " foo123bar foo xxx111ccc xxx ^Z 234` [download]	[reply] [d/l]
Re^3: reset of match variables by bart (Canon) on Oct 04, 2011 at 11:31 UTC
Jeezes, who would want to use strict and a workaround for a one-liner??	[reply]
Re^4: reset of match variables by ambrus (Abbot) on Oct 04, 2011 at 12:34 UTC
Re^2: reset of match variables by hbm (Hermit) on Oct 03, 2011 at 18:22 UTC
Or even just this: `$SUM += /$regex/;` [download] Update: Sorry, mine only counts the number of matches (like `grep -c`). To accumulate the values of matches, I still instinctively reach for (something like) this: `nawk '/([0-9]*)/{tot+=$1}END{print tot}' file` [download]	[reply] [d/l] [select]
Re^2: reset of match variables by bart (Canon) on Oct 04, 2011 at 11:35 UTC
Warnings do nothing: if the regexp fails then it'll return false, which is 0 when treated as a number, not undef, and without a warning.	[reply]