Detlef Lindenthal wrote:
> ### Richard Smith wrote:
> ### "... but the previous value of $1 still seems to
> ### persist."
> ###
> ###
> ### Hi Richard,
> ###
> ### In RegEx, the variables $1, $2, $3 ... might carry on
> ### unwanted values from previous uses.
> ### How can we erase them?
> ### You can do this:
>
> $_ = "anything";
> m,(a),;
> print "\n(1.) \$1: ", $1; ## now $1 carries "a".
>
> ### You cannot erase the $1 variable in this way:
>
> # $1 = "";
> ### Error message: " # Modification of a read-only value attempted."
>
> ### But you can erase the $1 and $2 and $3 ... variables in this way:
>
> $_ = "anything";
> ### $1, $2, $3 do look for "nothing" and do find "nothing", thus
> ### $1, $2, $3 are erased:
> m,()()(),;
> print "\n(2.) \$1: ", $1;
>
>
> =Result:
>
> (1.) $1: a
> (2.) $1:
There are other ways of avoiding this problem. Consider the following:
#!/usr/bin/perl -l
use warnings;
use strict;
my @array;
while (<DATA>)
{
chomp;
next unless m/(\w+)\sstring/;
push @array, $1;# now we don't push a previous match into the array
print;
}
print join ", ", @array;
__DATA__
mystring
the string
any old string
some nonmatchingstring
sting like a bee
stringing me along
yet another string that matches
results:
3:55am {3} pcp02404936pcs:/home/webdragon>$ perl -c testperl2
testperl2 syntax OK
3:55am {4} pcp02404936pcs:/home/webdragon>$ perl testperl2
the string
any old string
yet another string that matches
the, old, another
basically you can avoid the effects of this by skipping unless there's a
match.
also things like:
,----[ surrounding loop (foreach, while, etc) ]
| #skip if no match
| next unless (my $name, $date) =~ m{^\s+(\w+\s+\w+)\s+(\d+\/\d+)\s+$}i;
| #do something with $name/$date...
`----
mean that those two variables will not be assigned to unless the regex
actually matches successfully, AND would be scoped to the loop around it
thus would not persist each iteration through the loop.
