On May 21, 2009, at 8:36 PM, Steve Revilak wrote:
A few days ago, I was helping a co-worker debug a Perl program. In
the process of doing this, we found a bit of behavior that seemed
odd. Here's a minimal program to reproduce what we saw.
m// only sometimes means "match the empty pattern". Depending on
context, it can also mean "match the same pattern again".
--------- f1.pl -------------------------
use strict;
use warnings;
my $line = "hello world\r";
print "\t<" . ("tri state airport" =~ m//) . ">\n";
$line =~ s/\r//gs;
print "\t<" . ("tri state airport" =~ m//) . ">\n";
exit 0;
-----------------------------------------
The first m// has no previous pattern to reuse, so works as "match the
empty pattern".
The second does have a previous pattern, from the substitution, so
works as m/\r/. "tri state airport" lacks carriage returns, so
doesn't match.
--------- f2.pl -------------------------
use strict;
use warnings;
my $line = "hello world\r";
print "\t<" . ("tri state airport" =~ m//) . ">\n";
$line =~ tr/\r//d;
print "\t<" . ("tri state airport" =~ m//) . ">\n";
exit 0;
-----------------------------------------
This time, there is never a "previous pattern", since tr does
translations, not matching. So both mean "match empty pattern".
Or read what Uri posted before I finished typing this.
--kag
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