Perl Gotcha II

Don't exit an each loop unless you really know what you do.
Consider the following code

  1 my %h = ( one => 1, two => 2, three => 3 );
  2 sub find_key {
  3   my $val = shift;
  4   while ( my ($k,$v) = each %h ) {
  5     return $k if $v == $val;
  6   }
  7 }
  8 find_key(2) eq 'two' or die "key not found";
  9 # and again
 10 find_key(2) eq 'two' or die "key not found";

The call to find_key in line 10 fails, because it does not iterate through %h again, but continues where the last each call left off.

This is the documented behavior, but one might not expect it.
perldoc says:

The next call to "each" after that will start iterating again.  There is a
single iterator for each hash, shared by all "each", "keys", and "values"
function calls in the program; it can be reset by reading all the elements
from the hash, or by evaluating "keys HASH" or "values HASH".

Perl Gotcha I

I'm trying to write some interesting, sometimes unexpected, things one can experience with Perl in the next time.
They are partly based on a talk I did at the German Perl Workshop 2008, see my talk in german.


How does the regex

  m/foo$\nbar/m

gets interpreted?

At first it looks like it matches "foo", then end of line with the $, then a newline with \n and finally "bar".
Let' see:

  my $string = "foo\nbar";
  print $string =~/foo$\nbar/m ? 'match':'no match'

Surprisingly the regex does not match like expected.

Explanation:
The regex is interpreted as

  /foo $\ nbar/mx

e.g. it matches "foo", then the content of the special variable $\ (usually undef) and finally "nbar"