I often find myself wanting to pass a regex as and argument to some function or another with the initial idea of passing the regex either as a string or a compiled regex (see the qr operator) and I invariably end up with problems. One example is for instance when I pass it as a string I mess up quoting ? / * . etc..
Another example of problems with this approach is the following, imagine that you have written a function to take a regex and you imagine searching all cells in an Excel sheet for cells matching this regex, as follows:
sub find_in_worksheet {
my $regex = shift;
my $workbook = shift;
my $worksheet_name = shift;
my $worksheet = $workbook->worksheet($worksheet_name);
my ( $row_min, $row_max ) = $worksheet->row_range();
my ( $col_min, $col_max ) = $worksheet->col_range();
for my $row ( $row_min .. $row_max ) {
for my $col ( $col_min .. $col_max ) {
my $cell = $worksheet->get_cell( $row, $col );
next unless $cell;
if( $cell->unformatted() =~ $regex ) {
return ($row, $col);
}
}
}
return undef;
}
That works fine, until inevetably you want the cell contents NOT to match the regex… *sigh*, what now…?
Well it turns out that a much more fruitful (and a more general) approach is as follows:
sub find_in_worksheet {
my $predicate = shift;
my $workbook = shift;
my $worksheet_name = shift;
my $worksheet = $workbook->worksheet($worksheet_name);
my ( $row_min, $row_max ) = $worksheet->row_range();
my ( $col_min, $col_max ) = $worksheet->col_range();
for my $row ( $row_min .. $row_max ) {
for my $col ( $col_min .. $col_max ) {
my $cell = $worksheet->get_cell( $row, $col );
next unless $cell;
if( $predicate->($cell->unformatted()) ) {
return ($row, $col);
}
}
}
return undef;
}
You would call this functions as follows:
my ($row, $col) = find_in_worksheet( sub { $_[0] =~ /regex to match cell contents/ } , $workbook, $worksheetname);
See the “sub { $_[0] =~ /regex to match cell contents/ }” part? There’s the magic. Now if you later on figure out that you want the cell contents NOT to match, you just call the method thusly:
my ($row, $col) = find_in_worksheet( sub { $_[0] !~ /regex to match cell contents/ } , $workbook, $worksheetname);
Volá!
Now you can even change the code for the predicate to something completely different, maybe not even involving regexes… for instance:
my ($row, $col) = find_in_worksheet( sub { $_[0] == 10 } , $workbook, $worksheetname);
This is just a small example of the power of using “sub’s” as arguments to functions and the very nice syntax in Perl for achieveing it. It also carries the benefit of giving you compile time checking of your regex! (Yes, Perl is a compiled language!)