Re: On using $_ in subroutines

Bryan Harris wrote:
> Is it not possible to use $_ in subroutines?

Yes it is, just not the way you seem to want to use it.


> For example, my temptation was to do this:
>
> **************************************
> sub isDate {
>
>     $_ = shift;
>     if (m!\d{2}/\d{2}/\d{2}!) { return 1; }
>     else { return 0; }
>
>
> } 

Why is this in a subroutine at all?  If you are using it like:

if ( isDate() ) {  #use $_ by default

Then just do this instead:

if ( m!\d{2}/\d{2}/\d{2}! ) {

Or use a variable instead:

my $isDate = qr!\d{2}/\d{2}/\d{2}!;

...

if ( /$isDate/ ) {


> **************************************
>
> ... but by modifying $_ I was clobbering $_ elsewhere in the larger program!

Yes because $_ is a special global variable.  This effect is called 
"action at a distance" which is why it is better to use named lexically 
scoped variables instead of $_.


> Oddly, perl won't let me do "my ($_) = shift;", so I'm stuck having to use
> another variable.

Perl 5.10 *will* let you do "my $_".


> Why can't we do that?  Is using $_ in subroutines discouraged??

Yes, precisely for the problems that you discovered.



John
--
Those people who think they know everything are a great
annoyance to those of us who do.        -- Isaac Asimov

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