Re: [MacPerl] regex help

[Ronald J Kimball <rjk@linguist.dartmouth.edu>]

On Sat, Nov 27, 1999 at 11:06:01AM -0800, Don Arbow wrote:
> On 11/27/99 at 1:31 AM, Nicholas G. Thornton wrote:

> :                          $back = $bug =~ tr/\.\.\//\.\.\//;

>  First, you're not using translate correctly.  You appear to want to
> count '../', but in this case that is not what translate does.  When you
> use =~, translate modifies $bug and assigns that modified string to
> $back, it does not return the count.  In order to count using tr///, your
> string needs to be in $_ and you must assign the count using =.  But
> translate only counts single character replacements, not patterns.  So
> each translation of '../' would return 3.

This is almostly entirely incorrect.

The =~ operator allows you to use tr/// (and m// and s///) on any scalar
value, not just $_.  However, use of =~ does _not_ affect tr///'s return
value.

$back = $bug =~ tr/\.\.\//\.\.\//; does not assign the modified string to
$back.  The expression modifies the value of $bug _in-place_, and assigns
the return value of tr/// to $back.

The return value of tr/\.\.\//\.\.\// here is not 3.  The target string in
the example is "../images/images.", which contains three periods and two
forward slashes, so the return value is 5.


> Use the substitute operator with the global option instead.  But first
> you'll have to move your string into $_.  So your code above should look
> like this:

You don't have to assign the string to $_, but you can if you want to.  If
you want to user another variable, the =~ operator allows you to do so.


> if ($_ =~ /<IMG usemap="#map1" src="(.*)titlebar\.GIF" width="467" height="60"
> border="0" align="top">/) {
>     $_ = $1;
>     $back = s/\.\.\//\.\.\//g;

It would be more correct to only count occurences of ../ at the beginning
of the string.

if ($_ =~ /<IMG\ usemap="#map1"\ src="(.*)titlebar\.GIF"\ width="467"
           \ height="60"\ border="0"\ align="top">/x) {
   my $path = $1;
   $count++ while $path =~ m,\G\.\./,g;



Ronald

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