Question about nested expressions...

I'm trying to understand how the nesting capabilities work
in Pm's new
core markup expressions function and think it has to do with
these
main lines...

  $expr = preg_replace('/\(\W/e',
"Keep(PSS('$2'),'P')", $expr);
  while (preg_match('/\((\w+)(\s[^()]*)?\)/', $expr,
$match)) {
    list($repl, $func, $params) = $match;

But for the life of me it doesn't make too much sense. 
Would anyone
be willing to try and interpret this briefly in a little bit
more
accessible english?

TIA, a bunch!

Cheers,
Dan

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On Sun, Apr 29, 2007 at 10:46:03AM -0400, The Editor wrote:
> I'm trying to understand how the nesting capabilities
work in Pm's new
> core markup expressions function and think it has to do
with these
> main lines...
> 
>   $expr = preg_replace('/\(\W/e',
"Keep(PSS('$2'),'P')", $expr);
>   while (preg_match('/\((\w+)(\s[^()]*)?\)/',
$expr, $match)) {
>     list($repl, $func, $params) = $match;
> 
> But for the life of me it doesn't make too much sense. 
Would anyone
> be willing to try and interpret this briefly in a
little bit more
> accessible english?

Short:  It's finding the smallest matching paren set for
"(funcname ... )",
and breaking it down into
    $repl    - the entire string to be replaced
    $func    - the name of the function to be called
    $params  - any arguments to that function

Longer:  

The first line escapes any opening parens that aren't
followed by
an identifier character -- this is in case some of the
params
contain an unquoted open parenthesis.  The while loop then 
looks for any instance of an open paren, a function name
(captured to $1), and optionally a whitespace character
followed
by any sequence of characters excluding parens (captured to
$2), 
and a close paren.

Pm

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On 4/29/07, Patrick R. Michaud <pmichaudpobox.com> wrote:
> On Sun, Apr 29, 2007 at 10:46:03AM -0400, The Editor
wrote:
> > I'm trying to understand how the nesting
capabilities work in Pm's new
> > core markup expressions function and think it has
to do with these
> > main lines...
> >
> >   $expr = preg_replace('/\(\W/e',
"Keep(PSS('$2'),'P')", $expr);
> >   while (preg_match('/\((\w+)(\s[^()]*)?\)/',
$expr, $match)) {
> >     list($repl, $func, $params) = $match;
> >
> > But for the life of me it doesn't make too much
sense.  Would anyone
> > be willing to try and interpret this briefly in a
little bit more
> > accessible english?
>
> Short:  It's finding the smallest matching paren set
for "(funcname ... )",
> and breaking it down into
>    $repl    - the entire string to be replaced
>    $func    - the name of the function to be called
>    $params  - any arguments to that function
>
> Longer:
>
> The first line escapes any opening parens that aren't
followed by
> an identifier character -- this is in case some of the
params
> contain an unquoted open parenthesis.  The while loop
then
> looks for any instance of an open paren, a function
name
> (captured to $1), and optionally a whitespace character
followed
> by any sequence of characters excluding parens
(captured to $2),
> and a close paren.
>
> Pm


Thanks so much. It's coming into focus now.  I take it these
lines
actually evaluate the nested functions somehow, is that
correct?

    $code = $MarkupExpr[$func];
    ...
    $out = eval("return ({$code});");
    if ($expr == $repl) { $expr = $out; break; }
    $expr = str_replace($repl, Keep($out, 'P'), $expr);

Could you help briefly parse this for me too?  I really
appreciate
your explanations!

Cheers,
Dan

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On Sun, Apr 29, 2007 at 12:45:41PM -0400, The Editor wrote:
> Thanks so much. It's coming into focus now.  I take it
these lines
> actually evaluate the nested functions somehow, is that
correct?
> 
>    $code = $MarkupExpr[$func];
>    ...
>    $out = eval("return ({$code});");
>    if ($expr == $repl) { $expr = $out; break; }
>    $expr = str_replace($repl, Keep($out, 'P'), $expr);
> 
> Could you help briefly parse this for me too?  

$func is the name of the markup expression function to be
evaluated.  Based on this, we look up the code to be
executed 
in the $MarkupExpr array (indexed by the name of the
function) 
and store that in $code.  We then use eval() to evaluate the

code and store the result in $out.

If the overall expression we were evaluating is exactly the
string to be replaced (i.e., this isn't a nested
expression),
then we return $out directly.  Otherwise, this was possibly
a nested expression, so we replace the nested expression
with its result (using Keep() to make sure the result is
treated as a single parameter even if it contains quotes or
spaces), and then loop around to evaluate another
expression.

Pm

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