SCRIPTS:
=====

regexp.pl (works like expected):
======

#!/usr/bin/perl -w

sub process_hashmark {
	my $s = shift @_;

	if ($s eq '') {
		print "Matched string:     empty string\n";
	} else {
		print "Matched string:     $s\n";
	}

	return 'XX' if ($s eq '');
	return 'ZZZ';
}

my $s;
my $regexp = 'X([^X]*?)X';

if (defined $ARGV[0]) {
	$s = $ARGV[0];
} else {
	$s = 'abcXdefXghi';
}

print "Regular Expression: $regexp\n";
print "String to process:  $s\n";

$s =~ s/$regexp/process_hashmark($1)/ge;

print "Processed String:   $s\n";
=====
regexp.pl (end)

regexp.php (works not like expected):
=====

function process_hashmark($string) {
	if ($string == "") {
		print "Matched string: empty string<br>\n";
	} else {
		print "Matched string: $string<br>\n";
	}
	
	if ($string == "") return "XX";
	return "ZZZ";
}

$regexp = "X([^X]*?)X";

if (empty($s)) $s = "abcXdefXghi";

echo "Regular Expression: $regexp<br>\n";
echo "String to process: $s<br>\n";

$s = preg_replace("/$regexp/e", "process_hashmark(\\1);", $s);

echo "Processed string: $s<br>\n";
=====
regexp.php (end)

OUTPUT:
=====
regexp.pl (case 1):
Regular Expression: X([^X]*?)X
String to process:  abcXdefXghi
Matched string:     def
Processed String:   abcZZZghi

regexp.pl (case 2):
Regular Expression: X([^X]*?)X
String to process:  abcXXdefXghi
Matched string:     empty string
Processed String:   abcXXdefXghi

regexp.php (case 1):
Regular Expression: X([^X]*?)X
String to process: abcXdefXghi
Matched string: def
Processed string: abcZZZghi

regexp.php (case 2):
Regular Expression: X([^X]*?)X
String to process: abcXXdefXghi
Matched string: XdefXghi
Processed string: abcZZZdefXghi

CONFIGURE LINE:
=====
./configure --with-apxs --with-pgsql=/usr/lib/pgsql --with-mysql=/usr

php.ini should be irrelevant and there is nothing special in our setup