and for possible further illumination: 
$t = 'abcd';
$t1 = 'ababxycdcd';
$t2 = 'ababxycdcd';
$t3 = 'ababxycdcd';     # same as $t2, which is identical to $t1
$t4 = 'ababxycdcd';     # still the same...

$t =~ s/bc|xy/pq/;
print "\$t is: $t\n";

$t1 =~ s/(ab|xy)/pq/;
print "\$t1 is: $t1\n";

$t2 =~ s/(ab|xy)/pq/g;
print "\$t2 is: $t2\n";

$t3 =~ s/(ab)|(xy)/pq/g;
print "\$t3 is: $t3\n";

$t4 =~ s/((ab)|(xy))/pq\1/g;  #capture regex -outer (); grouping () in
+side
print "\$t4 is: $t4\n";

=head1 OUT
Output of C:\_perl\pl_test>perl 440581.pl
$t is: apqd             matched the 'b'
$t1 is: pqabxycdcd      parenthesdized the alt; now matches the FIRST 
+'ab'
$t2 is: pqpqpqcdcd      added /g (match globally) replaces both 'ab's 
+and the xy
$t3 is: pqpqpqcdcd      Shifting parens ==> no change, here
$t4 is: pqabpqabpqxycdcd  Capturing regex -- 3 'pq' pairs, each pair f
+ol by 'ab' or 'xy'
=cut

[download]
additional example added

In reply to Re^3: Regexp dashes at boundaries by ww
in thread Regexp dashes at boundaries by cormanaz

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