Thanks. My earlier code explained:

• my ($pre, $post) is short for (my $pre, my $post).

• A list-like expression on the LHS of = (such as my ($pre, $post)) causes the list assignment (aassign) operator to be used instead of a scalar assignment (sassign) operator.

• The list assignment operator evaluates its RHS operand in list context.

• In list context, m// (no "g") returns

  • () if the match failed,
  • the captures if the match succeeded and the pattern contains captures, or
  • 1 (one) if the match succeeded and the pattern contains no captures.

• The list assignment operator returns the number of elements returned by its RHS operand.

• The if condition will be false when the match fails because the list assignment operator returned 0 (zero) because the match operator returned zero elements.

• The if condition will be true when the match succeeds because the list assignment operator returned 2 (two) because the match operator returned two captures.

• And finally, each level of recursion of the function get its own pad, which means my ($pre, $post) creates new variables for each level of recursion, which means the they keep their value even through a recursive call.