Re: Perl Quiz of the Week #23

Mark Jason Dominus <[email protected]> writes:

[...]

> Write a program, 'parens', which gets a command line argument, n',
> which is an integer.  The program should print all the
> properly-balanced strings of parentheses of length 2n.  For example,
> given the argument '3', the program should print these five lines:

Here's a recursive solution using the regexp engine.
Basically I thought of a line like

        (()(()))

as being a regexp match by inserting an "x" after each "("

        (x(x)(x(x)))

Then I adapted the balanced parens pattern in perlre to
this particular problem, using $^R to keep track of the
pattern match.

#!/usr/bin/perl -l
# Usage: perl qotw23.pl n - generate list of balanced parens of length 2n

use re 'eval';
$r = qr/
    (?{ "$^R(" })      # insert "(" before first matching "x"
    (x(??{$r})* )      # actual (recursive) pattern
    (?{ "$^R)" }) /x;  # insert ")" after final matching "x"

/^$r*$(??{print $^R})/ for "x" x shift;


-- 
Joe Schaefer