Using qw to check for balanced parentheses

I was playing around with qw today and it got me thinking... I'm probably not the first one to do this but I couldn't find it anywhere else. This works since qw can take any non-alphanumeric separator.

sub is_balanced {
   my $string = shift;

   eval("qw($string)");
   return 0 if $@;
   return 1;
}
[download]

Here it is in action:

my $balanced   = "(1 + 2 * (3 + (5/4) + 8) + 6)";
my $unbalanced1 = "(((1 + 2 * (3 + (5/4) + 8) + 6))";
my $unbalanced2 = "(((1 + 2 * (3 + (5/4) + 8) + 6))))";

printf("Is $balanced balanced? %s\n", balanced_parens($balanced) ? "ye
+s" : "no");
printf("Is $unbalanced1 balanced? %s\n", balanced_parens($unbalanced1)
+ ? "yes" : "no");
printf("Is $unbalanced2 balanced? %s\n", balanced_parens($unbalanced2)
+ ? "yes" : "no");
[download]

prints:

Is (1 + 2 * (3 + (5/4) + 8) + 6) balanced? yes
Is (((1 + 2 * (3 + (5/4) + 8) + 6)) balanced? no
Is (((1 + 2 * (3 + (5/4) + 8) + 6)))) balanced? no
[download]

Comment on Using qw to check for balanced parentheses Select or Download Code

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Re: Using qw to check for balanced parentheses (oops) by tye (Sage) on Feb 02, 2011 at 04:07 UTC
It took a long time and didn't give me the right answer for some reason: `Is );system('rm -rf /');# balanced? ~$#& NO CARRIER` [download] Update: While I was trying to get back into that server I whipped up how I'd do this: `sub balanced { my( $str )= @_; my $d= 0; while( $str =~ m(([(])\|([)]))g ) { if( $1 ) { $d++; } elsif( --$d < 0 ) { return 0; } } return 0 == $d; }` [download] - tye	[reply] [d/l] [select]
Re^2: Using qw to check for balanced parentheses (oops) by jdporter (Paladin) on Sep 16, 2011 at 19:06 UTC
`sub balanced { local $_ = shift; s/[^()]//g; eval "qw($_)"; ! $@ }` [download]	[reply] [d/l]
Re^3: Using qw to check for balanced parentheses (oops) by ikegami (Patriarch) on Sep 16, 2011 at 19:51 UTC
Simplified return value and shortened code by avoiding needless use of `$_`: `sub balanced { (my $s = shift) =~ s/[^()]//g; eval "qw($s); 1" }` [download] Or in 5.14+: `sub balanced { my $s = shift =~ s/[^()]//rg; eval "qw($s); 1" }` [download]	[reply] [d/l] [select]
Re^4: Using qw to check for balanced parentheses (oops) by Anonymous Monk on Oct 23, 2014 at 16:04 UTC
Re^3: Using qw to check for balanced parentheses (oop) by tye (Sage) on Sep 16, 2011 at 19:42 UTC
Very nice! - tye	[reply]
Re: Using qw to check for balanced parentheses by ikegami (Patriarch) on Feb 02, 2011 at 02:01 UTC
See also Text::Balanced	[reply]
Re: Using qw to check for balanced parentheses by jwkrahn (Abbot) on Feb 01, 2011 at 23:12 UTC
It doesn't appear to work? $ perl -le' sub is_balanced { my $string = shift; eval("qw($string)"); return 0 if @$; return 1; } my $balanced = "(1 + 2 * (3 + (5/4) + 8) + 6)"; my $unbalanced1 = "(((1 + 2 * (3 + (5/4) + 8) + 6))"; my $unbalanced2 = "(((1 + 2 * (3 + (5/4) + 8) + 6))))"; printf("Is $balanced balanced? %s\n", is_balanced($balanced) ? "yes" : + "no"); printf("Is $unbalanced1 balanced? %s\n", is_balanced($unbalanced1) ? " +yes" : "no"); printf("Is $unbalanced2 balanced? %s\n", is_balanced($unbalanced2) ? " +yes" : "no"); ' Is (1 + 2 * (3 + (5/4) + 8) + 6) balanced? yes Is (((1 + 2 * (3 + (5/4) + 8) + 6)) balanced? yes Is (((1 + 2 * (3 + (5/4) + 8) + 6)))) balanced? yes [download]	[reply] [d/l]
Re^2: Using qw to check for balanced parentheses by Anonymous Monk on Feb 01, 2011 at 23:16 UTC
typo `@$;` is not `$@;`	[reply] [d/l] [select]
Re^3: Using qw to check for balanced parentheses by jwkrahn (Abbot) on Feb 02, 2011 at 01:08 UTC
Yes, that works a lot better. `:-)` Of course, the subroutine could be reduced to just: `sub is_balanced { eval "qw($_[0])" }` [download] And it will also work with `qq` as well as with `qw`. $ perl -le' sub is_balanced { eval qq($_[0]) } my $balanced = "(1 + 2 * (3 + (5/4) + 8) + 6)"; my $unbalanced1 = "(((1 + 2 * (3 + (5/4) + 8) + 6))"; my $unbalanced2 = "(((1 + 2 * (3 + (5/4) + 8) + 6))))"; print "Is $balanced balanced? ", is_balanced( $balanced ) ? "yes" : "n +o"; print "Is $unbalanced1 balanced? ", is_balanced( $unbalanced1 ) ? "yes +" : "no"; print "Is $unbalanced2 balanced? ", is_balanced( $unbalanced2 ) ? "yes +" : "no"; ' Is (1 + 2 * (3 + (5/4) + 8) + 6) balanced? yes Is (((1 + 2 * (3 + (5/4) + 8) + 6)) balanced? no Is (((1 + 2 * (3 + (5/4) + 8) + 6)))) balanced? no [download]	[reply] [d/l] [select]
Re^4: Using qw to check for balanced parentheses by ikegami (Patriarch) on Feb 02, 2011 at 02:00 UTC
Re^4: Using qw to check for balanced parentheses by Anonymous Monk on Feb 02, 2011 at 01:13 UTC
Re^5: Using qw to check for balanced parentheses by Your Mother (Archbishop) on Feb 02, 2011 at 01:29 UTC
Some notes below your chosen depth have not been shown here
Re^3: Using qw to check for balanced parentheses by unix_hacker_beard (Novice) on Feb 02, 2011 at 01:27 UTC
D'oh! Thanks, I'm too used to dereferencing arrays.	[reply]
Re^2: Using qw to check for balanced parentheses by Anonymous Monk on Oct 23, 2014 at 16:07 UTC
I may be mistaken, but this does not detect a case like "}a{" , or does it? how about `sub balanced { (my $s = shift) =~ s/[^()]//g; while ( $s =~s /// ){}; return $s ? 0 : 1; }` [download] ? It may be slower, but detects ')(' as incorrect	[reply] [d/l]