Hi,

I tried to solve problem of nested parentheses. I have read in J.Friedl's book (Mastering Regular Expressions II ed.) about dynamic regexes - (??{code}).

The problem is so: You are given a sequence of (opening|closing) parentheses. What is the longest subsequence which is valid nested parentheses <for example: (), ()(), (()), ((())())>, and how much subsequences of that length occurs in the sequnce?

Here is the code how I am solving that problem: 

@lines = <>;

$n=1000;
$long = '';
srand;
# generating one random sequence of length $n:
while ($n--){
    $long .= qw<( )>[int (rand()*2) ]
    }
    
$m;
$m = qr/\((?:|(??{$m})++)*\)/; # main thing

for (@lines, $long){
    undef %occ;
    map { ++$occ{(length)} } /$m+/g;
    $max = 0;
    $max < $_ and $max = $_ for keys %occ;
    print $max," ",$occ{$max} || 1,$/;
    }

[download]
# Example input:
)((())))(()())
))(
()(())()
((((()(((
(()())()(())()()())())()((()(()(())()()())((()(())()(()()()()))()(())(
+)(((()())()(()((())()(())(()))

# Example output:
6 2
0 1
8 1
2 1
28 1
662 1

[download]

You can test it in: http://ideone.com/X8BUwy

Program works in different time, although $n is set to 1000. Sometimes it works 0.0 sec, sometimes >5 sec.

Any suggestions how to speed up regex? And how can someone determine when (with what sequences?) program will run slow?

In reply to how to speed up that dynamic regex? by rsFalse

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