Rewriting a recursive regex for older Perl version

Question

The following piece of code works just fine with Perl (v5.16.2). However, when I run it using Perl v5.8.9, it complains about the following regex. How can I rewrite this regex in a way that works with Perl v5.8.9. (I can't update the version).

REGEX:

use strict;
use warnings;

our %formula_per_k;
INIT {
    # List all functions that you want to allow in formulas.  All other words will be interpretted as variables.
    my @FORMULA_FUNCS = qw(sqrt exp log);

    # Load the data via a file.
    my $data = do {local $/; <DATA>};

    # Parse K blocks
    while ($data =~ m{
        ^K \s+ (\w+) \s* \{
            ( (?: [^{}]+ | \{(?2)\} )* )         # Matched braces only.
        \}
    }mgx) {
        my ($name, $params) = ($1, $2);

        # Parse LOL block
        next if $params !~ m{
            LOL \s* \{ 
                ( (?: [^{}]+ | \{(?1)\} )*? )    # Matched braces only.
            \}
        }mx;
        my $lol = $1;

        # Start building anonymous subroutine
        my $conditions = '';

        # Parse Conditions and Formulas
        while ($lol =~ m{
            COND \s* \{ (.*?) \} \s* 
            FORMULA \s* \{ (.*?) \}
        }gx) {
            my ($cond, $formula) = ($1, $2);

            # Remove Excess spacing and translate variable into perl scalar.
            for ($cond, $formula) {
                s/^\s+|\s+$//g;
                s{([a-zA-Z]+)}{
                    my $var = $1;
                    $var = "\$hashref->{$var}" if ! grep {$var eq $_} @FORMULA_FUNCS;
                    $var
                }eg;
            }

            $conditions .= "return $formula if $cond; ";
        }

        my $code = "sub {my \$hashref = shift; ${conditions} return; }";

        my $sub = eval $code;
        if ($@) {
            die "Invalid formulas in $name: $@";
        }

        $formula_per_k{$name} = $sub;
    }
}

sub formula_per_k {
    my ($k, $vars) = @_;

    die "Unrecognized K value '$k'" if ! exists $formula_per_k{$k};

    return $formula_per_k{$k}($vars);
}

print "'K1', {d => .1}   = " . formula_per_k('K1', {d => .1}) . "\n";
print "'K1', {d => .05}  = " . formula_per_k('K1', {d => .05}) . "\n";
print "'K3', {d => .02}  = " . formula_per_k('K3', {d => .02}) . "\n";
print "'K3', {d => .021} = " . formula_per_k('K3', {d => .021}) . "\n";


__DATA__
... #OTHER STUFFS
K K1 {
    LOL {
        COND { d < 0.01 }
        FORMULA { -0.2 + 3.3*sqrt(d) }
        COND { d >= 0.01 }
        FORMULA { -0.2 + 3.3*sqrt(d+0.4) }
    }
}
... #OTHER STUFFS
K K2 {
    LOL {
        COND { d < 0.03 }
        FORMULA { -2.2 + 1.3*sqrt(d) }
        COND { d >= 0.03 }
        FORMULA { -2.2 + 1.3*sqrt(d+0.8) }
    }
}
... #OTHER STUFFS
K K3 {
    LOL {
        COND { d < 0.02 }
        FORMULA { -4.3 + 0.3*sqrt(d) }
        COND { d >= 0.02 }
        FORMULA { -4.3 + 0.3*sqrt(d+0.3) }
    }
}
... #OTHER STUFF

Outputs:

'K1', {d => .1}   = 2.13345237791561
'K1', {d => .05}  = 2.01370729772479
'K3', {d => .02}  = -4.13029437251523
'K3', {d => .021} = -4.13002941430942

ERROR:

Sequence (?1...) not recognized in regex; marked by <-- HERE in m/
            ^K \s+ M3 \s* {
                ( (?: [^{}]+ | {(?2 <-- HERE )} )* )         # Matched braces only.
            }
        / at ./code.pl line 215, <RFILE> line 12.

UPDATE: Code is updated. This was originally suggested by https://stackoverflow.com/users/1733163/miller

Answer 1

Before the introduction of (?PARNO), we had to use (??{ code }) to create recursive regular expressions. An example can be found in perlre - Extended Patterns.

The following is tested on v5.16.2, v5.20.0, and locally on a v5.8.9 perlbrew:

our $braces_re;
$braces_re = qr{
    \{
    (?:
        (?> [^{}]+ )
    |
        (??{ $braces_re })
    )*
    \}
}sx;

# parse FOO block
while (
    $data =~ m{
    ^FOO \s+ (\w+) \s* \{
        ( (?: [^{}]+ | (??{ $braces_re }) )* )         # Matched braces only.
    \}
}mgx
    )
{
    my $params = $1;

    # parse BAR block
    next if $params !~ m{
        BAR \s* \{ 
            ( (?: [^{}]+ | (??{ $braces_re }) )*? )    # Matched braces only.
        \}
    }mx;
    # SOME CODE
}

Note, I intentionally separated out the declaration of the _re variable and its initialization. There are some versions of perl that will let you declare a recursive regular expression in the same statement as the initialization, but v5.8.9 is not one of them.

Also, if you're comfortable altering your original regex more than just dropping in a replacement for (?PARNO) notation, then the above can be reduced to the following. Also confirmed on v5.16.2:

my $braces_re;
$braces_re = qr{
    (?:
        (?> [^{}]+ )
    |                                   # The following is a "postponed" regular subexpression.
        \{ (??{ $braces_re }) \}        # Deferred execution enables recursive regex
    )*
}sx;

# parse FOO block
while ( $data =~ m{^FOO \s+ (\w+) \s* \{ ( $braces_re ) \} }mgx ) {
    my $params = $1;

    # parse BAR block
    next if $params !~ m{BAR \s* \{ ( $braces_re ) \}}mx;
    # SOME CODE
}