Rewriting a recursive regex for older Perl version - regex

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
}