Regular Expression Inception (a match within a match) - regex

Question

I am trying to create a regular expression that captures a named group and then looks within that named group to check if it contains certain qualities.
For example. I have a regular expression that matches a code block and I can use it to match and capture code blocks:
test.pl:
use strict;
use warnings;
my $text = <<'END_TEXT';
block {
// random stuff
}
block {
dog
}
END_TEXT
my $code_block_rx = qr{(?(DEFINE)
(?<code_block>
block\h\{ (?: [^{}]++ | (?&code_block) )*+ \}
)
)}xms;
while ($text =~ m/(?<match>(?&code_block))$code_block_rx/g) {
print $+{match}."\n";
}
This code will print both code blocks. But what if I only want to capture code blocks that contain the word "dog"?
Is there a way (in a single regular expression) to capture a code block, and then if that is found, look within the code block for the word "dog"?
I've tried modifying the regex to use a look ahead assertion, but it just causes the whole thing to fail: /(?<match>(?=dog)(?&code_block))$code_block_rx/g
What am I missing?

Answer 1

You tried to match dog at the position where the match starts.
Instead, you can check if it's in the block you matched.
while ($text =~ /(
\b block \h*+ ( (?&code_block) )
(?(DEFINE)
(?<code_block> \{ (?&code_block_body) \} )
(?<code_block_body> (?: [^{}]++ | (?&code_block) )*+ )
)
)/xg) {
my $block_stmt = $1;
my $block_stmt_block = $2;
if ($block_stmt_block =~ /\b dog \b/x) {
say $block_stmt;
}
}
It can be done in a single pattern by using (?(?{!( assertion() )})(*FAIL)) to match against something you already captured.
while ($text =~ m{(
\b block \h*+
# A code_block that contains the word 'dog'.
( (?&code_block) ) (?(?{!( "$^N" =~ /\b dog \b/x )})(*FAIL))
(?(DEFINE)
(?<code_block> \{ (?&code_block_body) \} )
(?<code_block_body> (?: [^{}]++ | (?&code_block) )*+ )
)
)}xg) {
say $1;
}