How do I use operations inside a Perl regex? - regex

Question

For example let's say I have something like this:
$_ = 23;
$a = 2;
print /$a $a+1/x;
should print 1. Basically, is it possible to use functions inside the regex string?

Answer 1

Variable interpolation in regexes works pretty much the same as variable interpolation in strings. Given my $x = 2, the string "$x $x+1" would be "2 2+1". The variable is expanded, but code in the string is not executed.
One trick around this is to use dereference a reference inside the string. This allows us to include arbitrary expressions, but the syntax is a bit cumbersome. Usually, we create an array reference with the value we want to include [$x + 1], then immediately dereference it: #{[$x + 1]}. This is similar to Ruby's #{...} interpolation, or to Bash $(...) command substitution.
So the regex /$x #{[$x + 1]}/x would work.
But in most cases, it's going to be much clearer to perform all calculations outside of the regex:
my $x = 2;
my $y = $x + 1;
/$x $y/x;
The Perl regex syntax also has syntax that can generate parts of the regex dynamically. With variable interpolation as above the variable contents are interpolated, and then the regex is compiled. But advanced regexes may change the value of a variable during the pattern match. These delayed regexes can be written with the (??{ ... }) syntax. Here: /$x (??{ $x + 1 })/x. However, this is a very advanced and error-prone regex feature. This will also be slower than an ordinary regex.

Answer 2

There is an extended pattern that provides for code execution in the match operator m/ or in the matching part of the substitution operator s///.
Its version that substitutes the code's return and goes on to treat it as a pattern is
/(??{ code })/
so in your case
$_ = 23;
my $x = 2;
my ($m) = /(2(??{ $x+1 }))/;
say $m;
or
RE_EVAL: {
use re 'eval';
my ($m) = /($x(??{ $x+1 }))/;
say $m;
}
matches and captures 23.
Here use re 'eval' specifically allows this, normally disallowed for security reasons.
This is a very involved capability which comes with complex warnings. Apart from its entry at the above link also follow the link in that text and read about Embedded Code Execution frequency.
Please don't use this complex tool for convenience, or to substitute for properly written code.