Raku regex: How to use capturing group inside lookbehinds

Question

How can I use capturing groups inside lookbehind assertions?

I tried to use the same formula as in this answer. But that does not seem to work with lookbehinds.

Conceptually, this is what I was trying to do.

say "133" ~~ m/ <?after $0+> (\d) $ /

I know this can be easily achieved without lookbehinds, but ignore that just for now :)

For this I tried with these options:

Use :var syntax:

say "133" ~~ m/ <?after $look-behind+> (\d):my $look-behind; $ /;
# Variable '$look-behind' is not declared

Use code block syntax defining the variable outside:

my $look-behind;
say "133" ~~ m/ <?after $look-behind+> (\d) {$look-behind=$0} $ /;
# False

It seems that the problem is that the lookbehind is executed before the "code block/:my $var", and thus the variable is empty for the lookbehind tree.

Is there a way to use capturing groups inside lookbehinds?

Pastose

A, er, curiosity: say .<after> given foo ~~ m/ $ <after bar> / will work for any string in foo that ends with the pattern bar, which is fair enough... and displays a capture that's the string in foo in its entirety (not just bar) and entirely flipped (backwards)! 🤪 – Heinrick

"I know this can be easily achieved without lookbehinds, but ignore that just for now :)" I've ignored that for a day, but now's a new now. :) So, what is it that you're really trying to do? – Heinrick

Regexes in Raku are code. You wouldn't expect a variable to have data in it before you set it, so why would you expect the same from a Regex? – Kile

Hi @Heinrick I tried to solve Day 15 of advent of code using regexes (and inline code in them). I finally reversed the input numbers and used lookeaheads instead of lookbehinds. But, I figured there would exist a way to reference capturing groups in lookbehinds using raku, so I asked... :) – Pastose

@BradGilbert Well, I was not expecting anything xD, that was something I tried just in case... – Pastose

@Pastose Thanks. I figured out what you were asking after I posted my comments. In case you haven't yet figured this out, using jnthn's answer to your previous Q about lookahead (/ (a) :my $lookahead; <?before b {$lookahead = $/}> /), a simple translation of that to a lookbehind version solving this Q would be / (a) :my $lookbehind; { $lookbehind = $/ } <?after $lookbehind ** 2> $ /;. (Which is more or less the same as .@WiktorStribiżew's answer.) – Heinrick

Answer 1

When you reference a captured value before it is actually captured, it is not initialized, hence you can't get a match. You need to define the capturing group before actually using the backreference to the captured value.

Next, you need to define a code block and assign the backreference to a variable to be used throughout the regex pattern, else, it is not visible to the lookbehind pattern. See this Capturing Raku reference:

This code block publishes the capture inside the regex, so that it can be assigned to other variables or used for subsequent matches

You can use something like

say "133" ~~ m/ (\d) {} :my $c=$0; <?after $c ** 2> $ /;

Here, (\d) matches and captures a digit, then a code block is used to assign this captured value to a $c variable, and then the <?after $c ** 2> lookbehind checks if the $c value appears at least twice immediately to the left of the current location, and then the $ anchor checks if the current position is the end of the string.

See this online Raku demo.

Mandler

Superb! Nice trick to postpone the lookbehind and read again the captured group on it! – Pastose

It would be worth pointing out that the reason {} is needed is so that $/ gets updated. ( $0 is really a shortcut for $/[0]) – Kile