Match reverse translation of a capture group in Perl regex - regex

Question

I am trying to find strings that match a certain pattern and then the reverse translation of that pattern followed by it separated by a letter O.
Translation rule is /ABC/XYZ.
Example of a match: CCBAOXYZZ
First section matches the pattern [ABC]{3,25}. Then there's a letter O which also matches. Then we see that XYZZ is the reverse of CCBA with the translation above applied.
I have managed to write the tr rule into my backreferencing. But I cannot figure out how to do the reverse as well.
while (my $input_string = <sample_input>) {
push #hit, $1 while $input_string
=~ m{
(([ABC]{3,25})
O
(??{ $2 =~ tr/ABC/XYZ/r}))
}xg;
}
Is it correct to add the 'reverse' function to the third line of the regex in this way: (??{ $2 =~ tr/ACGT/TGCA/r;reverse}))?
How do I match the reverse tr of $2?

Answer 1

Your tr///r returns the transliterated string. So you simply need to stick your reverse in front of the tr///r and you're good to go.
push #hit, $1 while $input_string
=~ m{
(([ABC]{3,25})
O
(??{ reverse $2 =~ tr/ABC/XYZ/r }))
}xg;
The return value of the tr///r does not go into $_, so ; reverse will reverse whatever is in $_. That makes the overall match fail.
You actually answered your own question in your last sentence.
How do I do the match the reverse tr of $2?
If you add use re 'debug' you can see the actual pattern that is being matched against.
With tr///; reverse, the second part of that debugging output, which relates to the regex compiled from the eval, is:
...
Compiling REx "ZZYXOABCC"
Final program:
1: EXACT <ZZYXOABCC> (5)
5: END (0)
anchored "ZZYXOABCC" at 0 (checking anchored isall) minlen 9
Matching embedded REx "ZZYXOABCC" against "XYZZ"
...
As we can see here, it took the full string as the second part of the match, after the O. It correctly reversed the left side of the string, but it returned the full string.
Now if we compare that to reverse tr///r, we see the difference.
...
Compiling REx "XYZZ"
Final program:
1: EXACT <XYZZ> (3)
3: END (0)
anchored "XYZZ" at 0 (checking anchored isall) minlen 4
Matching embedded REx "XYZZ" against "XYZZ"
...
It now only returns the transliterated left side of the string, which then matches.