how to eval regular expression with embedded perl code

Question

So I tested out a regular expression that utilizes the experimental embedded code features. My tests worked, so I expounded upon it to do a more sophisticated script, but ran into errors. I traced the errors to a simple use of a variable in the regular expression not in the embedded code. I tried doing the regex in the suggested eval, but discovered that that wouldn't work because I could not access special variables after the eval'ed regular expression. I eventually re-wrote the code to not use the embedded code strategy, but am left curious as to why it wouldn't work. I simplified the problem in a pair of perl one-liners below: This works: ``` perl -e '$_ = "The brown fox jumps over the lazy dog ABC god yzal eht revo spmuj xof nworb ehT"; while (/(.{10,41})(?{$cap = $^N;$rev = r($cap);})(...)(??{$rev})/ig { print("$1
") } sub r { return(join("",reverse(split("",$_[0])))) }' ``` So why doesn't this?: ``` perl -e '$_ = "The brown fox jumps over the lazy dog ABC god yzal eht revo spmu…

Accepted Answer

You need to use

```
use re 'eval';
```

It lets Perl know you are aware that the pattern being interpolated can evaluate arbitrary code and that you're ok with that. It's lexically-scoped, so it will only affect the regular expressions in the file or in the curlies where it is used.

Since you have a one-liner, you can do the same using the command line option

```
-Mre=eval
```

how to eval regular expression with embedded perl code

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