When I wrote this post last night I originally intended to add comments to the compiled regular expressions using extended syntax. However, when I did that the code suddenly started generating a "Use of uninitialized value in concatenation (.) or string" warning in the expression and I couldn't see why. I can now see what the problem is but I'm puzzled as to why it is happening. If I put a variable name in the comment the warning appeared and I had to escape the $ sigil to make the warning go away, which is a bit worrying in what is supposed to be of no significance to the compiler. Here are examples of the regex and what happens in each case.

Original regular expression, no warning generated

$rxNumberGrps = qr
   {(?x)
       \D*
       (\d+)
       (?=,|\))
       (?{$deCommafied .= $^N})
       (?:
          (??{$rxNumberGrps})
          |
          \)\z
       )
   };
[download]

With offending comment added, warning generated

$rxNumberGrps = qr
   {(?x)
       \D*
       (\d+)
       (?=,|\))
       # append capture to $deCommafied
       (?{$deCommafied .= $^N})
       (?:
          (??{$rxNumberGrps})
          |
          \)\z
       )
   };
[download]

escape sigil, no warning generated

$rxNumberGrps = qr
   {(?x)
       \D*
       (\d+)
       (?=,|\))
       # append capture to \$deCommafied
       (?{$deCommafied .= $^N})
       (?:
          (??{$rxNumberGrps})
          |
          \)\z
       )
   };
[download]

use (?# ... ) style comment, no warning generated

$rxNumberGrps = qr
   {(?x)
       \D*
       (\d+)
       (?=,|\))
       (?# append capture to $deCommafied)
       (?{$deCommafied .= $^N})
       (?:
          (??{$rxNumberGrps})
          |
          \)\z
       )
   };
[download]

Other than the warning being generated, there was no effect on the running of the script and the same output was produced each run. It looks to me like the extended syntax comments aren't stopping the interpolation of variables in some way or another. Is this some sort of bug or am I missing something?

Cheers,

JohnGG

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