Can you salvage my negative lookbehind example for commifying numbers?

Question

In the \"Advanced Regular Expresssion\" chapter in Mastering Perl, I have a broken example for which I can\'t figure out a nice fix. The example is perhaps trying to be too clev

Answer 1

I don't think it's possible without some form of variable-width look-behind. The addition of the \K assertion in 5.10 provides a way of faking variable-width positive look-behind. What we really need is variable-width negative look-behind but with a little creativity and a lot of ugliness we can make it work:

use 5.010;
$_ = '$1234567890.123456789';
s/(?

If there was ever a pattern that begged for the /x notation it's this one:

s/
  (?