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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 clever for its own good, but maybe someone can fix it for me. There could be a free copy of the book in it for working fixes. :) In the section talking about lookarounds, I wanted to use a negative lookbehind to implement a commifying routine for numbers with fractional portions. The point was to use a negative lookbehind because that was the topic. I stupidly did this: $_ = '$1234.5678'; s/(?<!\.\d)(?<=\d)(?=(?:\d\d\d)+\b)/,/g; # $1,234.5678 The (?<!\.\d) asserts that the bit before the (?=(?:\d\d\d)+\b) is not a decimal point and a digit. The stupid thing is not trying hard enough to break it. By adding another digit to the end, there is now a group of three digits not preceded by a decimal point and a digit: $_ = '$1234.56789'; s/(?<!\.\d)(?<=\d)(?=(?:\d\d\d)+\b)/,/g; # $1,234.56,789 If lookbehinds could be variable width in Perl, this would have been really easy. But they can't. Note that it's easy to do this without a negative lookbehind, but that's not the point of the example. Is there a way to salvage this example?

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