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 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?

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/(?<!\.)(?:\b|\G)\d+?\K(?=(?:\d\d\d)+\b)/,/g;
say;  # $1,234,567,890.123456789

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

s/
  (?<!\.)        # Negative look-behind assertion; we don't want to match
                 # digits that come after the decimal point.

  (?:            # Begin a non-capturing group; the contents anchor the \d
                 # which follows so that the assertion above is applied at
                 # the correct position.

    \b           # Either a word boundary (the beginning of the number)...

    |            # or (because \b won't match at subsequent positions where
                 # a comma should go)...

    \G           # the position where the previous match left off.

  )              # End anchor grouping

  \d+?           # One or more digits, non-greedily so the match proceeds
                 # from left to right. A greedy match would proceed from
                 # right to left, the \G above wouldn't work, and only the
                 # rightmost comma would get placed.

  \K             # Keep the preceding stuff; used to fake variable-width
                 # look-behind

                 # <- This is what we match! (i.e. a position, no text)

  (?=            # Begin a positive look-ahead assertion

    (?:\d\d\d)+  # A multiple of three digits (3, 6, 9, etc.)

    \b           # A word (digit) boundary to anchor the triples at the
                 # end of the number.

  )              # End positive look-ahead assertion.
/,/xg;