++ I like the thinking outside the box. That said, you can only do that if $str comes from a trusted source. If, for example, you get it from user input, try this as input: 

  abbr(?{system qw(format c:)})ev

[download]
Of course, that's only really a problem if you have enabled use re 'eval'; - but if you already need that for other purposes, you'll need to be very careful.

In fact, your "outside the box" is almost exactly described in the documentation for the (?{ code }) construct in perlre where it goes into a bit more detail and talks about ways to mitigate it.

In reply to Re^2: Abbreviation regex? by Tanktalus
in thread Abbreviation regex? by bsb

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