I've just been to demerphq's 5.10 regex talk at YAPC::Europe and I've been overwhelmed by the number of new features in the regex syntax. However, unless I'm missing something in the talk and perlre there still seems no simple way to use Perl code as an assertion. When I say "as an assertion" I mean that I assert that some condition must be met if the (sub)pattern is to match.

The way I would assert in a 5.8 regex that some expression, say whatever(), must be true would be.

/(?(?{! whatever() })(?!))/
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Or

/(?(?{ whatever() })|(?!))/
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The latter can, in 5.10, be written...

/(?(?{ whatever() })|(*FAIL))/
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... which almost reads naturally as whatever or fail but this still seems unduly complex for what I would have thought would be a common desire. Indeed I've always felt that this should be the semantics of simple code constructs. (After all the documentation does call them assertions).

/(?{ whatever() })/
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The above, of course, does not work because code assertions are defined to always succeed.

Is there a neater way to do this in 5.10 (or indeed 5.8) than the ways I'd currently use in 5.8?

In previous discussion Ilya Zakharevich suggested that we could add flags between the closing brackets so perhaps we could use:

/(?{ whatever() }?)/
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