"Assertion" is the wrong word here because unless you do gymnastics the code block has no affect on whether the expression succeeds or fails. On the rare occasion that I want to use perl code in an assertion (and this never happens for "real" code) you have to use the eval block in a conditional and then use zero-width assertions to simulate a true-false.

/(? # Use the conditional construct

   (?{ perl code goes here})  # Perl code that will assert something

   (?=)  # empty positive assertion.

   |

   (?!)  # empty negative assertion

 )/x

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In reply to Re: Re^6: Replace zero-width grouping? by diotalevi
in thread Replace zero-width grouping? by tinypig

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