Hi

I think the X is to prevent the regex from matching. Isn't there a better way to do that?

That is a good observation

Maybe, there are http://perldoc.perl.org/perlre.html#Verbs for that, more on this at the end of this post

Taking a second look at output of (??{ versus (?{

v5.16.1 
[1 3]
[3 8]
[8 14]
DELETED:[1 3 8 ] REMAINS:[14 19 26 33
]
[14 19]
[19 26]
DELETED:[14 19 ] REMAINS:[26 33
]
[26 33]
DELETED:[26 ] REMAINS:[33
]
NUMBERS:[1:33
]
----------

[download]
[1 3]
DELETED:[1 ] REMAINS:[3 8 14 19 26 33
]
[3 8]
DELETED:[3 ] REMAINS:[8 14 19 26 33
]
[8 14]
DELETED:[8 ] REMAINS:[14 19 26 33
]
[14 19]
DELETED:[14 ] REMAINS:[19 26 33
]
[19 26]
DELETED:[19 ] REMAINS:[26 33
]
[26 33]
DELETED:[26 ] REMAINS:[33
]
NUMBERS:[1:33
]
----------

[download]

v5.18.2 
[1 3]
DELETED:[1 ] REMAINS:[3 8 14 19 26 33
]
[3 8]
DELETED:[3 ] REMAINS:[8 14 19 26 33
]
[8 14]
DELETED:[8 ] REMAINS:[14 19 26 33
]
[14 19]
DELETED:[14 ] REMAINS:[19 26 33
]
[19 26]
DELETED:[19 ] REMAINS:[26 33
]
[26 33]
DELETED:[26 ] REMAINS:[33
]
NUMBERS:[1:33
]
----------

[download]
[1 3]
[3 8]
[8 14]
DELETED:[1 3 8 ] REMAINS:[14 19 26 33
]
[14 19]
[19 26]
DELETED:[14 19 ] REMAINS:[26 33
]
[26 33]
DELETED:[26 ] REMAINS:[33
]
NUMBERS:[3:""33
]
----------

[download]

As you can see all three versions end the final result of 

NUMBERS:[1:33
]

[download]
they just get there by different order of add/delete, not sure how important this is

Only the crashing/buggy version returns something different when its able to complete under -Mre=debug

Now back to verbs, using 

$2 - $1 <= $c ? '(*FAIL)' : '(*ACCEPT)'

[download]
makes it fail in the exact same way, which is not too surprising

Using *PRUNE or $2 - $1 <= $c ? '(*SKIP)' : '(*ACCEPT)' and its just like using (?{

In reply to Re^3: Can't coerce UNKNOWN to string in substitution iterator (splain, (??{})) by beech
in thread Can't coerce UNKNOWN to string in substitution iterator by rsFalse

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