I've also been trying to interprete the output from use re 'debug';, but the message "Match possible, but length=0 is smaller than requested=1, failing!" has me stumped.

But I did find a workaround for the problem. Adding a non-greedy 0 or 1 quantifier to the code assertion prevents it from being executed twice: 

my $rxBetween = qr/
    (?<= []>})] )
    (?=  [[<{(] )
    (?{ print "'$`'$&'$''" })??
/x;

[download]

This simplified (to reduce the trace output) version of your regex now produces 

C:\test>junk6 2>junk6.trace
Before: (x1)[x2]{x3}(x4)
----------------------------------------
'(x1)''[x2]{x3}(x4)'
'(x1)[x2]''{x3}(x4)'
'(x1)[x2]{x3}''(x4)'
----------------------------------------
After: (x1)+[x2]+{x3}+(x4)

[download]

I've had a go at coming up with an explanation ,from looking at the way it changes the debug trace...but I decided that I am well up on my mental torture quota for this month, so I'll let someone else have that pleasure :)

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.
"Too many [] have been sedated by an oppressive environment of political correctness and risk aversion."

In reply to Re: Regex code block executes twice per match using look-arounds by BrowserUk
in thread Regex code block executes twice per match using look-arounds by johngg

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