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

##</code><code>##

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)