Question

I’m not sure if this problem solvable with regular expressions (in Perl5 syntax), but here is self-explanatory example:

smth Y1 test X foo X Y2 bar X Y1 X X Y2
s/?/Z/g
smth Y1 test Z foo Z Y2 bar X Y1 Z Z Y2

Consider that Y1 always have a matching Y2 and there is no overlapping.

Answer 1

Here you go:

$str = 'smth Y1 test X foo X Y2 bar X Y1 X X Y2';
$str =~ s/X(?=((?!Y1).)*Y2)/Z/g;
print $str; #smth Y1 test Z foo Z Y2 bar X Y1 Z Z Y2