Here are a handful of ways:
  1. Break the look-behind into two look-behinds. Make sure it's not preceded by a Y, and then make sure it's not preceded by the beginning of the string: 
    $string =~ s/(?<!Y)(?<!^)X//g;

    [download]
  2. Reverse the string and reverse the sense of the regex. Match an X not followed by an optional Y and then the end of the string: 
    my $rstr = reverse $string;
$rstr =~ s/X(?!Y|\Z)//g;  # note \Z, not $
$string = reverse $rstr;

    [download]
  3. Use my Regexp::Keep module (which I hope can be refactored to a standard regex assertion). It provides an "anchor", \K, which saves you from having to replace what you've matched with what you've matched. You'll see the difference here: 
    # old:
# $string =~ s/([^Y])X/$1/g;
# new:
$string =~ s/[^Y]\KX//;

    [download]
Regexp::Keep's \K anchor basically resets where Perl thinks it has started matching. See its documentation for more explanation.
_____________________________________________________
Jeff[japhy]Pinyan: Perl, regex, and perl hacker, who'd like a job (NYC-area)
s++=END;++y(;-P)}y js++=;shajsj<++y(p-q)}?print:??;

In reply to Re: Variable-width negative lookbehind by japhy
in thread Variable-width negative lookbehind by dragonchild

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