m/(?(?=Z|Y)[Z]|[Y])/

##</code><code>##

(?(DEFINE)
    (?<NAME_PAT>....)
)

##</code><code>##

print "Yes"
  if $string =~ m/(?(?=')'[^']|)
                  (?(?=")"[^"]|)/x;

##</code><code>##

m/(?:'[^']|"[^"])/