s/
   .
   (?<!J(?=AZZ))
   (?<!JA(?=ZZ))
   (?<!JAZ(?=Z))
   (?<!JAZZ)
/a/xgs;

##</code><code>##

s/~/a/g;
s/JAZZ/~/g;
s/[^~]/a/g;
s/~/JAZZ/g;

##</code><code>##

s/(JAZZ|.)/$1 eq 'JAZZ' ? $1 : 'a'/egs;

##</code><code>##

s/(JAZZ)|./defined($1) ? $1 : 'a'/egs;

##</code><code>##

s/((?:JAZZ)*)./$1a/gs;