how to replace everything except string of interest - regex

Question

file.txt
fruits:banana,apple,grape,limon,orange,tomate,
fruits:apple,limon,
fruits:banana,grape,limon,
fruits:orange,tomate,grape,
fruits:banana,
fruits:apple,
fruits:banana,apple,
I need to replace everything that is different than "banana" for FRUIT, and get output like this:
fruits:banana,FRUIT,FRUIT,FRUIT,FRUIT,FRUIT,
fruits:FRUIT,FRUIT,
fruits:banana,FRUIT,FRUIT,
fruits:FRUIT,FRUIT,FRUIT,
fruits:banana,
fruits:FRUIT,
fruits:FRUIT,apple,
I tried using awk, but I can only replace the fields of specific strings.
Example replace all strings "apple" by fruit2, or all strings "apple" by fruit2 and all strings "tomate"or "orange" by fruit3
awk -F":" '{ gsub(/apple/,"FRUIT2",$2); print }' OFS="," file.tx
or
awk -F":" '{ gsub(/apple/,"FRUIT2",$2);;gsub(/tomate|orange/,"FRUIT3",$2); print }' OFS="," file.txt |sed "s/./:/7"
fruits:banana,FRUIT2,grape,limon,FRUIT3,FRUIT3,
fruits:FRUIT2,limon,
fruits:banana,grape,limon,
fruits:FRUIT3,FRUIT3,grape,
fruits:banana,
fruits:FRUIT2,
fruits:banana,FRUIT2
but I really need is to replace everything that is different from that for any string, ex: fruit4
How to generate output like this?
fruits:FRUIT4,FRUIT2,FRUIT4,FRUIT4,FRUIT3,FRUIT3,
fruits:FRUIT2,FRUIT4,
fruits:FRUIT4,FRUIT4,FRUIT4,
fruits:FRUIT3,FRUIT3,FRUIT4,
fruits:FRUIT4,
fruits:FRUIT2,
fruits:FRUIT4,FRUIT2

Answer 1

This awk should work:
awk -F, -v OFS=, '{
for (i=1; i<=NF; i++)
if ($i !~ /(^|:)banana$/)
sub(/[^:]+$/, "FRUIT", $i)
} 1' file
Output:
fruits:banana,FRUIT,FRUIT,FRUIT,FRUIT,FRUIT,
fruits:FRUIT,FRUIT,
fruits:banana,FRUIT,FRUIT,
fruits:FRUIT,FRUIT,FRUIT,
fruits:banana,
fruits:FRUIT,
fruits:banana,FRUIT,

Answer 2

To make the process automated, you can do
awk -F '[:,]' -v OFS=, '
{
for (i=2; i<=NF; i++)
if ($i)
if (seen[$i])
$i = seen[$i]
else
$i = seen[$i] = "FRUIT" ++n
sub(OFS, ":")
print
}
END {
print "map:"
for (key in seen)
print key "\t" seen[key]
}
' file
fruits:FRUIT1,FRUIT2,FRUIT3,FRUIT4,FRUIT5,FRUIT6,
fruits:FRUIT2,FRUIT4,
fruits:FRUIT1,FRUIT3,FRUIT4,
fruits:FRUIT5,FRUIT6,FRUIT3,
fruits:FRUIT1,
fruits:FRUIT2,
fruits:FRUIT1,FRUIT2,
map:
orange FRUIT5
tomate FRUIT6
apple FRUIT2
limon FRUIT4
banana FRUIT1
grape FRUIT3

Answer 3

If you'd like some flexibility in being able to specify your mapping of old to new names on the command line:
$ cat tst.awk
BEGIN {
FS="[:,]"; OFS=","
split(map,t)
for (i=1; i in t; i+=2) {
m[t[i]] = t[i+1]
}
}
{
printf "%s:", $1
for (i=2;i<=NF;i++) {
if ($i in m ) { $i = m[$i] }
else if ("*" in m) { $i = m["*"] }
printf "%s%s", $i, (i<NF?OFS:ORS)
}
}
.
$ awk -v map='apple,FRUIT2,tomate,FRUIT3,*,FRUIT4' -f tst.awk file
fruits:FRUIT4,FRUIT2,FRUIT4,FRUIT4,FRUIT4,FRUIT3,FRUIT4
fruits:FRUIT2,FRUIT4,FRUIT4
fruits:FRUIT4,FRUIT4,FRUIT4,FRUIT4
fruits:FRUIT4,FRUIT3,FRUIT4,FRUIT4
fruits:FRUIT4,FRUIT4
fruits:FRUIT2,FRUIT4
fruits:FRUIT4,FRUIT2,FRUIT4
$ awk -v map='apple,BAZINGA,*,VEGGIE' -f tst.awk file
fruits:VEGGIE,BAZINGA,VEGGIE,VEGGIE,VEGGIE,VEGGIE,VEGGIE
fruits:BAZINGA,VEGGIE,VEGGIE
fruits:VEGGIE,VEGGIE,VEGGIE,VEGGIE
fruits:VEGGIE,VEGGIE,VEGGIE,VEGGIE
fruits:VEGGIE,VEGGIE
fruits:BAZINGA,VEGGIE
fruits:VEGGIE,BAZINGA,VEGGIE
$ awk -v map='apple,FRUIT2,tomate,FRUIT3' -f tst.awk file
fruits:banana,FRUIT2,grape,limon,orange,FRUIT3,
fruits:FRUIT2,limon,
fruits:banana,grape,limon,
fruits:orange,FRUIT3,grape,
fruits:banana,
fruits:FRUIT2,
fruits:banana,FRUIT2,