Replace and increment letters and numbers with awk or sed - bash

Question

I have a string that contains
fastcgi_cache_path /var/run/nginx-cache15 levels=1:2 keys_zone=MYSITEP:100m inactive=60m;
One of the goals of this script is to increment nginx-cache two digits based on the value find on previous file. For doing that I used this code:
# Replace cache_path
PREV=$(ls -t /etc/nginx/sites-available | head -n1) #find the previous cache_path number
CACHE=$(grep fastcgi_cache_path $PREV | awk '{print $2}' |cut -d/ -f4) #take the string to change
SUB=$(echo $CACHE |sed "s/nginx-cache[0-9]*[0-9]/&#/g;:a {s/0#/1/g;s/1#/2/g;s/2#/3/g;s/3#/4/g;s/4#/5/g;s/5#/6/g;s/6#/7/g;s/7#/8/g;s/8#/9/g;s/9#/#0/g;t a};s/#/1/g") #increment number
sed -i "s/nginx-cache[0-9]*/$SUB/g" $SITENAME #replace number
Maybe not so elegant, but it works.
The other goal is to increment last letter of all occurrences of MYSITEx (MYSITEP, in that case, should become MYSITEQ, after MYSITEP, etc. etc and once MYSITEZ will be reached add another letter, like MYSITEAA, MYSITEAB, etc. etc.
I thought something like:
sed -i "s/MYSITEP[A-Z]*/MYSITEGG/g" $SITENAME
but it can't works cause MYSITEGG is a static value and can't be used.
How can I calculate the last letter, increment it to the next one and once the last Z letter will be reached, add another letter?
Thank you!

Answer 1

Perl's autoincrement will work on letters as well as digits, in exactly the manner you describe
We may as well tidy your nginx-cache increment as well while we're at it
I assume SITENAME holds the name of the file to be modified?
It would look like this. I have to assign the capture $1 to an ordinary variable $n to increment it, as $1 is read-only
perl -i -pe 's/nginx-cache\K(\d+)/ ++($n = $1) /e; s/MYSITE\K(\w+)/ ++($n = $1) /e;' $SITENAME
If you wish, this can be done in a single substitution, like this
perl -i -pe 's/(?:nginx-cache|MYSITE)\K(\w+)/ ++($n = $1) /ge' $SITENAME

Answer 2

Note: The solution below is needlessly complicated, because as Borodin's helpful answer demonstrates (and #stevesliva's comment on the question hinted at), Perl directly supports incrementing letters alphabetically in the manner described in the question, by applying the ++ operator to a variable containing a letter (sequence); e.g.:
$ perl -E '$letters = "ZZ"; say ++$letters'
AAA
The solution below may still be of interest as an annotated showcase of how Perl's power can be harnessed from the shell, showing techniques such as:
use of s///e to determine the replacement string with an expression.
splitting a string into a character array (split //, "....")
use of the ord and chr functions to get the codepoint of a char., and convert a(n incremented) codepoint back to a char.
string replication (x operator)
array indexing and slices:
getting an array's last element ($chars[-1])
getting all but the last element of an array (#chars[0..$#chars-1])
A perl solution (in effect a re-implementation of what ++ can do directly):
perl -pe 's/\bMYSITE\K([A-Z]+)/
#chars = split qr(), $1; $chars[-1] eq "Z" ?
"A" x (1 + scalar #chars)
:
join "", #chars[0..$#chars-1], chr (1 + ord $chars[-1])
/e' <<'EOF'
...=MYSITEP:...
...=MYSITEZP:...
...=MYSITEZZ:...
EOF
yields:
...=MYSITEQ:... # P -> Q
...=MYSITEZQ:... # ZP -> ZQ
...=MYSITEAAA:... # ZZ -> AAA
You can use perl's -i option to replace the input file with the result
(perl -i -pe '...' "$SITENAME").
As Borodin's answer demonstrates, it's not hard to solve all tasks in the question using perl alone.
The s function's /e option allows use of a Perl expression for determining the replacement string, which enables sophisticated replacements:
$1 references the current MYSITE suffix in the expression.
#chars = split qr(), $1 splits the suffix into a character array.
$chars[-1] eq "Z" tests if the last suffix char. is Z
If so: The suffix is replaced with all As, with an additional A appended
("A" x (1 + scalar #chars)).
Otherwise: The last suffix char. is replaced with the following letter in the alphabet
(join "", #chars[0..$#chars-1], chr (1 + ord $chars[-1]))