Playing around with pattern subtitution

The other day I was given a cool task that I should replace the every second occurance of a character in a line. If there are only one of that special char (e.g. a colon) then do nothing. The list itself had tousands of newlines. Digging deep into this task I've collected some nice tricks around the net I wanted to record here.
#!/bin/bash

xxx="This:is:a:test"
echo "0:" `grep -o ":" <<< "$xxx" | wc -l ` # simple count
y="${xxx//[^:]}"        #pattern matching, y= all the chars that matches the char itself
echo "1: " "$y" # prints :::
echo "2: " ${#y} # stands for the lenght of a string = 3
echo "3: " `echo $xxx | awk -F":" '{print $NF}'` # finds the last occurence and cut the original string after there = test
echo "4: " `echo $xxx | awk -F":" '{print length($0)-length($NF)}' ` # similar to above but prints the found char position in the string = 14
end=${xxx##*:}
echo "5: Last : is in column $((${#xxx} - ${#end}))" # same as above
echo "6: " `sed 's/\(.*\):.*/\1/' <<< $xxx` # cuts the string at the last occurence of : and prints the first part
echo "7: " `sed 's/.*\:/\ /g' <<< $xxx` # cuts the string at the last occurence of : and prints the rest = test
echo "8: " `sed 's/\(.*\):/\1!/' <<< $xxx` # replaces the _last_ occurence of : with a !
echo "9: " $xxx| sed 's/t$/!/' # same as above what have to specify the last char
echo "10: " $xxx| sed 's/:/!/2' # replaces the second occurence of : with !
echo "11: " ${xxx##*:} # cuts the string at the last : and prints the rest = test
echo "12: " "${xxx#*:}" # cuts out the first word, prints the rest = "is:a:test"
echo "13: " ${xxx%:*}!!!${xxx##*:} # replaces the last occurence of : with the string: !!!
echo "14: " "${xxx%?}!"  # replaces the very last character of the string with !
echo "15: " ${xxx%:*} # cuts out the last part of the string using separator : ,selecting the first parts.
echo "16: " $xxx | sed "s/:[^:]*$//"  # cuts out the last part of the string using separator : ,selecting the first parts.
echo "17: " `sed -r "s/([^:]*:){2}//" <<< $xxx` # removes the first two parts separeted by : and prints the rest= "a:test"
echo "18: " "${xxx/:/!}" # replace the first occurence without using sed
echo "19: " "${xxx//:/!}" # replace all occurences of : without using sed
echo "20: " ${xxx:5:2} # for the sake of completion, prints = is. (2 chars from the 6th char)
echo "21: " ${xxx,} # converts the first char to lowercase
echo "22: " ${xxx,,} # concerts all to lowercase
echo "23: " "${0##*/}" # prints the name of the script without using basename
#echo $xxx | awk -F: '{print $1 $2 FS $3 $4}'