#!/bin/ksh
# Usage: tester [count [from [to]]]
#       Change the "count"th occurrence of the character specified by "from"
#       to the character specified by "to".  If not given on the command line,
#               "count" defaults to 150,
#               "from" defaults to "T", and
#               "to" defaujlts to "C".
awk -v cnt=${1:-150} -v from=${2:-T} -v to=${3:-C} 'BEGIN{tmpc = "\a"}
cnt>0 {
        # See if changing every "from" character on this line will go too far.
        if((n = gsub(from, from)) < cnt) {
                # No.  Reduce cnt by the number of "from" characters found and
                # print the unchanged line.
                cnt -= n
        } else {
                # The "from" character we need to change is on this line.
                # Change cnt - 1 "from" characters to "tmpc" characters.
                for(i = 1; i < cnt; i++) sub(from, tmpc)
                # Change the desired "from" charaacter to the "to" character.
                sub(from, to)
                # Change the "tmpc" characters inserted above back to "from"
                # characters.
                gsub(tmpc, from)
                # Note that we are done looking.
                cnt = 0
        }
        # Fall through to next action to print the processed line.
}
1
END {   if(cnt) {
                printf("Still looking for %d %s characters when EOF found.\n",
                        cnt, from)
                exit 1
        }
        exit 0
}' in