Basic Regexp Question: How to modify only the lines starting with a STRING with sed

Question

I have to replace all ocurrences of: 5.6xx and 5.5xx (where x is a 0-9 digit) on a textfile with 5.500, but only when the line that contains the match starts with a string (e.g. STARTSTRING). That means

STARTSTRING 5.610 4.500 3.550 5.530
OTHERSTRING 5.600 5.500 5.500 5.600

should become

STARTSTRING 5.500 4.500 3.550 5.500
OTHERSTRING 5.600 5.500 5.500 5.600

I am not sure how can I do this. I am using sed for the editing.

Thanks a lot! :)

PS: If necessary, I am running the latest debian version.

Answer 1

sed -i -e '/^STARTSTRING/s/5\.[56][0-9]{2}/5.500/g' $file

Answer 2

sed -i -e '/^STARTSTRING/s/5\.[56][0-9][0-9]/5.500/g' file

Answer 3

you can use awk as well

$ cat file
STARTSTRING 5.610 4.500 3.550 5.530 35.530
OTHERSTRING 5.600 5.500 5.500 5.600 35.500

$ awk '/STARTSTRING/{  for(i=1;i<=NF;i++) if($i ~/^5\.[56][0-9][0-9]/) $i="5.500"} 1 ' file
STARTSTRING 5.500 4.500 3.550 5.500 35.530
OTHERSTRING 5.600 5.500 5.500 5.600 35.500

Note that the sed solutions doesn't take care of numbers like 35.530.