Replace value if just first character of line matches in sed command - sed

Question

I am trying to replace the value with N.A if the first column is less than or equal to 5 means between 0 and 5 using this command :
sed -e '/^[0-5]/ s/2/N.A/g' snp.example.1 > result2
For instance input line,
4 2 0 0 2 0 0 2 0 2 0
Converted to:
4 N.A 0 0 N.A 0 0 N.A 0 N.A 0
But instead of just checking for the first character it also looks for the second character and replaces the values with N.A.
For instance input line should not be change as it's first column contains value (33) which is greater than 5:
33 2 2 2 2 2 2 2 2 2 2
But its also get converted:
33 N.A N.A N.A N.A N.A N.A N.A N.A N.A N.A
Your kind help will be highly appreciated.

Answer 1

You can do this with sed, but awk seems a better choice for making numerical comparisons:
awk '$1 < 6{gsub("2","N.A")}1' input

Answer 2

To restrict the changes to lines whose first number is 5 or less, try:
sed -e '/^[0-5] / s/2/N.A/g' example
Note the space afer [0-5].
For example, consider this input file:
$ cat example
4 2 0 0 2 0 0 2 0 2 0
33 2 2 2 2 2 2 2 2 2 2
Our command produces:
$ sed -e '/^[0-5] / s/2/N.A/g' example
4 N.A 0 0 N.A 0 0 N.A 0 N.A 0
33 2 2 2 2 2 2 2 2 2 2