Replace only the first character of a matched pattern

Question

I need to write a sed command that reads from a file and replaces the first character in each line with 0, only in the event that the first and second character in that line is a digit (and leave the rest of the lines intact).

Answer 1

You can find lines that begin with two digits using either ^[0-9][0-9] or ^[0-9]\{2\}

Then you can replace a single character with 0 using s/./0/

Putting it together,

sed '/^[0-9][0-9]/ s/./0/' somefile

or

sed '/^[0-9]\{2\}/ s/./0/' somefile