Question

I was looking for some help replacing particular characters if based on the first 10 characters of a line. For example:

If the line is

test  (    2 ) [123456789]

I would like to replace it with

test  (    2 ) [123MMM789]

i.e, if the line starts with test ( 2 ) replace columns 19,20,21 with M

if the line starts with anything else, it can be left as is. Thank you in advance!

Answer 1

You can use a match and substr:

while (<>) {
  substr($_, 19, 3) = 'MMM' if /^test  \(    2 \)/;
  print;
}

This can also be applied to the command line directly, since it is pretty simple:

$ perl -pe 'substr($_, 19, 3) = "MMM" if /^test  \(    2 \)/' file.txt