How do I substitute with an evaluated expression in Perl?

Question

There\'s a file dummy.txt

The contents are:

 9/0/2010
 9/2/2010
 10/11/2010

I have to change the month portion (0,2,11) to +1, ie, (1,3

Answer 1

Three changes:

• You'll have to use the e modifier to allow an expression in the replacement part.
• To make the replacement globally you should use the g modifier. This is not needed if you've one date per line.
• You use $1 on the replacement side, not a backreference

This should work:

$line =~ s{/(\d+)/}{'/'.($1+1).'/'}eg;

Also if your regex contains the delimiter you're using(/ in your case), it's better to choose a different delimiter ({} above), this way you don't have to escape the delimiter in the regex making your regex clean.