18. Greediness always favours matching

• Here is another classic example of where you need to be careful with greediness. Let's say you have some floating point numbers that you want to truncate to something reasonable - say to 2 decimal places if the third decimal place is zero, or to three decimal places otherwise.
• That seems pretty straight forward:

    #!/usr/bin/perl
    $number = "2.3451245678";
    $number =~ s/(\.\d\d[1-9]?)\d*/$1/;
    print "$number\n";
    $number = "5.190653417532";
    $number =~ s/(\.\d\d[1-9]?)\d*/$1/;
    print "$number\n";
  

• In other words, look for the literal decimal point, then two digits, then an optional "[1-9]", keeping all of that in $1, and finally any more digits, replacing it all with what we kept in $1. Seems pretty straight forward, huh?
• Indeed, the result is what we want:

    2.345
    5.19
  

• But what happens if we were to give it a number that was already truncated like we want? Say "2.562"? Well, the required literal decimal place, digit, digit, and then optional digit from "[1-9]" match, and then the "\d*" has nothing to match, and thus settles for matching zero times, and then the substitution replaces the number with itself - essentially a no-op.
• So, it's pretty tempting to re-write the regex so that it only does the substitution when it's necessary, and leaves already truncated numbers alone.
• Something like:

    $number =~ s/(\.\d\d[1-9]?)\d+/$1/;
  

  makes sense, yeah? Let's make those digits on the end that we don't want required, so that it only does the substitution when they are there.