19. Greediness always favours matching - continued

• 'Fraid not.
• While our new regex of:

    #!/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";
  

  still gives the same result:

    2.345
    5.19
  

  we now have a problem with:

    #!/usr/bin/perl
    $number = "2.562";
    $number =~ s/(\.\d\d[1-9]?)\d+/$1/;
    print "$number\n";
  

  which gives:

    2.56
  

• Yikes! That number was truncated right - where did that third decimal place go?
• Let's work the example to see why:

    Position in the regex | Position in the string
    ----------------------------------------------
     (\.\d\d[1-9]?)\d+    | 2.562
       ^                  |^
    ----------------------------------------------
     (\.\d\d[1-9]?)\d+    | 2.562
       ^                  | ^
    ----------------------------------------------
     (\.\d\d[1-9]?)\d+    | 2.562
       ^                  |  ^
    ----------------------------------------------
     (\.\d\d[1-9]?)\d+    | 2.562
       ^ ^                |  ^^
    ----------------------------------------------
     (\.\d\d[1-9]?)\d+    | 2.562
       ^ ^ ^              |  ^^^
    ----------------------------------------------
     (\.\d\d[1-9]?)\d+    | 2.562
       ^ ^ ^ ^^^          |  ^^^^
    ----------------------------------------------
     (\.\d\d[1-9]?)\d+    | 2.562
       ^ ^ ^ ^^^    ^     |  ^^^^
    ----------------------------------------------
  

• Nope, that's no good. There's nothing for the "\d+" to match - and "+" means one or more times - it's required. So, the engine will backtrack to the last good place:

    ----------------------------------------------
     (\.\d\d[1-9]?)\d+    | 2.562
       ^ ^ ^ ^^^          |  ^^^^
    ----------------------------------------------
  

  where it can try something different - which is to drop that optional "[1-9]" digit - which it does:

    ----------------------------------------------
     (\.\d\d[1-9]?)\d+    | 2.562
       ^ ^ ^              |  ^^^
    ----------------------------------------------
  

  and then it carries on from there:

    ----------------------------------------------
     (\.\d\d[1-9]?)\d+    | 2.562
       ^ ^ ^        ^     |  ^^^^
    ----------------------------------------------
  

• See how greediness always favours a match?