Re-arranging a timestamp with a Perl regex

Question

I'd like to re-arrange a timestamp with a Perl regex with the least code possible. The original format of the time stamp is as follows:

2011/12/29 20:19:45

All I need to do is convert it so that the year at the front instead comes after the month/date as follows:

12/29/2011 20:19:45

I was able to achieve this with the 3 lines of code below. I'm just wondering if there is a way to do this with less code. In particular I tried to do away with the middle line saving $1 into an intermediate variable, and specifying $1 from the first substitution directly in the regex for the second substitution, but this resulted in the error: "Use of uninitialized value $1 in concatenation (.) or string."

If the second line cannot be gotten rid of, then it would seem like this can't be gotten down to one line either?

#my $ts = '2011/12/29 20:19:45'; #input to a subroutine

$ts =~ s/^(\d{4})\///;
my $year = $1;
$ts =~ s/ /\/$year /;

Answer 1

Here are you go:

$ts =~ s|^(\d{4})/(\d{2})/(\d{2})(.+)$|$2/$3/$1$4|;

Please note that the above expression expects timestamps having exactly 2 digits for months and days and 4 digits for years. But you can make it even shorter yet more reliable:

$ts =~ s|^(\d+)/(\d+)/(\d+)(.+)$|$2/$3/$1$4|;

This one will handle timestamps like 1/12/98 12:34:56 properly.

Answer 2

Using Back References you can get the desired formatting -

[jaypal:~/Temp] echo "2011/12/29 20:19:45" | 
perl -pe 's@([0-9]{4})/([0-9]{2})/([0-9]{2})@$3/$1/$2@'
29/2011/12 20:19:45

Answer 3

For more generic timestamp manipulation you possibly want to roundtrip via strptime/strftime. In this case it's just field reordering, but for more complex cases such as turning month numbers into names, strftime will be required.

my @t = strptime "2011/12/29 20:19:45", "%Y/%m/%d %H:%M:%S";

say strftime "%d/%m/%Y %H:%M:%S", @t;    #  29/12/2011 20:19:45
say strftime "%d %b %Y, %H:%M:%S", @t;   #  29 December 2011, 20:19:45