Convert date from file in Perl - perl

Question

On my program, I need to open my text file. Next I need to find date and convert. I don't know how to convert date with my code. Now in result I have mm / dd / yyyy. I'd like to change it to dd-mm-yyyy. Is it possible to do it with my code. How should I fix it?
#my program
use strict;
use warnings;
use feature 'say';
open my $file, 'file.txt' or die "Error\n";
my $re = qr/\d{1,2}\/\d{1,2}\/\d{1,4}/i; #mm/dd/yyyy
#my $re = qr/\d{1,2}-\d{1,3}/i; #postcode
while(my $fh = <$file>) {
if (my #match = $fh =~ /$re/g){;
say for #match;
}
}
#my file.txt
Today is 03.02.2020. Tommorow will be 03.03.2020.

Answer 1

To get the results of a match captured, you need to write parentheses around the parts you want to keep:
my $re = qr/(\d{1,2})\/(\d{1,2})\/(\d{1,4})/i; #mm/dd/yy
Also, your sample file.txt separates the components with dots and not slashes, so you need either change the file or the regular expression.

Answer 2

If the goal to change date format in file then this can be achieved with following one liner
perl -0777 -pe "s/\b(\d{2})\.(\d{2})\.(\d{2,4})\b/$1-$2-$3/g" -i file.txt
Options:
-0777 read file at once
-pe execute script in "..." and print out result
-i do replacement in place (original file modified to desired result)
Note:
MS Windows assume the script to be wrapped into "perl script", UNIX systems assume the script to be wrapped into 'perl script'.
\b can be omitted if there is not strict requirements for date be separated by punctuation.
Attention:
Please make a copy of original file just in case you make a mistake.

Answer 3

Goal:
substitute in file all dates represented in form 'dd.mm.yyyy' to 'dd-mm-yyyy'
Errors:
In OP's code regex is tuned to date format 'dd/mm/yyyy' what does not match data in input file 'dd.mm.yyyy'
Procedure:
open file, read file line by line, substitute date in 'dd.mm.yyyy' format to 'dd-mm-yyyy' format in each line all occurrances, output result to console, close the file, done
Note:
OP outputs date by one digit on each line to console -- definitely not what he intended [corrected], to make in place substitute in input file the code should be modified or see one liner provided.
use strict;
use warnings;
use feature 'say';
# Uncomment bellow to read from file
# my $filename = 'file.txt';
# open my $file, '<', $filename
# or die "Error\n";
# NOTE:
# In provided file you have dates in following format
# 03.02.2020
# but in question you refer to
# 03/02/2020
# The code adjusted accordingly, substitution applies
# only to dates matching re format in line (no need for 'if')
while(<DATA>) { # replace DATA with $file to read from a file
chomp; # snip eol
s/(\d{1,2})\.(\d{1,2})\.(\d{1,4})/$1-$2-$3/g; # substitute all date 0ccurrences in line
say;
}
__DATA__
#my file.txt
Today is 03.02.2020. Tommorow will be 03.03.2020.
Output
#my file.txt
Today is 03-02-2020. Tommorow will be 03-03-2020.