PHP regex prevent repeated string

Question

Im trying to create a regular expression in PHP which is checking that a string conforms to the following rules:

• Has 1 - 7 occurrences of one of the following strings (Mon,Tues,Wed,Thurs,Fri,Sat,Sun)
• Strings separated by a ,
• not case sensitive
• No strings are repeated.

I believe I have fulfilled the first three aspects of this with the following:

/^(Mon|Tues|Wed|Thurs|Fri|Sat|Sun)(,(Mon|Tues|Wed|Thurs|Fri|Sat|Sun)){0,6}$/i

I am struggling to get to grips with preventing any repeats. Can anyone advise?

Answer 1

^(Mon|Tues|Wed|Thurs|Fri|Sat|Sun)(?:,(?!\1|\2)(Mon|Tues|Wed|Thurs|Fri|Sat|Sun)){0,6}$

You could use this if regex is an absolute requirement but I'd rather recommend Martijn's answer. It is much more flexible and easier to read.

Here is how i tested this in PHP:

<?php

$subject1 = "Mon,Mon";
$subject2 = "Sun,Mon,Fri,Sun";
$subject3 = "Sat";
$subject4 = "Mon,Wed,Tues,Fri,Wed";
$subject5 = "Mon,Tues";
$pattern = '/^(Mon|Tues|Wed|Thurs|Fri|Sat|Sun)(?:,(?!\1|\2)(Mon|Tues|Wed|Thurs|Fri|Sat|Sun)){0,6}$/i';
print_r(preg_match($pattern, $subject1, $matches) . " " . $subject1 . "\n");
print_r(preg_match($pattern, $subject2, $matches) . " " . $subject2 . "\n");
print_r(preg_match($pattern, $subject3, $matches) . " " . $subject3 . "\n");
print_r(preg_match($pattern, $subject4, $matches) . " " . $subject4 . "\n");
print_r(preg_match($pattern, $subject5, $matches) . " " . $subject5 . "\n");

?>

This outputs:

0 Mon,Mon
0 Sun,Mon,Fri,Sun
1 Sat
1 Mon,Wed,Tues,Fri,Wed
1 Mon,Tues