Why preg_match(“/[^(22|75)]/”, “25”) returns false?

Question

I want to test that a given string does not belong to the following group of strings: 22 75.

Could anyone please tell why PHP\'s preg_match(\"/[^(22|75)]/\

Answer 1

What are character classes

A character class is a collection of characters (not strings). With a character class, you're telling the regex engine to match only one out of several characters.

For example, if you wanted to match an a or e, you'd write [ae]. If you wanted to match grey or gray, you'd write gr[ae]y.

Explanation for first regex

[^(22|75)]

As said above, character classes match a single character from the list. Here, you're using ^ to get a negated character class, so this will match a single character that's not in the supplied list. In this case, our list contains the following characters:

( 2 2 | 7 5 )

Multiple characters are only counted once. So this effectively becomes:

( 2 | 7 5 )

25 is the string you're matching against. The regular expression asks: Does the supplied string contains a single character that's not in the above list? 2 and 5 are in the list, so the answer is No. That explains why preg_match() returns false (not false, 0 to be precise).

Explanation for second regex

/[^(22|76)]/

It is same as above. The only difference here is that 5 changed to 6. It now checks for the absense of any of the following characters:

( 2 | 7 6 )

The supplied string is still the same as before - 25. Does the string contain any character that's not in the list above? Yes! It does contain 5 (which is not in the list anymore). That explains why preg_match() returns 1.

Difference between character classes and alternation

They look similar but they do different things. Alternation can be used when you want to match a single regular expression out of several possible regular expressions. Unlike character classes, alternation works with a regex. A simple string, say foo is also a valid regular expression. It matches f followed by o, followed by o.

Use character class when you want to match one of the included characters. Use alternation when you want to match between n number of strings.

How should you modify the regex to obtain correct results

Negate your preg_match() call and use the regex (22|75):

if (!preg_match('/(22|75)/', '25')) {
    # code...
}

This is the easiest approach. If you want to achieve this directly using a regex, then you may want to use look-arounds.

Alternative solution

If this is exactly what you're trying to do, then you don't need a regular expression at all. Leverage PHP's built-in functions for string manipulation! Not only it will be faster, it will be more readable too.

In this case, a simple in_array() should suffice:

if(!in_array('25', array(25,75))) {
    # code ...
}