CODEWITHSUNDEEP
phpregex
Anax
Anax

Reputation: 9372

Ensure a string contains specific characters, each at most once

I would like to test if a string is empty or if it only contain specific characters, each at most once.

Example:

Given $valid = 'ABCDE', the following strings are:

$a = ''; // valid, empty
$b = 'CE'; // valid, only contains C and E, each once
$c = 'AZ'; // invalid, contains Z
$d = 'DAA'; // invalid, contains A twice

Any quick way of doing this, (possibly) using regex?

Upvotes: 1

Views: 70

Answers (1)

Tim Biegeleisen
Tim Biegeleisen

Reputation: 520978

We can try using the following regex pattern:

^(?!.*(.).*\1)[ABCDE]{0,5}$

Here is an explanation of the regex:

^                    from the start of the string
    (?!.*(.).*\1)    assert that the same letter does not repeat
    [ABCDE]{0,5}     then match 0-5 letters
$                    end of the string

Sample PHP script:

$input = "ABCDE";
if (preg_match("/^(?!.*(.).*\1)[ABCDE]{0,5}$/", $input)) {
    echo "MATCH";
}

The negative lookahead (?!.*(.).*\1) works by checking if it can capture any single letter, and then also find it again later on in the string. Let's take the OP's invalid input DAA. The above negative lookahead would ffail when it matches and captures the first A, and then sees it again. Note carefully that lookarounds can have their own capture groups.

Upvotes: 4

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