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Asked: 2026-05-16T01:43:15+00:00

I need a regular expression that will allow only a to z and 0

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I need a regular expression that will allow only a to z and 0 to 9. I came across the function below on this site, but it allows a few symbols thru (#.-). How should it be done if it has to allow only a to z (both upper and lower case) and 0 to 9? I’m scared to edit it since I know nothing about regular expressions.

Also is this regular expression good to check for a to z and 0 to 9, or is there any way it can be bettered.

function isValid($str) {
    return !preg_match('/[^A-Za-z0-9.#\\-$]/', $str);
}

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1 Answer

  1. Editorial Team

    The following seems to be what you need in this case:

    function isValid($str) {
    return !preg_match('/[^A-Za-z0-9]/', $str);
}

    The […] regex construct is called a character class. Something like [aeiou] matches one of any of the vowels.

    The [^…] is a negated character class, so [^aeiou] matches one of anything but the vowels (which includes consonants, digits, symbols, etc).

    The -, depending on where/how it appears in a character class definition, is a range definition, so 0-9 is the same as 0123456789.

    Thus, the regex [^A-Za-z0-9] actually matches a character that’s neither a letter nor a digit. This is why the result of preg_match is negated with !.

    That is, the logic of the above method uses double negation:

    isValid = it's not the case that
              there's something other than a letter or a digit
                  anywhere in the string

    You can alternatively get rid of the double negation and use something like this:

    function isValid($str) {
    return preg_match('/^[A-Za-z0-9]*$/', $str);
}

    Now there’s no negation. The ^ and $ are the beginning and of the string anchors, and * is a zero-or-one-of repetition metacharacter. Now the logic is simply:

    isValid = the entire string from beginning to end
              is a sequence of letters and digits

    References

    Related questions

    Non-regex alternative

    Some languages have standard functions/idiomatic ways to validate that a string consists of only alphanumeric characters (among other possible string "types").

    In PHP, for example, you can use ctype_alnum.

    bool ctype_alnum ( string $text )

    Checks if all of the characters in the provided string , text, are alphanumeric.

    API links

    • PHP Ctype Functions – list of entire family of ctype functions
      • ctype_alpha, digit, lower, upper, space, etc
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