Validating Strings as Half width Katakana Using Regex in PHP

2025-08-08 2025-08-08

Tak@

To check if a string consists of half-width Katakana characters using regular expressions, use the pattern ^[ｦ-ﾟ]+$. This regex means "a string composed of one or more half-width Katakana characters." It will match strings like "ﾊﾟﾀｰﾝ" (pattern) and "ｳﾞｨｰﾅｽ" (Venus), which include chōonpu (long sound marks), dakuten (voicing marks), and handakuten (semi-voicing marks).

For this pattern to work correctly in PHP, you need to specify the PCRE_UTF8 modifier "u" to indicate that the character encoding is UTF-8, like this: /^[ｦ-ﾟ]+$/u.

Below are common regular expression patterns for basic half-width Katakana validation:

#	Match Condition	Regular Expression Pattern
1	All Half-width Katakana	^[ｦ-ﾟ]+$
2	Fixed-length n-digit Half-width Katakana	^[ｦ-ﾟ]{n}$
3	At least n-digit Half-width Katakana	^[ｦ-ﾟ]{n,}$
4	At most m-digit Half-width Katakana	^[ｦ-ﾟ]{1,m}$
5	Between n and m-digit Half-width Katakana	^[ｦ-ﾟ]{n,m}$

Regex Pattern List and Conditions

Source Code

Next, we introduce a PHP function that determines if an input string consists solely of half-width Katakana characters. This function can validate strings based on the following conditions:

If `minLength` is omitted: Checks if the string consists only of half-width Katakana characters and does not exceed the specified maximum length.
If `maxLength` is omitted: Checks if the string consists only of half-width Katakana characters and meets or exceeds the specified minimum length.
If both `minLength` and `maxLength` are omitted: Checks if the entire string consists solely of half-width Katakana characters.

/**
 * Checks if a string consists only of half-width Katakana characters.
 *
 * This function verifies if the string is composed entirely of half-width Katakana
 * characters and falls within the specified length range.
 *
 * @param string $str The input string.
 * @param ?int $minLength Minimum length (defaults to 1 if null).
 * @param ?int $maxLength Maximum length (defaults to the string's length if null).
 * @return bool Returns true if the input string meets the conditions, false otherwise.
 * @throws InvalidArgumentException Throws an exception if $minLength is less than 1, or if $maxLength is less than $minLength.
 */
function isHalfWidthKatakana(string $str, ?int $minLength = null, ?int $maxLength = null): bool {
    // Set default values
    $min = $minLength ?? 1;
    $max = $maxLength ?? mb_strlen($str);

    // Validate arguments
    if ($min < 1) {
        // Throw an exception if minLength is less than 1
        throw new InvalidArgumentException('Minimum length must be an integer of 1 or more.');
    }

    if (!is_null($maxLength) && $max < $min) {
        // Throw an exception if maxLength is less than minLength
        throw new InvalidArgumentException('Minimum length must be less than or equal to maximum length.');
    }

    // Construct the half-width Katakana regex pattern
    $pattern = is_null($maxLength)
        ? sprintf('/^[ｦ-ﾟ]{%d,}$/u', (int)$min) // If maxLength is unlimited
        : sprintf('/^[ｦ-ﾟ]{%d,%d}$/u', (int)$min, (int)$max);

    // Perform the regex check
    return (bool)preg_match($pattern, $str);
}