src/Zend/Filter/Alpha.php
public function filter($value)
    {
        $whiteSpace = $this->allowWhiteSpace ? '\s' : '';
        if (!self::$_unicodeEnabled) {
            // POSIX named classes are not supported, use alternative a-zA-Z match
            $pattern = '/[^a-zA-Z' . $whiteSpace . ']/';
        } else if (self::$_meansEnglishAlphabet) {
            //The Alphabet means english alphabet.
            $pattern = '/[^a-zA-Z'  . $whiteSpace . ']/u';
        } else {
            //The Alphabet means each language's alphabet.
            $pattern = '/[^\p{L}' . $whiteSpace . ']/u';
        }

        return preg_replace($pattern, '', (string) $value);
    }
public function filter($value)
    {
        $whiteSpace = $this->allowWhiteSpace ? '\s' : '';
        if (!self::$_unicodeEnabled) {
            // POSIX named classes are not supported, use alternative a-zA-Z match
            $pattern = '/[^a-zA-Z' . $whiteSpace . ']/';
        } else if (self::$_meansEnglishAlphabet) {
            //The Alphabet means english alphabet.
            $pattern = '/[^a-zA-Z'  . $whiteSpace . ']/u';
        } else {
            //The Alphabet means each language's alphabet.
            $pattern = '/[^\p{L}' . $whiteSpace . ']/u';
        }

        return preg_replace($pattern, '', (string) $value);
    }
Defined by Zend_Filter_Interface

Returns the string $value, removing all but alphabetic characters

@param  string $value
@return string