private function isValidLicenseString($license)
    {
        if (isset($this->licenses[strtolower($license)])) {
            return true;
        }

        $licenses = $this->getLicensesExpression();
        $exceptions = $this->getExceptionsExpression();

        $regex = <<<REGEX
{
(?(DEFINE)
    # idstring: 1*( ALPHA / DIGIT / - / . )
    (?<idstring>[\pL\pN.-]{1,})

    # license-id: taken from list
    (?<licenseid>${licenses})

    # license-exception-id: taken from list
    (?<licenseexceptionid>${exceptions})

    # license-ref: [DocumentRef-1*(idstring):]LicenseRef-1*(idstring)
    (?<licenseref>(?:DocumentRef-(?&idstring):)?LicenseRef-(?&idstring))

    # simple-expresssion: license-id / license-id+ / license-ref
    (?<simple_expression>(?&licenseid)\+? | (?&licenseid) | (?&licenseref))

    # compound-expression: 1*(
    #   simple-expression /
    #   simple-expression WITH license-exception-id /
    #   compound-expression AND compound-expression /
    #   compound-expression OR compound-expression
    # ) / ( compound-expression ) )
    (?<compound_head>
        (?&simple_expression) ( \s+ WITH \s+ (?&licenseexceptionid))?
            | \( \s* (?&compound_expression) \s* \)
    )
    (?<compound_expression>
        (?&compound_head) (?: \s+ (?:AND|OR) \s+ (?&compound_expression))?
    )

    # license-expression: 1*1(simple-expression / compound-expression)
    (?<license_expression>(?&compound_expression) | (?&simple_expression))
) # end of define

^(NONE | NOASSERTION | (?&license_expression))$
}xi
REGEX;

        $match = preg_match($regex, $license);

        if (0 === $match) {
            return false;
        }

        if (false === $match) {
            throw new \RuntimeException('Regex failed to compile/run.');
        }

        return true;
    }
private function isValidLicenseString($license)
    {
        if (isset($this->licenses[strtolower($license)])) {
            return true;
        }

        $licenses = $this->getLicensesExpression();
        $exceptions = $this->getExceptionsExpression();

        $regex = <<<REGEX
{
(?(DEFINE)
    # idstring: 1*( ALPHA / DIGIT / - / . )
    (?<idstring>[\pL\pN.-]{1,})

    # license-id: taken from list
    (?<licenseid>${licenses})

    # license-exception-id: taken from list
    (?<licenseexceptionid>${exceptions})

    # license-ref: [DocumentRef-1*(idstring):]LicenseRef-1*(idstring)
    (?<licenseref>(?:DocumentRef-(?&idstring):)?LicenseRef-(?&idstring))

    # simple-expresssion: license-id / license-id+ / license-ref
    (?<simple_expression>(?&licenseid)\+? | (?&licenseid) | (?&licenseref))

    # compound-expression: 1*(
    #   simple-expression /
    #   simple-expression WITH license-exception-id /
    #   compound-expression AND compound-expression /
    #   compound-expression OR compound-expression
    # ) / ( compound-expression ) )
    (?<compound_head>
        (?&simple_expression) ( \s+ WITH \s+ (?&licenseexceptionid))?
            | \( \s* (?&compound_expression) \s* \)
    )
    (?<compound_expression>
        (?&compound_head) (?: \s+ (?:AND|OR) \s+ (?&compound_expression))?
    )

    # license-expression: 1*1(simple-expression / compound-expression)
    (?<license_expression>(?&compound_expression) | (?&simple_expression))
) # end of define

^(NONE | NOASSERTION | (?&license_expression))$
}xi
REGEX;

        $match = preg_match($regex, $license);

        if (0 === $match) {
            return false;
        }

        if (false === $match) {
            throw new \RuntimeException('Regex failed to compile/run.');
        }

        return true;
    }
@param string $license

@throws \RuntimeException

@return bool