XMLDecl from http://www.w3.org/TR/xml/#xmldoc

XML Declaration Regex.php

// XMLDecl from http://www.w3.org/TR/xml/#xmldoc
$regex = <<<'REGEX'
/
(?(DEFINE)
    (?<S> [\x20\x9\xD\xA]+ )
    (?<Eq> (?&S)? = (?&S)? )
)
^(?<XMLDecl>
    <\?xml
    (?<VersionInfo>
        (?&S) version (?&Eq)
        (?|
            '(?<VersionNum> 1\.[0-9]+ )' |
            "(?<VersionNum> 1\.[0-9]+ )"
        )
    )
    (?<EncodingDecl>
        (?&S) encoding (?&Eq)
        (?|
            '(?<EncName> [A-Za-z] (?:[A-Za-z0-9._] | -)*)' |
            "(?<EncName> [A-Za-z] (?:[A-Za-z0-9._] | -)*)"
        )
    )?
    (?<SDDecl>
        (?&S) standalone (?&Eq)
        (?|
            '(?<Standalone> yes | no)' |
            "(?<Standalone> yes | no)"
        )
    )?
    (?&S)? \?>
)
/x
REGEX;