rule xml :i {
    ^
    \s*
    [
        # Single tags like <foo/>
        \<
        \s*
        <[a-z:]>+
        (?:
            \s*<[a-z:]>*
            \s* = \s*
            (?:' <[^']>* ' | " <[^"]>* ")
        )*
        \s*
        /\s*
        \>
    |
        # Tags in pairs like <foo>content</foo>
        \<
        \s*
        $1 := (<[a-z:]>+)
        [
            \s*<[a-z:]>*
            \s* = \s*
            [ ' <[^']>* ' | " <[^"]>* " ]
        ]*
        \s*
        \>

        [ <[^<>]>* | <xml> ]*

        \< \s* / \s* $1 \s* \>
    ]
    \s*
}