%
    ^
    \s*
    (                               # <1>
        <
        \s*
        ([a-zA-Z:]+)                # <2/>
        (?:
            \s*[a-zA-Z:]*
            \s* = \s*
            (?:'[^']*'|"[^"]*")
        )*
        \s*
        (/\s*)?                     # <3/>
        >

        (?:[^<>]* | (?1))*

        (?(3)|
            <\s*/\s*\2\s*>
        )
    )                               # </1>
    \s*
    $
%x

##</code><code>##

<foo><bar></bar></foo> # Match
<foo><bar></foo></bar> # No match
<foo><bar/></foo>      # Match *
<foo><bar></foo>       # No match
<foo bar=baz/>         # No match
<foo bar="baz">        # No match
<foo bar="baz"/>       # Match
<    fooo   /  >       # Match
<foo/>foo              # No match
foo<foo/>              # No match
<foo>foo</foo>         # Match
<foo><bar/>foo</foo>   # Match *
<a><b><c></c></b></a>  # Match *

This one matches as well * (wow)
<wml>
    <card id="mail" title="C: WAP POP3">
        <onevent type="onenterforward">
            <refresh>
                <setvar name="host" value="mail.some-domain"/>
            </refresh>
        </onevent>
        <p>
            Host:
            <input type="text" format="*m" name="host" value="$(host)"/>
            <br/>

            User:
            <input type="text" format="*m" name="user"/>
            <br/>

            Pass:
            <input type="password" format="*m" name="pass"/>
            <br/>

            <a href="pop3.cgi?host=$(host:e)&amp;user=$(user:e)&amp;pass=$(pass:e)">
                Login
            </a>
        </p>
    </card>
</wml>

This single regex can actually test XML well-formedness :)
(Doesn't take <?xml?> and <!DOCTYPE> etc, though)