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copied!<p>This is actually a very subtle problem and I think a great question.</p> <p>My understanding is that an (abbreviated) XPATH points to an attribute if and only its <em>last</em> <code>@</code> is not within a predicate, that is, something of the form <code>[...]</code>, and has no steps after it (something like <code>/...</code>). I think this has the relatively simple regular expression <code>@[^]/]*$</code>, that is, there must be an <code>@</code> that has no <code>]</code>s nor <code>/</code>s after it. Also, if you want to cover unabbreviated XPATHs, you can use <code>(@|attribute::)[^]/]*$</code></p> <p>I've included a test harness that may prove useful in checking this or other tests. Note also that there may be whitespace in between tokens which can complicate some regexs.</p> <h3>Positive (an attribute)</h3> <ul> <li><code>@*</code> or <code>@a</code> or <code>../@a</code> or <code>a/@b</code></li> <li><code>a[@b and @c]/@d</code></li> <li><code>a[b[@c="d"]/e[@f and @g]]/h[@i="j"]/@k</code></li> </ul> <h3>Negative (not an attribute)</h3> <ul> <li><code>a[@b]</code> or <code>a[@b and @c]</code></li> <li><code>a[b[@c and @d]/@e]</code></li> <li><code>a[b[@c="d"]/e[@f and @g]]/h[@i="j"]/k[5][@l="m"]</code></li> </ul> <p>I can't think of a legal example where there is a <code>/</code> but not a <code>]</code> after the last example, but I think there might be one.</p> <p>Hopefully these examples make it at least a little clear that there can be arbitrary nesting of <code>[</code> and <code>]</code> together with <code>@</code>s anywhere in between. Luckily, I think only the very last <code>@</code> and its nesting level matters.</p> <p>(For reference, the OP's regex fails on <code>@a</code>. My original regex failed on <code>a[@b and @c]</code>.)</p> <p><em>Edit</em>: It turns out that there are more corner cases, which convinces me that there is no perfectly-correct regular expression. For example, once you have an attribute node, there are many ways of keeping it, <em>e.g.</em> <code>//@a//</code> or <code>//@a/.</code> in the abbreviated syntax. There are also a variety of more creative ways, such as <code>//@f//[node()]</code>. All in all, it seems that if you want to cover these cases, you need to be able to match <code>[</code> and <code>]</code>, which a basic regular expression cannot do. On the other hand, you could decide this is too contrived ...</p>

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