Regex: Does an XPATH string point to an attribute?

Question

Here's what I'm using: ".+/@[^/]+$". Can you think of a reason why this might not work?

Answer 1

This is actually a very subtle problem and I think a great question.

My understanding is that an (abbreviated) XPATH points to an attribute if and only its last @ is not within a predicate, that is, something of the form [...], and has no steps after it (something like /...). I think this has the relatively simple regular expression @[^]/]*$, that is, there must be an @ that has no ]s nor /s after it. Also, if you want to cover unabbreviated XPATHs, you can use (@|attribute::)[^]/]*$

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.

Positive (an attribute)

• @* or @a or ../@a or a/@b
• a[@b and @c]/@d
• a[b[@c="d"]/e[@f and @g]]/h[@i="j"]/@k

Negative (not an attribute)

• a[@b] or a[@b and @c]
• a[b[@c and @d]/@e]
• a[b[@c="d"]/e[@f and @g]]/h[@i="j"]/k[5][@l="m"]

I can't think of a legal example where there is a / but not a ] after the last example, but I think there might be one.

Hopefully these examples make it at least a little clear that there can be arbitrary nesting of [ and ] together with @s anywhere in between. Luckily, I think only the very last @ and its nesting level matters.

(For reference, the OP's regex fails on @a. My original regex failed on a[@b and @c].)

Edit: 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, e.g. //@a// or //@a/. in the abbreviated syntax. There are also a variety of more creative ways, such as //@f//[node()]. All in all, it seems that if you want to cover these cases, you need to be able to match [ and ], which a basic regular expression cannot do. On the other hand, you could decide this is too contrived ...