ruby - xpath expression for regex-like matching?

Question

I want to search div id in an html doc with certain pattern. I want to match this pattern in regex:

foo_([[:digit:]]{1.8})

using xpath. What is the xpath equivalent for the above pattern?

I'm stuck with //div[@id="foo_ and then what? If someone could continue a legal expression for it.

EDIT

Sorry, I think I have to elaborate more. Actually it's not foo_, it's post_message_

Btw, I use mechanize/nokogiri ( ruby )

Here's the snippet :

html_doc = Nokogiri::HTML(open(myfile))
message_div =  html_doc.xpath('//div[substring(@id,13) = "post_message_" and substring-after(@id, "post_message_") => 0 and substring-after(@id, "post_message_") <= 99999999]') 

Still failed. Error message:

Couldn't evaluate expression '//div[substring(@id,13) = "post_message_" and substring-after(@id, "post_message_") => 0 and substring-after(@id, "post_message_") <= 99999999]' (Nokogiri::XML::XPath::SyntaxError)

Answer 1

How about this (updated):

XPath 1.0:

"//div[substring-before(@id, '_') = 'foo' 
       and substring-after(@id, '_') >= 0 
       and substring-after(@id, '_') <= 99999999]"

Edit #2: The OP made a change to the question. The following, even more reduced XPath 1.0 expression works for me:

"//div[substring(@id, 1, 13) = 'post_message_' 
       and substring(@id, 14) >= 0 
       and substring(@id, 14) <= 99999999]"

XPath 2.0 has a convenient matches() function:

"//div[matches(@id, '^foo_d{1,8}$')]"

Apart from the better portability, I would expect the numerical expression (XPath 1.0 style) to perform better than the regex test, though this would only become noticeable when processing large data sets.

---

Original version of the answer:

"//div[substring-before(@id, '_') = 'foo' 
       and number(substring-after(@id, '_')) = substring-after(@id, '_') 
       and number(substring-after(@id, '_')) >= 0 
       and number(substring-after(@id, '_')) <= 99999999]"

The use of the number() function is unnecessary, because the mathematical comparison operators coerce their arguments to numbers implicitly, any non-numbers will become NaN and the greater than/less than tests will fail.

I also removed the encoding of the angle brackets, since this is an XML requirement, not an XPath requirement.