RegExp problem

Question

Hi.

I have a problem creating regular expression for the following task:

Suppose we have HTML-like text of the kind:

<x>...<y>a</y>...<y>b</y>...</x>

I want to get a collection of values inside "y" tags located inside given "x" tag, so the result of the above example would be a collection of two elements ["a","b"].

additionaly we know that:

• "y" tags cannot be enclosed in other "y" tags
• ... can include any text or other tags.

Please, help with regexp.

Answer 1

This is a job for an HTML/XML parser. You could do it with regular expressions, but it would be very messy. There are examples in the page I linked to.

Answer 2

Short and simple: Use XPath :)

Answer 3

I'm taking your word on this:

"y" tags cannot be enclosed in other "y" tags

input looks like: <x>...<y>a</y>...<y>b</y>...</x>

and the fact that everything else is also not nested and correctly formatted. (Disclaimer: If it is not, it's not my fault.)

First, find the contents of any X tags with a loop over the matches of this:

<x[^>]*>(.*?)</x>

Then (in the loop body) find any Y tags within match group 1 of the "outer" match from above:

<y[^>]*>(.*?)</y>

Pseudo-code:

input = "<x>...<y>a</y>...<y>b</y>...</x>"
x_re  = "<x[^>]*>(.*?)</x>"
y_re  = "<y[^>]*>(.*?)</y>"

for each x_match in input.match_all(x_re)
  for each y_match in x_match.group(1).value.match_all(y_re)
    print y_match.group(1).value
  next y_match
next x_match

Pseudo-output:

a
b

---

Further clarification in the comments revealed that there is an arbitrary amount of Y elements within any X element. This means there can be no single regex that matches them and extracts their contents.

Answer 4

It would help if we knew what language or tool you're using; there's a great deal of variation in syntax, semantics, and capabilities. Here's one way to do it in Java:

String str = "<y>c</y>...<x>...<y>a</y>...<y>b</y>...</x>...<y>d</y>";
String regex = "<y[^>]*+>(?=(?:[^<]++|<(?!/?+x\\b))*+</x>)(.*?)</y>";
Matcher m = Pattern.compile(regex).matcher(str);
while (m.find())
{
  System.out.println(m.group(1));
}

Once I've matched a <y>, I use a lookahead to affirm that there's a </x> somewhere up ahead, but there's no <x> between the current position and it. Assuming the pseudo-HTML is reasonably well-formed, that means the current match position is inside an "x" element.

I used possessive quantifiers heavily because they make things like this so much easier, but as you can see, the regex is still a bit of a monster. Aside from Java, the only regex flavors I know of that support possessive quantifiers are PHP and the JGS tools (RegexBuddy/PowerGrep/EditPad Pro). On the other hand, many languages provide a way to get all of the matches at once, but in Java I had to code my own loop for that.

So it is possible to do this job with one regex, but a very complicated one, and both the regex and the enclosing code have to be tailored to the language you're working in.