How to find a repeated string and the value between them using regexes?

Question

How would you find the value of string that is repeated and the data between it using regexes? For example, take this piece of XML:

<tagName>Data between the tag</tagName>

What would be the correct regex to find these values? (Note that tagName could be anything).

I have found a way that works that involves finding all the tagNames that are inbetween a set of < > and then searching for the first instance of the tagName from the opening tag to the end of the string and then finding the closing </tagName> and working out the data from between them. However, this is extremely inefficient and complex. There must be an easier way!

EDIT: Please don't tell me to use XMLReader; I doubt I will ever use my custom class for reading XML, I am trying to learn the best way to do it (and the wrong ways) through attempting to make my own.

Thanks in advance.

Answer 1

with Perl:

my $tagName = 'some tag';
my $i; # some line of XML
$i =~ /\<$tagName\>(.+)\<\/$tagname\>/;

where $1 is now filled with the data you captured

Answer 2

You can use: <(\w+)>(.*?)<\/\1>

Group #1 is the tag, Group #2 is the content.

Answer 3

You can use a backreference like \1 to refer to an earlier match:

@"<([^>]*)>(.*)</\1>"

The \1 will match what was captured by the first parenthesized group.

Answer 4

Using regular expressions to parse XML is a terrible error.

This is efficient (it doesn't parse the XML into a DOM) and simple enough:

string s = "<tagName>Data between the tag</tagName>";

using (XmlReader xr = XmlReader.Create(new StringReader(s)))
{
    xr.Read();
    Console.WriteLine(xr.ReadElementContentAsString());
}

Edit:

Since the actual goal here is to learn something by doing, and not to just get the job done, here's why using regular expressions doesn't work:

Consider this fairly trivial test case:

<a><b><a>text1<b>CDATA<![<a>text2</a>]]></b></a></b>text3</a>

There are two elements with a tag name of "a" in that XML. The first has one text-node child with a value of "text1", and the second has one text-node child with a value of "text3". Also, there's a "b" element that contains a string of text that looks like an "a" element but isn't because it's enclosed in a CDATA section.

You can't parse that with simple pattern-matching. Finding <a> and looking ahead to find </a> doesn't begin to do what you need. You have to put start tags on a stack as you find them, and pop them off the stack as you reach the matching end tag. You have to stop putting anything on the stack when you encounter the start of a CDATA section, and not start again until you encounter the end.

And that's without introducing whitespace, empty elements, attributes, processing instructions, comments, or Unicode into the problem.

Answer 5

Going forward, if you get stuck check out regexlib.com

It's the first place I go when i get stuck on regex