Regex: Extracting readable (non-code) text and URLs from HTML documents

Question

I am creating an application that will take a URL as input, retrieve the page's html content off the web and extract everything that isn't contained in a tag. In other words, the textual content of the page, as seen by the visitor to that page. That includes 'masking' out everything encapsuled in <script></script>, <style></style> and <!-- -->, since these portions contain text that is not enveloped within a tag (but is best left alone).

I have constructed this regex:

(?:<(?P<tag>script|style)[\s\S]*?</(?P=tag)>)|(?:<!--[\s\S]*?-->)|(?:<[\s\S]*?>)

It correctly selects all the content that i want to ignore, and only leaves the page's text contents. However, that means that what I want to extract won't show up in the match collection (I am using VB.Net in Visual Studio 2010).

Is there a way to "invert" the matching of a whole document like this, so that I'd get matches on all the text strings that are left out by the matching in the above regex?

So far, what I did was to add another alternative at the end, that selects "any sequence that doesn't contain < or >", which then means the leftover text. I named that last bit in a capture group, and when I iterate over the matches, I check for the presence of text in the "text" group. This works, but I was wondering if it was possible to do it all through regex and just end up with matches on the plain text.

This is supposed to work generically, without knowing any specific tags in the html. It's supposed to extract all text. Additionally, I need to preserve the original html so the page retains all its links and scripts - i only need to be able to extract the text so that I can perform searches and replacements within it, without fear of "renaming" any tags, attributes or script variables etc (so I can't just do a "replace with nothing" on all the matches I get, because even though I am then left with what I need, it's a hassle to reinsert that back into the correct places of the fully functional document).

I want to know if this is at all possible using regex (and I know about HTML Agility Pack and XPath, but don't feel like).

Any suggestions?

Update: Here is the (regex-based) solution I ended up with: http://www.martinwardener.com/regex/, implemented in a demo web application that will show both the active regex strings along with a test engine which lets you run the parsing on any online html page, giving you parse times and extracted results (for link, url and text portions individually - as well as views where all the regex matches are highlighted in place in the complete HTML document).

Answer 1

what I did was to add another alternative at the end, that selects "any sequence that doesn't contain < or >", which then means the leftover text. I named that last bit in a capture group, and when I iterate over the matches, I check for the presence of text in the "text" group.

That's what one would normally do. Or even simpler, replace every match of the markup pattern with and empty string and what you've got left is the stuff you're looking for.

It kind of works, but there seems to be a string here and there that gets picked up that shouldn't be.

Well yeah, that's because your expression—and regex in general—is inadequate to parse even valid HTML, let alone the horrors that are out there on the real web. First tip to look at, if you really want to chase this futile approach: attribute values (as well as text content in general) may contain an unescaped > character.

I would like to once again suggest the benefits of HTML Agility Pack.

ETA: since you seem to want it, here's some examples of markup that looks like it'll trip up your expression.

<a href=link></a> - unquoted
<a href= link></a> - unquoted, space at front matched but then required at back
<a href="~/link"></a> - very common URL char missing in group
<a href="link$!*'link"></a> - more URL chars missing in group
<a href=lïnk></a> - IRI
<a href
    ="link"> - newline (or tab)
<div style="background-image: url(link);"> - unquoted
<div style="background-image: url( 'link' );"> - spaced
<div style="background-image: u&#114;l('link');"> - html escape
<div style="background-image: ur\l('link');"> - css escape
<div style="background-image: url('link\')link');"> - css escape
<div style="background-image: url(\
'link')"> - CSS folding
<div style="background-image: url
('link')"> - newline (or tab)

and that's just completely valid markup that won't match the right link, not any of the possible invalid markup, markup that shouldn't but does match a link, or any of the many problems with your other technique of splitting markup from text. This is the tip of the iceberg.

Answer 2

Regex is not reliable for retrieving textual contents of HTML documents. Regex cannot handle nested tags. Supposing a document doesn't contain any nested tag, regex still requires every tags are properly closed.

If you are using PHP, for simplicity, I strongly recommend you to use DOM (Document Object Model) to parse/extract HTML documents. DOM library usually exists in every programming language.

Answer 3

If you're looking to extract parts of a string not matched by a regex, you could simply replace the parts that are matched with an empty string for the same effect.

Note that the only reason this might work is because the tags you're interested in removing, <script> and <style> tags, cannot be nested.

However, it's not uncommon for one <script> tag to contain code to programmatically append another <script> tag, in which case your regex will fail. It will also fail in the case where any tag isn't properly closed.

Answer 4

OK, so here's how I'm doing it:

Using my original regex (with the added search pattern for the plain text, which happens to be any text that's left over after the tag searches are done):

(?:(?:<(?P<tag>script|style)[\s\S]*?</(?P=tag)>)|(?:<!--[\s\S]*?-->)|(?:<[\s\S]*?>))|(?P<text>[^<>]*)

Then in VB.Net:

Dim regexText As New Regex("(?:(?:<(?<tag>script|style)[\s\S]*?</\k<tag>>)|(?:<!--[\s\S]*?-->)|(?:<[\s\S]*?>))|(?<text>[^<>]*)", RegexOptions.IgnoreCase)
Dim source As String = File.ReadAllText("html.txt")
Dim evaluator As New MatchEvaluator(AddressOf MatchEvalFunction)
Dim newHtml As String = regexText.Replace(source, evaluator)

The actual replacing of text happens here:

Private Function MatchEvalFunction(ByVal match As Match) As String
    Dim plainText As String = match.Groups("text").Value
    If plainText IsNot Nothing AndAlso plainText <> "" Then
        MatchEvalFunction = match.Value.Replace(plainText, plainText.Replace("Original word", "Replacement word"))
    Else
        MatchEvalFunction = match.Value
    End If
End Function

Voila. newHtml now contains an exact copy of the original, except every occurrence of "Original word" in the page (as it's presented in a browser) is switched with "Replacement word", and all html and script code is preserved untouched. Of course, one could / would put in a more elaborate replacement routine, but this shows the basic principle. This is 12 lines of code, including function declaration and loading of html code etc. I'd be very interested in seeing a parallel solution, done in DOM etc for comparison (yes, I know this approach can be thrown off balance by certain occurrences of some nested tags quirks - in SCRIPT rewriting - but the damage from that will still be very limited, if any (see some of the comments above), and in general this will do the job pretty darn well).

Answer 5

You cannot parse HTML with regular expressions.

Parsing HTML with regular expressions leads to sadness.

I know you're just doing it for fun, but there are so many packages out there than actually do the parsing the right way, AND do it reliably, AND have been tested.

Don't go reinventing the wheel, and doing it a way that is all but guaranteed to frustrate you down the road.

Answer 6

I suggest you read http://stackoverflow.com/questions/1732348/regex-match-open-tags-except-xhtml-self-contained-tags.