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Softpanorama Bulletin
Vol 23, No.09 (September, 2011)

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Perl regex HTML Matching Examples

A good example of usefulness of lazy quantifies is the solving of the problem of removing or modifing a specific tag from HTML.

So .*? matches zero or more times, but rather than skip as much text as possible before finding what following it in regex, it skips (consumes) as little as possible and stops at the first match. The following example well illustrates the difference:

# greedy pattern
s/<.*>/ /g;  # incorrect way to remove tag with all attributes
# non-greedy pattern
s/<.*?>/ /g; # more correct regex, althouth it still have problems

If there are more the one HTML tags on the line greedy regex will stops only at last ">" "eating" extra text in between.

Not let's try to remove some HTML tag, for example <tt>. Although the following regex looks promising

s{<tt>(.*)</tt>}{\1}gsi

it actually deletes everything from the first open <tt> tag through the last closing one. Which in case we have several <tt> tags on the line will lead to a nasty mistake.

For example this regex will turn

"Advanced editors like <tt>Slickedit</tt> can edit <tt>Perl</tt> code effectively." 

into

"Advanced editors like Slickedit </tt> can edit <tt>Perl code effectively."

I we can see opening <tt> was matched with </tt> from a different phrase producing broken HTML. This is a classic case where non-greedy quantifiers have an edge. If we change out regex to:

 s{<tt>(.*?)</tt>}{\1}gi

we have better chances, althouth it is easy to create an example where this regex also fail.

This approach doesn't remove tags from all possible HTML correctly, because a single regular expression is not an acceptable replacement for a real lexical parser.

Imagine if we were trying to pull out everything between bold-italic pairs:

<b><i>this</i> and <i>that</i> are important</b> Oh, <b><i>me too!</i></b>

A pattern to find only text between bold-italic HTML pairs, that is, text that doesn't include and closing tags, might appear to be this one:

m{<b></i>(.*?)</i></b>}sx

You might be surprised to learn that the pattern doesn't do that. Many people incorrectly understand this as matching a "<b><i>" sequence, then something that's not "<b>", and then "</b>", leaving the intervening text in $1. While often it works out that way due to the input data, that's not really what it says. This regex does not provides neither for nesting (and those tags can be nested at least to the level 2 in the string) nor for separate close of individual tags. It just matches the shortest leftmost substring that satisfies the entire pattern. In this case, that's the entire string. If you expect it to extract only stuff between "<b>" and its corresponding "</b>" on the same nesting level, ignoring any nested tags with bold tags, it would be incorrect.

To get correct result we need to use "negative matching" capability of Perl. Applying this to the regex above we will get something like:

m{<b><i>((?:(?!</b>|</i>).)*)</i></b>}sx

or better:

m{<b><i>((?:(?!</[ib]>).)*)</i></b>}sx

Jeffrey Friedl points out that this quick-and-dirty method isn't particularly efficient. He suggests crafting a more elaborate pattern when speed really matters, such as:

m{
    <b><i> 
    [^<]*  # stuff not possibly bad, and not possibly the end.
    (?:
 # at this point, we can have '<' if not part of something bad
     (?!  </?[ib]>  )   # what we can't have
     <                  # okay, so match the '<'
     [^<]*              # and continue with more safe stuff
    ) *
    </i></b>
 }sx

Substituting Bold Text HTML tags for Italic

HTML tags are enclosed in angle brackets "<" and ">". For example

<H3>culture</H3>

or if it contains additional attributes:

<title color="red">my bookmarks</TITLE>.
<a href="www.yahoo.com"> Yahoo </A>

In general, we cannot match all of these, with one regular expression, since they can be recursive, like:

<ul> Case 1  
       <ul> 
          <li> Subcase 1 
          <li> Subcase 2 
       </ul> 
</ul>
That means that closing tag for the particular opening tag can't be found by simple search -- you need to take into account the level of nesting of particular tag.

This can be done directly by calculating the nesting level, or indirectly by using recursion.

For many tags this nesting does not make sense and in well-formed HTML they are never nested. For example such HTML tags as <b>, <i> are never nested in well-formed HTML. The problem is that you never know if the HTML is well formed or not unless you pipe it via some sort of verifier.

To match:

<B> text <\B>

we could say:

m"<B>(.*?)<\B>"

Assuming, of course, that this tag isn't recursive and does not have any attributes.

A regular expression to match HTML tags:

If we now take (B|I) and substitute it with $pattern, where $pattern = 'B|I', we get:

m"<($pattern)(\s.*?)?>"

To match the first tag <B>. And to match the whole enchilada:

m"<($patterns)(\s.*?){0,1}>.*?</\1>"sg;

Here, \1 is whatever pattern we had found in '$patterns'. Hence, this matches the whole tag (in this case, the 'B' tag.)

<B>hold</B>

For our code example, Lets be a little bit simple, and only consider two possible strings to match: <B> and <I>. Furthermore, in this case, lets substitute bold text for italic, and vice-versa.

Decoding http, ftp tags

In <a> tag the href attribute contains the url of the object you are linking in the page. For example:

<a href="http://www.yahoo.com/index.html"
<a href="ftp://www.softpanorama.org/Scripting/best_practices.shtml">

For simplicity, lets suppose that they aren't split between lines. In order to extract them you need to detect the string <a href=" and then consume everything from it to closeind doble quote("). 

This translates into something that looks like this:

m{((?:ftp|http)\://\w+(?:\.\w+)*)}g

This did take a little trial and error. We use "(?:"  ")" brackets so we don't get any backreferences, and the (?:ftp|http) is self explanatory

Note that this pattern is not perfect. If you have backslashes spaces in the http tag in particular, this won't work. Or, if you have a http daemon running on a different port (http://site:8080 for example) it won't work. However, it is as we say 'close enough': if you want to improve it to handle such cases, go right ahead.

Let's now make a loop which extracts all http, and ftp tags from a given file, for example a bookmarks file:

undef $/; # read all the file as a single string
my $fd = new FileHandle("$ARGV[0]");
$line = <$fd>; # now the while file is in $line
while ( $line =~ m{((?:ftp|http)\://\w+(?:\.\w+)*)}g ){
   my $tag = $1;
   chop($tag);
   push(@tags, $tag);
}

Here, we chop off the last character, since in a bookmarks file these tags are in double quoted strings. We shall approach this problem more directly next, when we consider matching a double quoted string.

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