SUMMARY
Using patterns on little, limited pieces of reasonably well-defined pieces of HTML is quick and easy. But using them on an entire document containing fully general, open-ended HTML of unforeseeable quirks is, while theoretically possible, in practice much too hard compared with using someone else’s parser that’s already been written for that express purpose. See also this answer for a more general discussion on using patterns on XML or HTML.
Naïve Regex Solution
You’ve asked for a regex solution, so I will provide you such.
#!/usr/bin/perl
use 5.10.0;
use strict;
use warnings;
$/ = undef;
$_ = <DATA>; # read all input
while (m{ < \s* img [^>]* src \s* = \s* ['"]? ([^<>'"]+) }gsix) {
print "IMG SRC=$1\n";
}
while (m{ < \s* a [^>]* href \s* = \s* ['"]? ([^<>'"]+) }gsix) {
print "A HREF=$1\n";
}
while (m{ < \s* strong [^>]* > (.*?) < \s* / \s* strong \s* > }gsix) {
print "STRONG=$1\n";
}
__END__
<td class="MODULE_PRODUCTS_CELL" align="center" valign="top" height="100">
<table width="100" summary="products">
<tr>
<td align="center" height="75">
<a href="https://stackoverflow.com/collections.php?prod_id=50">
<img src="https://stackoverflow.com/questions/4284176/files/products_categories50_t.txt" border="0" alt="products" />
</a>
<br/>
</td>
</tr>
<tr>
<td align="center">
<a href="https://stackoverflow.com/collections.php?prod_id=50">
<strong>Buffer</strong><br />
</a>
<td>
</tr>
</table>
</td>
That program, when run, produces this output:
IMG SRC=files/products_categories50_t.txt
A HREF=/collections.php?prod_id=50
A HREF=/collections.php?prod_id=50
STRONG=Buffer
If you are quite certain that works for the particular specimen of HTML that you wish it to, then by all means use it. Notice several things that I do which you didn’t. One of them is not dealing with the HTML a line at a time. That virtually never works.
However, this sort solutions works only on extremely limited forms of valid HTML. You can only use it when you can guarantee that the HTML you’re working with really looks like what you expect it to.
The problem is that it quite often does not look all neat and tidy. For these situations, you are strongly advised to use an HTML parsing class. However, no one seems to have shown you the code to do that. That’s not very helpful.
Wizard-Level Regex Solution
And I’m going to be one of them myself. Because I am going to show you a more general solution for approaching what I believe your take to be, but unlike anyone else who ever posts on Stack Overflow, I’m going to use regexes to do it, just to show you that it can be done, but that you do not wish to do it this way:
#!/usr/bin/perl
use 5.10.0;
use strict;
use warnings;
$/ = undef;
$_ = <DATA>; # read all input
our(
$RX_SUBS,
$tag_template_rx,
$script_tag_rx,
$style_tag_rx,
$strong_tag_rx,
$a_tag_rx,
$img_tag_rx,
);
# strip stuff we aren't supposed to look at
s{ <! DOCTYPE .*? > }{}sx;
s{ <! \[ CDATA \[ .*? \]\] > }{}gsx;
s{ $style_tag_rx .*? < (?&WS) / (?&WS) style (?&WS) > }{}gsix;
s{ $script_tag_rx .*? < (?&WS) / (?&WS) script (?&WS) > }{}gsix;
s{ <!-- .*? --> }{}gsx;
while (/$img_tag_rx/g) {
my $tag = $+{TAG};
printf "IMG tag at %d: %s\n", pos(), $tag;
while ($tag =~
m{
$RX_SUBS
\b src (?&WS) = (?&WS)
(?<VALUE>
(?: (?"ed_value) | (?&unquoted_value) )
)
}gsix)
{
my $value = dequote($+{VALUE});
print "\tSRC is $value\n";
}
}
while (/$a_tag_rx/g) {
my $tag = $+{TAG};
printf "A tag at %d: %s\n", pos(), $tag;
while ($tag =~
m{
$RX_SUBS
\b href (?&WS) = (?&WS)
(?<VALUE>
(?: (?"ed_value) | (?&unquoted_value) )
)
}gsix)
{
my $value = dequote($+{VALUE});
print "\tHREF is $value\n";
}
}
while (m{
$strong_tag_rx (?&WS)
(?<BODY> .*? ) (?&WS)
< (?&WS) / (?&WS) strong (?&WS) >
}gsix)
{
my ($tag, $body) = @+{ qw< TAG BODY > };
printf "STRONG tag at %d: %s\n\tBODY=%s\n",
pos(), $+{TAG}, $+{BODY};
}
exit;
sub dequote {
my $string = shift();
$string =~ s{
^
(?<quote> ["'] )
(?<BODY>
(?: (?! \k<quote> ) . ) *
)
\k<quote>
$
}{$+{BODY}}gsx;
return $string;
}
sub load_patterns {
$RX_SUBS = qr{ (?(DEFINE)
(?<any_attribute>
\b \w+
(?&WS) = (?&WS)
(?:
(?"ed_value)
| (?&unquoted_value)
)
)
(?<unquoted_value>
(?&unwhite_chunk)
)
(?<quoted_value>
(?<quote> ["'] )
(?: (?! \k<quote> ) . ) *
\k<quote>
)
(?<unwhite_chunk>
(?:
# (?! [<>'"] )
(?! > )
\S
) +
)
(?<WS> \s * )
(?<end_tag>
(?&html_end_tag)
| (?&xhtml_end_tag)
)
(?<html_end_tag> > )
(?<xhtml_end_tag> / > )
) # end DEFINE
}six;
my $_TAG_SUBS = $RX_SUBS . q{ (?(DEFINE)
(?<attributes>
(?:
(?&WS)
(?&one_attribute)
) *
)
(?<one_attribute>
(?= (?&legal_attribute) )
(?&any_attribute)
)
(?<optional_attribute>
(?&permitted_attribute)
| (?&deprecated_attribute)
)
(?<legal_attribute>
(?: (?&required_attribute)
| (?&optional_attribute)
| (?&standard_attribute)
| (?&event_attribute)
# for LEGAL parse only, comment out next line
| (?&illegal_attribute)
)
)
(?<optional_attribute>
(?&permitted_attribute)
| (?&deprecated_attribute)
)
(?<illegal_attribute> \b \w+ \b )
(?<tag>
(?&start_tag)
(?&WS)
(?&attributes)
(?&WS)
(?&end_tag)
)
) # end DEFINE
}; # this is a q tag, not a qr
$tag_template_rx = qr{
$_TAG_SUBS
(?<TAG> (?&XXX_tag) )
(?(DEFINE)
(?<XXX_tag> (?&tag) )
(?<start_tag> < (?&WS) XXX \b )
(?<required_attribute> (*FAIL) )
(?<standard_attribute> (*FAIL) )
(?<event_attribute> (*FAIL) )
(?<permitted_attribute> (*FAIL) )
(?<deprecated_attribute> (*FAIL) )
) # end DEFINE
}six;
$script_tag_rx = qr{
$_TAG_SUBS
(?<TAG> (?&script_tag) )
(?(DEFINE)
(?<script_tag> (?&tag) )
(?<start_tag> < (?&WS) style \b )
(?<required_attribute> type )
(?<permitted_attribute>
charset
| defer
| src
| xml:space
)
(?<standard_attribute> (*FAIL) )
(?<event_attribute> (*FAIL) )
(?<deprecated_attribute> (*FAIL) )
) # end DEFINE
}six;
$style_tag_rx = qr{
$_TAG_SUBS
(?<TAG> (?&style_tag) )
(?(DEFINE)
(?<style_tag> (?&tag) )
(?<start_tag> < (?&WS) style \b )
(?<required_attribute> type )
(?<permitted_attribute> media )
(?<standard_attribute>
dir
| lang
| title
| xml:lang
)
(?<event_attribute> (*FAIL) )
(?<permitted_attribute> (*FAIL) )
(?<deprecated_attribute> (*FAIL) )
) # end define
}six;
$strong_tag_rx = qr{
$_TAG_SUBS
(?<TAG> (?&strong_tag) )
(?(DEFINE)
(?<strong_tag> (?&tag) )
(?<start_tag>
< (?&WS)
strong
\b
)
(?<standard_attribute>
class
| dir
| ltr
| id
| lang
| style
| title
| xml:lang
)
(?<event_attribute>
on click
on dbl click
on mouse down
on mouse move
on mouse out
on mouse over
on mouse up
on key down
on key press
on key up
)
(?<required_attribute> (*FAIL) )
(?<permitted_attribute> (*FAIL) )
(?<optional_attribute> (*FAIL) )
(?<deprecated_attribute> (*FAIL) )
) # end DEFINE
}six;
$a_tag_rx = qr{
$_TAG_SUBS
(?<TAG> (?&a_tag) )
(?(DEFINE)
(?<a_tag> (?&tag) )
(?<start_tag>
< (?&WS)
a
\b
)
(?<permitted_attribute>
charset
| coords
| href
| href lang
| name
| rel
| rev
| shape
| rect
| circle
| poly
| target
)
(?<standard_attribute>
access key
| class
| dir
| ltr
| id
| lang
| style
| tab index
| title
| xml:lang
)
(?<event_attribute>
on blur
| on click
| on dbl click
| on focus
| on mouse down
| on mouse move
| on mouse out
| on mouse over
| on mouse up
| on key down
| on key press
on key up
)
(?<required_attribute> (*FAIL) )
(?<deprecated_attribute> (*FAIL) )
) # end define
}xi;
$img_tag_rx = qr{
$_TAG_SUBS
(?<TAG> (?&image_tag) )
(?(DEFINE)
(?<image_tag> (?&tag) )
(?<start_tag>
< (?&WS)
img
\b
)
(?<required_attribute>
alt
| src
)
# NB: The white space in string literals
# below DOES NOT COUNT! It's just
# there for legibility.
(?<permitted_attribute>
height
| is map
| long desc
| use map
| width
)
(?<deprecated_attribute>
align
| border
| hspace
| vspace
)
(?<standard_attribute>
class
| dir
| id
| style
| title
| xml:lang
)
(?<event_attribute>
on abort
| on click
| on dbl click
| on mouse down
| on mouse out
| on key down
| on key press
| on key up
)
###########################
) # end DEFINE
}six;
}
UNITCHECK { load_patterns() }
__END__
<td class="MODULE_PRODUCTS_CELL" align="center" valign="top" height="100">
<table width="100" summary="products">
<tr>
<td align="center" height="75">
<a href="https://stackoverflow.com/collections.php?prod_id=50">
<img src="https://stackoverflow.com/questions/4284176/files/products_categories50_t.txt" border="0" alt="products" />
</a>
<br/>
</td>
</tr>
<tr>
<td align="center">
<a href="https://stackoverflow.com/collections.php?prod_id=50">
<strong>Buffer</strong><br />
</a>
<td>
</tr>
</table>
</td>
That program, when run, produces this output:
IMG tag at 304: <img src="https://stackoverflow.com/questions/4284176/files/products_categories50_t.txt" border="0" alt="products" />
SRC is files/products_categories50_t.txt
A tag at 214: <a href="https://stackoverflow.com/collections.php?prod_id=50">
HREF is /collections.php?prod_id=50
A tag at 451: <a href="https://stackoverflow.com/collections.php?prod_id=50">
HREF is /collections.php?prod_id=50
STRONG tag at 491: <strong>
BODY=Buffer
The Choice Is Yours — Or Is It?
Both those solve your problem with regexes. It is possible that you will be able to use the first of my two approaches. I cannot say, because like seemingly all such questions asked here, you haven’t told us enough about the data for us (and perhaps also you) to know for sure whether the naïve approach will suffice.
When it doesn’t, you have two choices.
- You can either use the more robust and flexible approach offered by my second technique. Just make certain that you understand it in all its aspects, because otherwise you won’t be able to maintain your code — and neither will anybody else.
- Use an HTML parsing class.
I find it unlikely that even 1 person in a 1000 would reasonably make the first of those two choices. In particular, I find it extremely unlikey that someone who asks for help with regexes as simple as those in my first solution would be a person capable of managing the regexes given in my second solution.
Which really leaves you with only one “choice” — if I may use that word so loosely.