PHP Regular expression to match keyword outside HTML tag

Question

I've been trying to do a regex to match and replace the occurrences of a keyword on a portion of HTML:

i want to match keyword and <strong>keyword</strong>
but <a href="someurl.html" target="_blank">keyword</a> and <a href="someur2.html">already linked keyword </a> should NOT be matched

I'm only interested in matching (and replacing) the keyword on the 1st line.

The reason I want this is to replace keyword with <a href="dictionary.php?k=keyword">keyword</s>, but ONLY if keyword it's not already inside an <a> tag.

Any help will be much appreciated!

I cleaned this up a bit, as the formatting was quite off, but I'm not sure my corrections are completely correct... tixastronauta, if my "fix" introduced mistakes, please edit and correct them. — eykanal, Oct 18 '11 at 00:10

Alan Moore · Answer 1 · 2011-10-18T16:42:38.360

3

$str = preg_replace('~Moses(?!(?>[^<]*(?:<(?!/?a\b)[^<]*)*)</a>)~i',
                    '<a href="novo-mega-link.php">$0</a>', $str);

The expression inside the negative lookahead matches up to the next closing </a> tag, but only if it doesn't see an opening <a> tag first. If that succeeds it means the word Moses is inside an anchor element, so the lookahead fails, and no match occurs.

Here's a demo.

edited Oct 18 '11 at 16:42

answered Oct 18 '11 at 16:08

Alan Moore

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Thank you Alan, but your regexp also replaces the keyword "`moses`" inside an `` tag.. So, in your sample: `but Moses supposes erroneously;` becomes: `but moses1.html">Moses supposes erroneously;` and I do not want this to happen. – tixastronauta Oct 18 '11 at 16:21
Sorry 'bout that. I was experimenting with using `\s` instead of `\b` in the lookahead (which didn't work), and accidentally left it in. – Alan Moore Oct 18 '11 at 16:44
How could this be amended to also NOT replace keywords within img alt="" attributes? – Martin Petts Jun 27 '13 at 15:23
Well, there's no point singling out `alt` attributes; any match that occurs inside a tag would be invalid for this purpose. The lookahead to prevent those matches is much simpler: `(?![^<]*+>)`. – Alan Moore Jun 27 '13 at 18:45
Thanks, but this had some unintended consequences where a word or phrase were matched by multiple keywords - it was putting tags within tags, e.g.: benefits">Compensation – Martin Petts Jul 04 '13 at 20:46

tixastronauta · Accepted Answer · 2011-10-18T15:20:52.203

I managed to do what I wanted (without using Regex) by:

parsing each character of my string
removing all <a> tags (copying them to a temporary array and keeping a placeholder on the string)
str_replace the new string in order to replace all the keywords
repopulating the placeholders by it's original <a> tags

Here's the code I used, in case someone else needs it:

$str = <<<STRA
Moses supposes his toeses are roses,
but <a href="original-moses1.html">Moses</a> supposes erroneously;
for nobody's toeses are posies of roses,
as Moses supposes his toeses to be.
Ganda <span class="cenas"><a href="original-moses2.html" target="_blank">Moses</a></span>!
STRA;

$arr1 = str_split($str);

$arr_links = array();
$phrase_holder = '';
$current_a = 0;
$goto_arr_links = false;
$close_a = false;

foreach($arr1 as $k => $v)
{
    if ($close_a == true)
    {
        if ($v == '>') {
            $close_a = false;
        } 
        continue;
    }

    if ($goto_arr_links == true)
    {
        $arr_links[$current_a] .= $v;
    }

    if ($v == '<' && $arr1[$k+1] == 'a') { /* <a */
        // keep collecting every char until </a>
        $arr_links[$current_a] .= $v;
        $goto_arr_links = true;
    } elseif ($v == '<' && $arr1[$k+1] == '/' && $arr1[$k+2] == 'a' && $arr1[$k+3] == '>' ) { /* </a> */
        $arr_links[$current_a] .= "/a>";

        $goto_arr_links = false;
        $close_a = true;
        $phrase_holder .= "{%$current_a%}"; /* put a parameter holder on the phrase */
        $current_a++;
    }    
    elseif ($goto_arr_links == false) {
        $phrase_holder .= $v;
    }
}

echo "Links Array:\n";
print_r($arr_links);
echo "\n\n\nPhrase Holder:\n";
echo $phrase_holder;
echo "\n\n\n(pre) Final Phrase (with my keyword replaced):\n";
$final_phrase = str_replace("Moses", "<a href=\"novo-mega-link.php\">Moses</a>", $phrase_holder);
echo $final_phrase;
echo "\n\n\nFinal Phrase:\n";
foreach($arr_links as $k => $v)
{
    $final_phrase = str_replace("{%$k%}", $v, $final_phrase);
}
echo $final_phrase;

The output:

Links Array:

Array
(
    [0] => <a href="original-moses1.html">Moses</a>
    [1] => <a href="original-moses2.html" target="_blank">Moses</a>
)

Phrase Holder:

Moses supposes his toeses are roses,
but {%0%} supposes erroneously;
for nobody's toeses are posies of roses,
as Moses supposes his toeses to be.
Ganda <span class="cenas">{%1%}</span>!

(pre) Final Phrase (with my keyword replaced):

<a href="novo-mega-link.php">Moses</a> supposes his toeses are roses,
but {%0%} supposes erroneously;
for nobody's toeses are posies of roses,
as <a href="novo-mega-link.php">Moses</a> supposes his toeses to be.
Ganda <span class="cenas">{%1%}</span>!

Final Phrase:

<a href="novo-mega-link.php">Moses</a> supposes his toeses are roses,
but <a href="original-moses1.html">Moses</a> supposes erroneously;
for nobody's toeses are posies of roses,
as <a href="novo-mega-link.php">Moses</a> supposes his toeses to be.
Ganda <span class="cenas"><a href="original-moses2.html" target="_blank">Moses</a></span>!

score 0 · Answer 3 · answered Oct 17 '11 at 19:59

0

$lines = explode( "\n", $content );
$lines[0] = stri_replace( "keyword", "replacement", $lines[0] );
$content = implode( "\n", $lines );

or if you explicitly want to use a regular expression

$lines = explode( "\n", $content );
$lines[0] = preg_replace( "/keyword/i", "replacement", $lines[0] );
$content = implode( "\n", $lines );

answered Oct 17 '11 at 19:59

Ben Swinburne

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those direct replaces replace all occurrences of "keyword". I just want to replace some of them. thanks nevertheless – tixastronauta Oct 17 '11 at 22:01

score -1 · Answer 4 · answered Oct 17 '11 at 20:01

Consider using an HTML parsing library rather than a regular expression, like simplehtmldom. You can use it to update the contents of specific HTML tags (therefore, ignoring ones you don't want to change). You wouldn't have to use a regex then; just use a function like str_replace once you've filtered the appropriate tags.

PHP Regular expression to match keyword outside HTML tag

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