Question

I’m new to pattern matching, having finally figured it out. I am stuck trying to find an approach to the following problem.

I need to return a match (with php preg_match) if any of a number html tags are present.

<p></p>
<br>
<h1></h1>
<h2></h2>

And return no match match otherwise. So anything not in the above list fails, e.g:

<script></script>
<table></table>

ect

…And ideally I want to operate a white list of safe tags if possible.

Anyone know a pattern that I can use/adapt?

Answer 1

preg_match_all('/<([a-z]*)\b[^>]*>(.*?)</\1>/i'$html,$matches);

Breaking down the expression

The first / is the delimiter

the < is the start of the tag, the very first <

the ([a-z]*) starts to match a tag name so fir instance < strong

the \b[^>]* says once you found a space, keep looking for all words

the > says it want the previous section to keep looking until it finds the very first >

the (.*?) says keep on looking and COLLECT ( .. ) the string inside but becuse we have a ? then stop looking when you find the next char after the closing brace.

the </\1> says i want to match but only if the value inside is the same as the very first match, this is done by \1 as in match , the value of this would be what's found with([a-z]*)`.

then you can use preg_match_all to find all them with contents, the array output would be something like

array(
    0 > THE WHOLE TAG
    1 > TAG NAME
    2 > TAG VALUE
)

Hope it helps 🙂

Exmaple

$allowed = array('b','strong','i','pre','code'); WHITELIST, never blacklist
foreach($matchas as $match)
{
    if(!in_array($match[1],$allowed))
    {
        echo sprintf('The tag %s is disallowed!',$match[1]);
    }
}