Strip all HTML tags except links
-
09-06-2019 - |
Question
I am trying to write a regular expression to strip all HTML with the exception of links (the <a href
and </a>
tags respectively. It does not have to be 100% secure (I am not worried about injection attacks or anything as I am parsing content that has already been approved and published into a SWF movie).
The original "strip tags" regular expression I'm using was <(.|\n)+?>
, and I tried to modify it to <([^a]|\n)+?>
, but that of course will allow any tag that has an a in it rather than one that has it in the beginning, with a space.
Not that it should really matter, but in case anyone cares to know I am writing this in ActionScript 3.0 for a Flash movie.
Solution
<(?!\/?a(?=>|\s.*>))\/?.*?>
Try this. Had something similar for p tags. Worked for them so don't see why not. Uses negative lookahead to check that it doesn't match a (prefixed with an optional / character) where (using positive lookahead) a (with optional / prefix) is followed by a > or a space, stuff and then >. This then matches up until the next > character. Put this in a subst with
s/<(?!\/?a(?=>|\s.*>))\/?.*?>//g;
This should leave only the opening and closing a tags
OTHER TIPS
I keep going on about it, but there's no way I can recommend regexr too often. It's fantastic for testing this type of things.
In general there are problems with this approach. Regexes are best for 'flat' text matches - nested data pushes regex engines into areas for which they are not designed. General HTML parsing needs a parser not a regex engine (Google for the difference between regular and context-free languages if you want the full technical details).
It is easy to strip out all tags by replacing /</ and />/ with the empty string or their entity equivalents but selectively filtering HTML using regexes will be vulnerable to a wide range of accidental or malicious inputs breaking things.
Here you go:
{<(?!i|b|h[1-6]|/i|/b|/h[1-6][\s|>|/])[^>]*>}
How about
<[^a](.|\n)+?>
?
strip_tags()
does this.
Here, I am including all <a><p><font><b><i><sup>
tags and outputting a tidied version:
cat input.htm | tr -d '\n' | php -r '$input=fgets(STDIN); echo strip_tags($input,"<a><p><font><b><i><sup>");' | tidy -i -wrap 0 -o output.htm