How can I create my own regex for "parse" HTML links?

Question

The strings looks like hyperlinks, such as http://somethings. This is what I need :

I need to check them only if they doesnt start with the character "; I mean, only that characters : if before there aren't characters it must check;
That somethings string means that every kind of characters can be used (of course, is a link) except a whitespace (The end marker link); I know, it's permitted by RFC, but is the only way I know to escape;
these string are previously filtered by using htmlentities($str, ENT_QUOTES, "UTF-8"), that's why every kind of characters can be used. Is it secure? Or I risk problems with xss or html broked?
the occurences of this replacement can me multiple, not only 1, and must be case insenstive;

This is my actual regex :

preg_replace('#\b[^"](((http|https|ftp)://).+)#', '<a class="lforum" href="$1">$1</a>', $str);

But it check only those string that START with ", and I want the opposite. Any helps answering to this question would be good, Thanks!

1 - you will need to split the words into individual strings. Then the regex would be [^"].* — Joe
– Joe, Commented May 9, 2011 at 11:27
[^"] should works, but if the word is the first word it doesnt (because before there isn't any characters. — kwichz
– kwichz, Commented May 9, 2011 at 11:30

ThiefMaster · Accepted Answer · 2011-05-09 11:28:39Z

2

For both of your cases you'll want lookbehind assertions.

\b(?<!")(\w)\b - negative lookbehind to match only if not preceded by "
(?<=ThisShouldBePresent://)(.*) - positive lookbehind to match only if preceded by the your string.

answered May 9, 2011 at 11:28

ThiefMaster

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kwichz Over a year ago

Edited the topic with some others further infos (because this in fact doesnt work so well); Please let me know, thanks for your time

Spudley · Accepted Answer · 2011-05-09 11:34:20Z

1

Something like this: preg_match('/\b[^"]/',$input_string);

This looks for a word-break (\b), followed by any character other than a double quote ([^"]).
Something like this: preg_match('~(((ThisShouldBePresent)://).+)~');

I've assumed the brackets you specified in the question (and the plus sign) were intended as part of the regex rather than characters to search for.

I've also taken @ThiefMaster's advice and changed the delimiter to ~ to avoid having to escape the //.

edited May 9, 2011 at 11:34

answered May 9, 2011 at 11:29

Spudley

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3 Comments

ThiefMaster Over a year ago

You might want to use a different delimiter for (2) so you don't have an escaping hell - # would do the job fine for example.

Spudley Over a year ago

@Thief - but I like having to escape my slashes; it makes regex syntax even more obtuse! yay! ;-) (but yes, I have edited it to use an alternative character)

kwichz Over a year ago

Tried your example, but it doesnt work (maybe we misunderstand). I edited my topic with a more concise explanation ;)

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