Question

A system I am writing uses Markdown to modify links, but I also want to make plain links active, so that typing http://www.google.com would become an active link. To do this, I am using a regex replacement to find urls, and rewrite them in Markdown syntax. The problem is that I can not get the regex to not also parse links already in Markdown syntax.

I’m using the following code:

$value = preg_replace('@((?!\()https?://([-\w\.]+)+(:\d+)?(/([\w/_\.]*(\?\S+)?)?)?)@', '[$1]($1)', $value); 

This works well for plain links, such as http://www.google.com, but I need it to ignore links already in the Markdown format. I thought the section (?!() would prevent it from matching urls which followed a parenthesis, but it would seem that I am in error.

I realize that even this is not an ideal solution (if it worked), but this is pushing beyond my regex abilities.

Answer 1

I think (?<!\() is what you meant. If the match position is at the beginning of http://www.google.com, it’s not the next character you need to check, but the previous one. In other words you need a negative lookbehind, not a negative lookahead.