php - regex to turn URLs into links without messing with existing links in the text

Question

I am trying to convert URLs in a piece of text into hyperlinks - using regular expressions. I have managed to achieve this but the problem is when there are already existing links in the text

so

bla bla blah www.google.com bla blah <a href="www.google.com">www.google.com</a>

should result in

bla bla blah <a href="http://www.google.com">www.google.com</a> bla blah <a href="www.google.com">www.google.com</a> 

not

bla bla blah <a href="http://www.google.com">www.google.com</a> bla blah <a href="<a href="http://www.google.com">www.google.com</a></a>"><a href="http://www.google.com">www.google.com</a></a>

Answer 1

This is almost impossible to do with a single regular expression. I would instead recommend a state-machine based approach. Something like this (in pseudo-code)

state = OUTSIDE_LINK
for pos (0 .. length input)
   switch state
   case OUTSIDE_LINK
     if substring at pos matches /<a/
       state = INSIDE_LINK
     else if substring at pos matches /(www.S+|S+.com|S+.org)/
       substitute link
   case INSIDE_LINK
     if substring at post matches /</a>/
       state = OUTSIDE_LINK