What's wrong with this subroutine?

[A.Newby]

First, here's what I'm trying to do ......

I'm working on a html, cgi based chat script. It's working pretty well so 
far. 

I want to get this subroutine so that the user just has to post a url into 
the chat box to get a post a picture or a link in chat. For e.g, to post a 
pic of a walrus, you would simply type ....

http://www.pbs.org/kratts/world/oceans/walrus/images/walrus.jpg

... and the script would take that, and before writing it to the chat log 
it would convert it to ...

<img src = http://www.pbs.org/kratts/world/oceans/walrus/images/walrus.jpg>

... or to post a link, the user would simply type ... 

http://www.goodvibes.com/

... and the script would convert it to ....

<a href = "http://www.goodvibes.com/>Link!</a>


Here's the subroutine I'm using to try and do this.... it is called from a 
function named "infilter" which filters the posts before writing them to 
the chat log ...

def search4URL(string):
    #the following bits are used to alter the post when it finds the URL
    beginpic = " <img src =" 
    endpic = ">"
    beginlink = " <a href ="
    endlink = " target = blank>Link</a>"
    
    URLStarts = string.find(' http:') #space is so when the function     
	    	    	    	    	    	  #repeats, it won't convert the same 
    	    	    	    	    	    	  #block of text again

    URLEnds = string.find(' ', URLStarts+1, len(string))
    
    plaything = string[URLStarts: URLEnds]
    if plaything[-4:] == ".jpg" or plaything[-4:] == ".gif": #if the URL is     
	    	    	    	    	    	    	    	    	    	 #a pic
        nstring = string.replace(plaything,beginpic+plaything[1:]+ endpic)

    elif not plaything[-4:] == ".jpg" or plaything[-4:] == ".gif": 
    	#for some reason, "else:" return a "ValueError: empty pattern string"
        nstring=string.replace(plaything,beginlink+plaything[1:]+ endlink)
    return nstring

...... The problem (aside from the fact that someone clever could probably 
neaten it up), is that it only works ONCE per post. That means, that if a 
chatter tries to post two pics, or a pic and a link, the URLs just come out 
as plain text. Any more than once and get a "ValueError: empty pattern 
string".

I have tried fixing the routine in "infilter" so it repeats the function as 
many times as it finds the substring " http:". I have tried doing this ...

x = searchstring.count(' http:')
for c in range(0, x):
    searchstring = search4URL(searchstring)

...  And I've also tried using a while loop instead. But I keep getting the 
dread ValueError. What am I missing here? 


-- 
cdewin at dingoblue.net.au