This code can be used to check if a url is valid or not and with some chances it can get url's from a text.

----

======
set blabla {http://www.ietf.org/rfc/rfc3986.txt}
if {[regexp -- {^(https?://[a-z0-9\-]+\.[a-z0-9\-\.]+(?:/|(?:/[a-zA-Z0-9!#\$%&'\*\+,\-\.:;=\?@\[\]_~]+)*)))$} $blabla match url]} {
        puts "$url is a valid url."
}
======

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Let's test it and post the results here...

PS: It doesn't work for IPv6 yet.