These snippets of code can be used to check if an URL is valid or not and with some changes it can get URL's from a text.

''"In general URI's as defined by http://www.ietf.org/rfc/rfc3986.txt%|%RFC 3986%|% (page 12) may contain any of the following characters: '''A-Z''', '''a-z''', '''0-9''', '''-''', '''.''', '''_''', '''~''', ''':''', '''/''', '''?''', '''#''', [, ], '''@''', '''!''', '''$''', '''&''', ', '''(''', ''')''', '''*''', '''+''', ''',''', ''';''' and '''='''. Any other character needs to be encoded with the percent-encoding ('''%hh'''). Each part of the URI has further restrictions about what characters need to be represented by an percent-encoded word."'' (Gumbo, 2009)

----

======
#
#        Checking if an URL is valid or not...
#

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."
}

#
#        Getting an URL from a HTML code...
#

set blabla {<div class="title">Edit <a href="http://wiki.tcl.tk/26255">URL Parser</a></div>}
if {[regexp -- {(https?://[a-z0-9\-]+\.[a-z0-9\-\.]+(?:/|(?:/[a-zA-Z0-9!#\$%&'\*\+,\-\.:;=\?@\[\]_~]+)*)))} $blabla match url]} {
        puts "$url found in the HTML code."
}
======

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

PS: It doesn't work for IPv6 yet.