wikilinks.  
local function is_url (scheme, domain)
if utilities.is_set (scheme) then -- if scheme is set check it and domain
return is_scheme (scheme) and is_domain_name (domain);
else
return is_domain_name (domain); -- scheme not set when URL is protocol-relative
end
end  
----------------------------< S P L I T _ U R L >------------------------------------------------------------  
Split a URL into a scheme, authority indicator, and domain.  
First remove Fully Qualified Domain Name terminator (a dot following TLD) (if any) and any path(/), query(?) or fragment(#).  
If protocol-relative URL, return nil scheme and domain else return nil for both scheme and domain.  
When not protocol-relative, get scheme, authority indicator, and domain. If there is an authority indicator (one
or more '/' characters immediately following the scheme's colon), make sure that there are only 2.  
Any URL that does not have news: scheme must have authority indicator (//). TODO: are there other common schemes
like news: that don't use authority indicator?  
Strip off any port and path;  
local function split_url (url_str)
local scheme, authority, domain;  
url_str = url_str:gsub ('([%a%d])%.?[/%?#].*$', '%1'); -- strip FQDN terminator and path(/), query(?), fragment (#) (the capture prevents false replacement of '//')  
if url_str:match ('^//%S*') then -- if there is what appears to be a protocol-relative URL
domain = url_str:match ('^//(%S*)')
elseif url_str:match ('%S-:/*%S+') then -- if there is what appears to be a scheme, optional authority indicator, and domain name
scheme, authority, domain = url_str:match ('(%S-:)(/*)(%S+)'); -- extract the scheme, authority indicator, and domain portions
if utilities.is_set (authority) then
authority = authority:gsub ('//', '', 1); -- replace place 1 pair of '/' with nothing;
if utilities.is_set(authority) then -- if anything (1 or 3+ '/' where authority should be) then
return scheme; -- return scheme only making domain nil which will cause an error message
end
else
if not scheme:match ('^news:') then -- except for news:..., MediaWiki won't link URLs that do not have authority indicator; TODO: a better way to do this test?
return scheme; -- return scheme only making domain nil which will cause an error message
end
end
domain = domain:gsub ('(%a):%d+', '%1'); -- strip port number if present
end  
return scheme, domain;
end  
--[[--------------------------< L I N K _ P A R A M _ O K >---------------------------------------------------  
checks the content of |title-link=, |series-link=, |author-link=, etc. for properly formatted content: no wikilinks, no URLs  
Link parameters are to hold the title of a Wikipedia article, so of the WP:TITLESPECIALCHARACTERS are allowed:
1. < > [ ] | { } _
except the underscore which is used as a space in wiki URLs and # which is used for section links  
returns false when the value contains any of these characters.  
When there are no illegal characters, this function returns TRUE if value DOES NOT appear to be a valid URL (the
|<param>-link= parameter is ok); else false when value appears to be a valid URL (the |<param>-link= parameter is NOT ok).  
local function link_param_ok (value)
local scheme, domain;
if value:find ('[<>%[%]|{}]') then -- if any prohibited characters
return false;
end  
scheme, domain = split_url (value); -- get scheme or nil and domain or nil from URL;
return not is_url (scheme, domain); -- return true if value DOES NOT appear to be a valid URL
end  
--<param>-link= value and its matching |<title>= value.  
|<title>= may be wiki-linked but not when |<param>-link= has a value. This function emits an error message when
that condition exists  
check <link> for inter-language interwiki-link prefix. prefix must be a MediaWiki-recognized language
code and must begin with a colon.  
local function link_title_ok (link, lorig, title, torig)
local orig;
if utilities.is_set (link) then -- don't bother if <param>-link doesn't have a value
if not link_param_ok (link) then -- check |<param>-link= markup
orig = lorig; -- identify the failing link parameter
elseif title:find ('%[%[') then -- check |title= for wikilink markup
orig = torig; -- identify the failing |title= parameter
elseif link:match ('^%a+:') then -- if the link is what looks like an interwiki
local prefix = link:match ('^(%a+):'):lower; -- get the interwiki prefix  
if cfg.inter_wiki_map[prefix] then -- if prefix is in the map, must have preceding colon
orig = lorig; -- flag as error
end
end
end  
if utilities.is_set (orig) then
link = ''; -- unset
utilities.set_message ('err_bad_paramlink', orig); -- URL or wikilink in |title= with |title-link=;
end  
return link; -- link if ok, empty string else
end  
----------------------------< C H E C K _ U R L >------------------------------------------------------------  
Determines whether a URL string appears to be valid.  
First we test for space characters. If any are found, return false. Then split the URL into scheme and domain
portions, or for protocol-relative (//example.com) URLs, just the domain. Use is_url to validate the two
portions of the URL. If both are valid, or for protocol-relative if domain is valid, return true, else false.  
Because it is different from a standard URL, and because this module used external_link to make external links
that work for standard and news: links, we validate newsgroup names here. The specification for a newsgroup name
is at https://tools.ietf.org/html/rfc5536#section-3.1.4  
local function check_url( url_str )
if nil == url_str:match ("^%S+$") then -- if there are any spaces in |url=value it can't be a proper URL
return false;
end
local scheme, domain;  
scheme, domain = split_url (url_str); -- get scheme or nil and domain or nil from URL;  
if 'news:' == scheme then -- special case for newsgroups
return domain:match('^[%a%d%+%-_]+%.[%a%d%+%-_%.]*[%a%d%+%-_]$');
end  
return is_url (scheme, domain); -- return true if value appears to be a valid URL
end  
--[=[-------------------------< I S _ P A R A M E T E R _ E X T _ W I K I L I N K >----------------------------  
Return true if a parameter value has a string that begins and ends with square brackets [ and ] and the first
non-space characters following the opening bracket appear to be a URL. The test will also find external wikilinks
that use protocol-relative URLs. Also finds bare URLs.  
The frontier pattern prevents a match on interwiki-links which are similar to scheme:path URLs. The tests that
find bracketed URLs are required because the parameters that call this test (currently |title=, |chapter=, |work=,
and |publisher=) may have wikilinks and there are articles or redirects like '//Hus' so, while uncommon, |title=//Hus
is possible as might be en://Hus.  
]=]  
local function is_parameter_ext_wikilink (value)
local scheme, domain;  
if value:match ('%f[%[]%[%a%S*:%S+.*%]') then -- if ext. wikilink with scheme and domain: [xxxx://yyyyy.zzz]
scheme, domain = split_url (value:match ('%f[%[]%[(%a%S*:%S+).*%]'));
elseif value:match ('%f[%[]%[//%S+.*%]') then -- if protocol-relative ext. wikilink: [//yyyyy.zzz]
scheme, domain = split_url (value:match ('%f[%[]%[(//%S+).*%]'));
elseif value:match ('%a%S*:%S+') then -- if bare URL with scheme; may have leading or trailing plain text
scheme, domain = split_url (value:match ('(%a%S*:%S+)'));
elseif value:match ('//%S+') then -- if protocol-relative bare URL: //yyyyy.zzz; may have leading or trailing plain text
scheme, domain = split_url (value:match ('(//%S+)')); -- what is should be the domain
else
return false; -- didn't find anything that is obviously a URL
end  
return is_url (scheme, domain); -- return true if value appears to be a valid URL
end  
---------------------------< C H E C K _ F O R _ U R L >-----------------------------------------------------  
loop through a list of parameters and their values. Look at the value and if it has an external link, emit an error message.