Format LCCN link and do simple error checking. LCCN is a character string 8-12 characters long. The length of
the LCCN dictates the character type of the first 1-3 characters; the rightmost eight are always digits.
https://oclc-research.github.io/infoURI-Frozen/info-uri.info/info:lccn/reg.html  
length = 8 then all digits
length = 9 then lccn[1] is lowercase alpha
length = 10 then lccn[1] and lccn[2] are both lowercase alpha or both digits
length = 11 then lccn[1] is lower case alpha, lccn[2] and lccn[3] are both lowercase alpha or both digits
length = 12 then lccn[1] and lccn[2] are both lowercase alpha  
local function lccn (options)
local lccn = options.id;
local handler = options.handler;
local err_flag; -- presume that LCCN is valid
local id = lccn; -- local copy of the LCCN  
id = normalize_lccn (id); -- get canonical form (no whitespace, hyphens, forward slashes)
local len = id:len; -- get the length of the LCCN  
if 8 == len then
if id:match("[^%d]") then -- if LCCN has anything but digits (nil if only digits)
err_flag = set_message ('err_bad_lccn'); -- set an error message
end
elseif 9 == len then -- LCCN should be adddddddd
if nil == id:match("%l%d%d%d%d%d%d%d%d") then -- does it match our pattern?
err_flag = set_message ('err_bad_lccn'); -- set an error message
end
elseif 10 == len then -- LCCN should be aadddddddd or dddddddddd
if id:match("[^%d]") then -- if LCCN has anything but digits (nil if only digits) ...
if nil == id:match("^%l%l%d%d%d%d%d%d%d%d") then -- ... see if it matches our pattern
err_flag = set_message ('err_bad_lccn'); -- no match, set an error message
end
end
elseif 11 == len then -- LCCN should be aaadddddddd or adddddddddd
if not (id:match("^%l%l%l%d%d%d%d%d%d%d%d") or id:match("^%l%d%d%d%d%d%d%d%d%d%d")) then -- see if it matches one of our patterns
err_flag = set_message ('err_bad_lccn'); -- no match, set an error message
end
elseif 12 == len then -- LCCN should be aadddddddddd
if not id:match("^%l%l%d%d%d%d%d%d%d%d%d%d") then -- see if it matches our pattern
err_flag = set_message ('err_bad_lccn'); -- no match, set an error message
end
else
err_flag = set_message ('err_bad_lccn'); -- wrong length, set an error message
end  
if not is_set (err_flag) and nil ~= lccn:find ('%s') then
err_flag = set_message ('err_bad_lccn'); -- lccn contains a space, set an error message
end  
if is_set (err_flag) then
options.coins_list_t['LCCN'] = nil; -- when error, unset so not included in COinS
end  
return external_link_id ({link = handler.link, label = handler.label, q = handler.q, redirect = handler.redirect,
prefix = handler.prefix, id = lccn, separator = handler.separator, encode = handler.encode});
end  
----------------------------< M E D R X I V >-----------------------------------------------------------------  
Format medRxiv ID and do simple error checking. Similar to later bioRxiv IDs, medRxiv IDs are prefixed with a
yyyy.mm.dd. date and suffixed with an optional version identifier. Ealiest date accepted is 2020.01.01  
The medRxiv ID is a date followed by an eight-digit number followed by an optional version indicator 'v' and one or more digits:
https://www.medrxiv.org/content/10.1101/2020.11.16.20232009v2 -> 10.1101/2020.11.16.20232009v2  
local function medrxiv (options)
local id = options.id;
local handler = options.handler;
local err_msg_flag = true; -- flag; assume that there will be an error  
local patterns = {
'%d%d%d%d%d%d%d%d$', -- simple 8-digit identifier; these should be relatively rare
'^10%.1101/(20%d%d)%.(%d%d)%.(%d%d)%.%d%d%d%d%d%d%d%dv%d+$', -- y.m.d. date + 8-digit identifier + version (2020-01-01 and later)
'^10%.1101/(20%d%d)%.(%d%d)%.(%d%d)%.%d%d%d%d%d%d%d%d$', -- y.m.d. date + 8-digit identifier (2020-01-01 and later)
}  
for _, pattern in ipairs (patterns) do -- spin through the patterns looking for a match
if id:match (pattern) then
local y, m, d = id:match (pattern); -- found a match, attempt to get year, month and date from the identifier  
if m then -- m is nil when id is the 8-digit form
if not is_valid_rxiv_date (y, m, d, 'b') then -- validate the encoded date; 'b' for medrxiv limit
break; -- date fail; break out early so we don't unset the error message
end
end
err_msg_flag = nil; -- we found a match so unset the error message
break; -- and done
end
end -- <err_msg_flag> remains set here when no match  
if err_msg_flag then
options.coins_list_t['MEDRXIV'] = nil; -- when error, unset so not included in COinS
set_message ('err_bad_medrxiv'); -- and set the error message
end  
return external_link_id ({link = handler.link, label = handler.label, q = handler.q, redirect = handler.redirect,
prefix = handler.prefix, id = id, separator = handler.separator,
encode = handler.encode, access = handler.access});
end  
----------------------------< M R >--------------------------------------------------------------------------  
A seven digit number; if not seven digits, zero-fill leading digits to make seven digits.  
local function mr (options)
local id = options.id;
local handler = options.handler;
local id_num;
local id_len;  
id_num = id:match ('^[Mm][Rr](%d+)$'); -- identifier with mr prefix  
if is_set (id_num) then
set_message ('maint_mr_format'); -- add maint cat
else -- plain number without mr prefix
id_num = id:match ('^%d+$'); -- if here id is all digits
end  
id_len = id_num and id_num:len or 0;
if (7 >= id_len) and (0 ~= id_len) then
id = string.rep ('0', 7-id_len) .. id_num; -- zero-fill leading digits
else
set_message ('err_bad_mr'); -- set an error message
options.coins_list_t['MR'] = nil; -- when error, unset so not included in COinS
end  
return external_link_id ({link = handler.link, label = handler.label, q = handler.q, redirect = handler.redirect,
prefix = handler.prefix, id = id, separator = handler.separator, encode = handler.encode});
end  
----------------------------< O C L C >----------------------------------------------------------------------  
Validate and format an OCLC ID. https://www.oclc.org/batchload/controlnumber.en.html
archived at: https://web.archive.org/web/20161228233804/https://www.oclc.org/batchload/controlnumber.en.html  
local function oclc (options)
local id = options.id;
local handler = options.handler;
local number;  
if id:match('^ocm%d%d%d%d%d%d%d%d$') then -- ocm prefix and 8 digits; 001 field (12 characters)
number = id:match('ocm(%d+)'); -- get the number
elseif id:match('^ocn%d%d%d%d%d%d%d%d%d$') then -- ocn prefix and 9 digits; 001 field (12 characters)
number = id:match('ocn(%d+)'); -- get the number
elseif id:match('^on%d%d%d%d%d%d%d%d%d%d+$') then -- on prefix and 10 or more digits; 001 field (12 characters)
number = id:match('^on(%d%d%d%d%d%d%d%d%d%d+)$'); -- get the number
elseif id:match('^%(OCoLC%)[1-9]%d*$') then -- (OCoLC) prefix and variable number digits; no leading zeros; 035 field
number = id:match('%(OCoLC%)([1-9]%d*)'); -- get the number
if 9 < number:len then
number = nil; -- constrain to 1 to 9 digits; change this when OCLC issues 10-digit numbers
end
elseif id:match('^%d+$') then -- no prefix
number = id; -- get the number
if tonumber (id) > handler.id_limit then
number = nil; -- unset when id value exceeds the limit
end
end  
if number then -- proper format
id = number; -- exclude prefix, if any, from external link
else
set_message ('err_bad_oclc') -- add an error message if the id is malformed
options.coins_list_t['OCLC'] = nil; -- when error, unset so not included in COinS
end  
return external_link_id ({link = handler.link, label = handler.label, q = handler.q, redirect = handler.redirect,
prefix = handler.prefix, id = id, separator = handler.separator, encode = handler.encode});
end  
----------------------------< O P E N L I B R A R Y >--------------------------------------------------------  
Formats an OpenLibrary link, and checks for associated errors.  
local function openlibrary (options)
local id = options.id;
local access = options.access;
local handler = options.handler;