String splitting based on unknown delimeters (rather, LENGTH of delimeter) - string

Question

I have a somewhat esoteric problem. My program wants to decode morse code.
The point is, I will need to handle any character. Any random characters that adhere to my system and can correspond to a letter should be accepted. Meaning, the letter "Q" is represented by "- - . -", but my program will treat any string of characters (separated by appropriate newchar signal) to be accepted as Q, for example "dj ir j kw" (long long short long).
There is a danger of falling out of sync, so I will need to implement a "new character" signal. I chose this to be "xxxx" as in 4 letters. For white, blank space symbol, I chose "xxxxxx", 6 chars.
Long story short, how can I split the string that is to be decoded into readable characters based on the length of the delimeter (4 continous symbols), since I can't really deterministically know what letters will make up the newchar delimeter?

Answer 1

The question is not very clearly worded.
For instance, here you show space as a delimeter between parts of the symbol Q:
for example "dj ir j kw" (long long short long)
Later you say:
For white, blank space symbol, I chose "xxxxxx", 6 chars.
Is that the symbol for whitespace, or the delimeter you use within a symbol (such as Q, above)? Your post doesn't say.
In this case, as always, an example is worth a thousands words. You should have shown a few examples of possible input and shown how you'd like them parsed.
If what you mean was that "dj ir j kw jfkl abpzoq jfkl dj ir j kw" should be decoded as "Q Q", and you just want to know how to match tokens by their length, then... the question is easy. There's a million ways you could do that.
In Lua, I'd do it in two passes. First, convert the message into a string containing only the length of each chunk of consequitive characters:
message = 'dj ir j kw jfkl abpzoq jfkl dj ir j kw'
message = message:gsub('(%S+)%s*', function(s) return #s end)
print(message) --> 22124642212
Then split on the number 4 to get each group
for group in message:gmatch('[^4]+') do
print(group)
end
Which gives you:
2212
6
2212
So you could convert something like this:
function translate(message)
local lengthToLetter = {
['2212'] = 'Q',
[ '6'] = ' ',
}
local translation = {}
message = message:gsub('(%S+)%s*', function(s) return #s end)
for group in message:gmatch('[^4]+') do
table.insert(translation, lengthToLetter[group] or '?')
end
return table.concat(translation)
end
print(translate(message))

Answer 2

This will split a string by any len continuous occurrences of char, which may be a character or pattern character class (such as %s), or of any character (i.e. .) if char is not passed.
It does this by using backreferences in the pattern passed to string.find, e.g. (.)%1%1%1 to match any character repeated four times.
The rest is just a bog-standard string splitter; the only real Lua peculiarity here is the choice of pattern.
-- split str, using (char * len) as the delimiter
-- leave char blank to split on len repetitions of any character
local function splitter(str, len, char)
-- build pattern to match len continuous occurrences of char
-- "(x)%1%1%1%1" would match "xxxxx" etc.
local delim = "("..(char or ".")..")" .. string.rep("%1", len-1)
local pos, out = 1, {}
-- loop through the string, find the pattern,
-- and string.sub the rest of the string into a table
while true do
local m1, m2 = string.find(str, delim, pos)
-- no sign of the delimiter; add the rest of the string and bail
if not m1 then
out[#out+1] = string.sub(str, pos)
break
end
out[#out+1] = string.sub(str, pos, m1-1)
pos = m2+1
-- string ends with the delimiter; bail
if m2 == #str then
break
end
end
return out
end
-- and the result?
print(unpack(splitter("dfdsfsdfXXXXXsfsdfXXXXXsfsdfsdfsdf", 5)))
-- dfdsfsdf, sfsdf, sfsdfsdfsdf