Lua: Substitute list of characters in string

Question

Is it possible to substitute characters according to a list in Lua, like tr in Perl? For example, I would like to substitute A to B and B to A (e.g. AABBCC becomes BBAACC).

In Perl, the solution would be $str ~= tr/AB/BA/. Is there any native way of doing this in Lua? If not, I think the best solution would be iterating through the entire string, since separate substitutions need to use a special symbol to distinguish characters that were already substituted and characters that weren't.

Edit: my goal was to calculate the reverse complement of a DNA string, as described here.

Dickey

Answer 1

string.gsub can take a table as the third argument. The table is queried for each match, using the first capture as the key, and the associated value is used as the replacement string. If the value is nil, the match is not changed.

So you can build a helper table like this:

local s = "AABBCC"
local t = {A = "B", B = "A"}
local result = string.gsub(s, "[AB]", t)
print(result)

or this same one-liner:

print((string.gsub("AABBCC", "[AB]", {A = "B", B = "A"})))

Output:

BBAACC

For a one character pattern like "[AB]", "." can work as well because whatever not found in the table won't be changed. (But I don't think that's more efficient) But for some more complicated cases, a good pattern is needed.

Here is an example from Programming in Lua: this function substitutes the value of the global variable varname for every occurrence of $varname in a string:

function expand (s)
    return (string.gsub(s, "$(%w+)", _G))
end

Nejd

+1 I wasn't aware of the table form of gsub. Much nicer than the function method I used. – Furriery

Great solution! Do you think it would be more efficient we replace "[A|B]" with "." (matching every character)? I've tested and the result is the same. – Mlawsky

@FábioPerez Yes. \@YuHao lua-patterns don't support the | matching like regex. Besides, [AB] would work just as well. – Kame

Answer 2

The code below will replace each character with a desired mapping (or leave alone if no mapping exists). You could modify the second parameter to string.gsub in tr to be more specific if you know the exact range of characters.

s = "AABBCC"
mappings = {["A"]="B",["B"]="A"}

function tr(s,mappings)
    return string.gsub(s,
        "(.)",
        function(m)
            -- print("found",m,"replace with",mappings[m],mappings[m] or m)
            if mappings[m] == nil then return m else return mappings[m] end
        end 
    )
end

print(tr(s,mappings))

Outputs

henry@henry-pc:~/Desktop$ lua replace.lua
found   A   replace with    B   B
found   A   replace with    B   B
found   B   replace with    A   A
found   B   replace with    A   A
found   C   replace with    nil C
found   C   replace with    nil C
BBAACC  6

Furriery