Very basic BASIC lexer in Lua

basic.lua

--@name BASIC
--@server

local code = [[
10 PRINT "Hello, World!"
20 END
]]

local lexer = {}
lexer.data = code
lexer.index = 1
lexer.len = #code
lexer.out = {}
function lexer:eof()
    return self.index > self.len
end
function lexer:nextChars(n)
    local index = self.index
    return string.sub(self.data, index, index+(n and n-1 or 0))
end
function lexer:consumeChars(n)
    n = n or 1
    local chars = self:nextChars(n)
    self.index = self.index+n
    return chars
end
function lexer:consumeWhile(cond)
    local out = {}
    local i = 1
    while true do
        local char = self:nextChars(1)
        if cond(char) then
            self.index = self.index+1
            out[i] = char
            i = i+1
        else
            break
        end
    end
    return table.concat(out)
end
function lexer.isNumber(str)
    return string.find(str, '%d')
end
function lexer.isWhitespace(str)
    return string.find(str, '%s')
end
function lexer.isAlphabetic(str)
    return string.find(str, '%a')
end
function lexer.isNotQuote(str)
    return str ~= '"'
end
function lexer:consumeString()
    assert(not self.isNotQuote(self:consumeChars()), "missing beginning quote")
    local str = self:consumeWhile(self.isNotQuote)
    self.index = self.index+1
    return str
end
function lexer:consumeLine()
    if self:nextChars() == '\n' then
        self.index = self.index+1
        return
    end
    local lineno = self:consumeWhile(self.isNumber)
    self:consumeWhile(self.isWhitespace)
    local keyword = self:consumeWhile(self.isAlphabetic)
    self:consumeWhile(self.isWhitespace)
    local arguments = {}
    if keyword == 'PRINT' then
        arguments[1] = self:consumeString()
    end
    self.out[lineno] = {
        keyword = keyword,
        arguments = arguments
    }
    assert(self:consumeChars() == '\n' or self:eof(), "too many parameters")
end
function lexer:consumeLines()
    while not self:eof() do
        self:consumeLine()
    end
end

lexer:consumeLines()

do return printTable(lexer.out) end