Lua模式验证DNS地址

Question

I am currently using this regular expression to loosely validate a DNS address:

^[A-Za-z0-9_]+(\.[A-Za-z0-9_]+)*$

Which would match things like hello.com , hello , and hello.com.com.com . I was trying to replicate it exactly as it is into a Lua pattern. I came up with the following Lua pattern:

^([%d%a_]+(%.[%d%a_]+)*)$

So that I can use the following code to validate the DNS address:

local s = "hello.com"
print(s:match("^([%d%a_]+(%.[%d%a_]+)*)$"))

For some reason this always fails, although it looks like a 1:1 copy of the regular expression.

Any ideas why?

Answer 1

Lua patterns are not regular expressions. You cannot translate the ^[A-Za-z0-9_]+(\\.[A-Za-z0-9_]+)*$ to ^([%d%a_]+(%.[%d%a_]+)*)$ , because you cannot apply quantifiers to groups in Lua (see Limitations of Lua patterns ).

Judging by the ^[A-Za-z0-9_]+(\\.[A-Za-z0-9_]+)*$ regex, the rules are:

• String can consist of one or more alphanumeric or underscore or dot characters
• String cannot start with a dot
• String cannot end with a dot
• String cannot contain 2 consecutive dots

You can use the following work-around:

function pattern_checker(v)
    return string.match(v, '^[%d%a_.]+$') ~= nil and -- check if the string only contains digits/letters/_/., one or more
           string.sub(v, 0, 1) ~= '.' and            -- check if the first char is not '.'
           string.sub(v, -1) ~= '.' and              -- check if the last char is not '.'
           string.find(v, '%.%.') == nil             -- check if there are 2 consecutive dots in the string
end

See IDEONE demo :

-- good examples
print(pattern_checker("hello.com")) -- true
print(pattern_checker("hello")) -- true
print(pattern_checker("hello.com.com.com")) -- true
-- bad examples
print(pattern_checker("hello.com.")) -- false
print(pattern_checker(".hello")) -- false
print(pattern_checker("hello..com.com.com")) -- false
print(pattern_checker("%hello.com.com.com")) -- false

Answer 2

You can translate the pattern to ^[%w_][%w_%.]+[%w_]$ , although that still allows for double dots. When using that pattern while checking for double dots, you end up with this:

function pattern_checker(v)
    -- Using double "not" because we like booleans, no?
    return not v:find("..",1,true) and not not v:match("^[%w_][%w_%.]+[%w_]$")
end

I used the same testcode as Wiktor Stribiżew (since it's good testcode) and it produces the same results. Mine is also 2 to 3 times faster, if that matters. (Doesn't mean I don't like Wiktor's code, his code also works. He also has a link to the limitations page, a nice touch to his answer)

(I like playing with string patterns in Lua)