Lua checks whether the email address is valid

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The valid email address format is as follows:
1. The first character must contain letters, and other characters can only contain 26 uppercase/lowercase letters, 0 ~ 9 and _-. @ symbol
2. It must contain one and only one symbol "@".
3. @ must contain at least one of the three symbols "."
4. the first character cannot be "@" or "." (the first step has been checked)
5. "@." Or ". @" cannot appear .@
6. The end cannot be the character "@" or "."

Code implementation:

 1 function isRightEmail(str) 2     if string.len(str or "") < 6 then return false end 3     local b,e = string.find(str or "", '@') 4     local bstr = "" 5     local estr = "" 6     if b then 7         bstr = string.sub(str, 1, b-1) 8         estr = string.sub(str, e+1, -1) 9     else10         return false11     end12 13     -- check the string before '@'14     local p1,p2 = string.find(bstr, "[%w_]+")15     if (p1 ~= 1) or (p2 ~= string.len(bstr)) then return false end16     17     -- check the string after '@'18     if string.find(estr, "^[%.]+") then return false end19     if string.find(estr, "%.[%.]+") then return false end20     if string.find(estr, "@") then return false end21     if string.find(estr, "[%.]+$") then return false end22 23     _,count = string.gsub(estr, "%.", "")24     if (count < 1 ) or (count > 3) then25         return false26     end27 28     return true29 end