String Manipulation
This library provides generic functions for string manipulation, such as finding and extracting substrings, and pattern matching. When indexing a string in Lua, the first character is at position 1 (not at 0, as in C). Indices are allowed to be negative and are interpreted as indexing backwards, from the end of the string. Thus, the last character is at position -1, and so on.
The string library provides all its functions inside the table string. It also sets a metatable for strings where the __index field points to the string table. Therefore, you can use the string functions in object-oriented style. For instance, string.byte(s, i) can be written as s:byte(i).
The string library assumes one-byte character encoding.
string.byte (s [, i [, j]])
Returns the internal numerical codes of the given characters s[i], s[i+1], ···, s[j]. The default value for i is 1; the default value for j is i.
Note. See ASCII codes of symbols at http://www.asciitable.com
Example
local str="Insat OPC";
val0,val1,val2=string.byte(str,2,4,6);
--val0=110 ("n"),val1=115("a"),val2=97 (" ")
string.char (···)
Receives zero or more integers. Returns a string with length equal to the number of arguments, in which each character has the internal numerical code equal to its corresponding argument.
Note. See ASCII codes of symbols at http://www.asciitable.com
Example
local str=string.char(0x30,0x31,0x32);
--str="123"
string.dump (function)
Returns a string containing a binary representation of the given function, so that a later loadstring on this string returns a copy of the function. function must be a Lua function without upvalues.
string.find (s, pattern [, init [, plain]])
Looks for the first match of pattern in the string s. If it finds a match, then find returns the indices of s where this occurrence starts and ends; otherwise, it returns nil. A third, optional numerical argument init specifies where to start the search; its default value is 1 and can be negative. A value of true as a fourth, optional argument plain turns off the pattern matching facilities, so the function does a plain "find substring" operation, with no characters in pattern being considered "magic". Note that if plain is given, then init must be given as well.
If the pattern has captures, then in a successful match the captured values are also returned, after the two indices.
Example
local str="Insat Modbus Universal";
local n,l=string.find(str,"Modbus");
--n=7, l=12
string.format (formatstring, ···)
Returns a formatted version of its variable number of arguments following the description given in its first argument (which must be a string). The format string follows the same rules as the printf family of standard C functions. The only differences are that the options/modifiers *, l, L, n, p, and h are not supported and that there is an extra option, q. The q option formats a string in a form suitable to be safely read back by the Lua interpreter: the string is written between double quotes, and all double quotes, newlines, embedded zeros, and backslashes in the string are correctly escaped when written. For instance, the call
string.format('%q', 'a string with "quotes" and \n new line')
will produce the string:
"a string with \"quotes\" and \
new line"
The options c, d, E, e, f, g, G, i, o, u, X, and x all expect a number as argument, whereas q and s expect a string.
This function does not accept string values containing embedded zeros, except as arguments to the q option.
Example
--Example of conversion of a DEC number to a string containing the respective HEX number
local StrHex=string.format("%x",10);
--StrHex="A"
string.gmatch (s, pattern)
Returns an iterator function that, each time it is called, returns the next captures from pattern over string s. If pattern specifies no captures, then the whole match is produced in each call.
As an example, the following loop
s = "hello world from Lua"
for w in string.gmatch(s, "%a+") do
print(w)
end
will iterate over all the words from string s, printing one per line. The next example collects all pairs key=value from the given string into a table:
t = {}
s = "from=world, to=Lua"
for k, v in string.gmatch(s, "(%w+)=(%w+)") do
t[k] = v
end
For this function, a ’^’ character at the start of a pattern does not work as an anchor, as this would prevent the iteration.
string.gsub (s, pattern, repl [, n])
Returns a copy of s in which all (or the first n, if given) occurrences of the pattern have been replaced by a replacement string specified by repl, which can be a string, a table, or a function. gsub also returns, as its second value, the total number of matches that occurred.
If repl is a string, then its value is used for replacement. The character % works as an escape character: any sequence in repl of the form %n, with n between 1 and 9, stands for the value of the n-th captured substring (see below). The sequence %0 stands for the whole match. The sequence %% stands for a single %.
If repl is a table, then the table is queried for every match, using the first capture as the key; if the pattern specifies no captures, then the whole match is used as the key.
If repl is a function, then this function is called every time a match occurs, with all captured substrings passed as arguments, in order; if the pattern specifies no captures, then the whole match is passed as a sole argument.
If the value returned by the table query or by the function call is a string or a number, then it is used as the replacement string; otherwise, if it is false or nil, then there is no replacement (that is, the original match is kept in the string).
The oprtioanl n is a maximal number of replacements. For instance, if n = 1, there will be performed not more than one replacement.
Here are some examples:
x = string.gsub("hello world", "world", "earth")
--x="hello earth"
x = string.gsub("hello world", "(%w+)", "%1 %1")
--> x="hello hello world world"
x = string.gsub("hello world", "%w+", "%0 %0", 1)
--> x="hello hello world"
x = string.gsub("hello world from Lua", "(%w+)%s*(%w+)", "%2 %1")
--> x="world hello Lua from"
x = string.gsub("home = $HOME, user = $USER", "%$(%w+)", os.getenv)
--> x="home = /home/roberto, user = roberto"
x = string.gsub("4+5 = $return 4+5$", "%$(.-)%$", function (s)
return loadstring(s)()
end)
--> x="4+5 = 9"
local t = {name="lua", version="5.1"}
x = string.gsub("$name-$version.tar.gz", "%$(%w+)", t)
--> x="lua-5.1.tar.gz"
string.len (s)
Receives a string and returns its length. The empty string "" has length 0. Embedded zeros are counted, so "a\000bc\000" has length 5.
Example
local str="Modbus OPC Server";
local len=string.len(str);
--len=17
string.lower (s)
Receives a string and returns a copy of this string with all uppercase letters changed to lowercase. All other characters are left unchanged. The definition of what an uppercase letter is depends on the current locale.
Example
local str="Modbus OPC Server";
local newstr=string.lower(str);
--newstr=modbus opc server
string.match (s, pattern [, init])
Looks for the first match of pattern in the string s. If it finds one, then match returns the captures from the pattern; otherwise it returns nil. If pattern specifies no captures, then the whole match is returned. A third, optional numerical argument init specifies where to start the search; its default value is 1 and can be negative.
Example
local str="Modbus OPC Server";
local match=string.match(str,"OPC");
--match="OPC"
string.rep (s, n)
Returns a string that is the concatenation of n copies of the string s.
Example
local str=string.rep("Insat ",3);
--str="Insat Insat Insat "
string.reverse (s)
Returns a string that is the string s reversed.
Example
local str=string.reverse("Insat");
--str=tasnI
string.sub (s, i [, j])
Returns the substring of s that starts at i and continues until j; i and j can be negative. If j is absent, then it is assumed to be equal to -1 (which is the same as the string length). In particular, the call string.sub(s,1,j) returns a prefix of s with length j, and string.sub(s, -i) returns a suffix of s with length i.
Example
local str="Insat OPC Server";
local SubStr=string.sub(str,7,10);
--SubStr="OPC"
string.upper (s)
Receives a string and returns a copy of this string with all lowercase letters changed to uppercase. All other characters are left unchanged. The definition of what a lowercase letter is depends on the current locale.
Example
local str="Insat OPC Server";
local NewStr=string.upper(str;
--NewStr="INSAT OPC SERVER"
When developing a protocol, often there is the task of string parsing. A string is a set of elements separated by a special symbol (comma, dot, colon, etc.) It is necessary to split a string to its elements and form an array. To do this, you can use the string.split function.
Input arguments of that function are two string parameters: a string to be split and a separator. The function returns a table. Each table element is a string enclosed between separators in the initial string. If the separator does not exist in the initial string, the table contains only one element, and that element is the whole initial string.
Example
local list={};
local st="Insat;Modbus;Universal;MasterOPC"
list=string.split(st,";")
-- The table 'list' will contain 4 elements: "Insat", "Modbus", "Universal" and "MasterOPC".
The function packages the string into a zip archive.
Input arguments:
string for packing.
Output arguments:
error status;
the result of packing is a ZIP string.
Example
local Shakespeare="To be, or not to be, that is the question:"; --source text
local err,Zip=string.tozip(Shakespeare); --zipping
if err==true then --check
server.Message("Compression error" );
return;
end
server.Message("ZIP=",Zip); --print to log
The function decompresses the ZIP archive and returns the result as a string.
Input arguments:
ZIP string to decompress;
Output arguments:
error status;
the result of unpacking as a string.
Example
local Shakespeare="To be, or not to be, that is the question:"; --source text
local err,Zip=string.tozip(Shakespeare); --zipping
if err==true then --check
server.Message("Compression error" );
return;
end
server.Message("ZIP=",Zip); --print to log
local err,UnZip=string.fromzip(Zip); --unzipping
if err==true then --check
server.Message("Uncompression error" );
return;
end
server.Message("From zip=",UnZip); --print to log
The function is similar to string.tozip, but forms a HEX string - this archiving method should be used if you want to transfer the compressed string to another OPC server instance using bytestring data type. Only bytestring can be used to transfer archive values between servers, since a simple string will not work because of the possible presence of terminal zeros in the archive string. The function should be paired with the function string.fromhexzip.
Input arguments:
string for packing.
Output arguments:
error status;
the result is a ZIP archive as a HEX string.
Example
local Shakespeare="To be, or not to be, that is the question:"; --source text
local err,Zip=string.tohexzip(Shakespeare); --zipping to HEX string (bytestring)
if err==true then --check
server.Message("Compression error" );
return;
end
server.Message("ZIP=",Zip); --print to log
The function is similar to string.fromzip, but takes a HEX string as an argument - this archiving method should be used if you want to get the compressed string from another OPC server instance, using the bytestring data type. Only bytestring can be used to transfer archive values between servers, since a simple string will not work due to the possible presence of terminal zeros in the archive string. The function should be paired with the function string.tohexzip.
Input arguments:
ZIP string as a HEX string to decompress;
Output arguments:
error status;
the result of unpacking as a string.
Example
local Shakespeare="To be, or not to be, that is the question:"; --source text
local err,Zip=string.tohexzip(Shakespeare); --zipping to HEX string (bytestring)
if err==true then --check
server.Message("Compression error" );
return;
end
server.Message("ZIP=",Zip); --print to log
local err,UnZip=string.fromhexzip(Zip); --unzipping from HEX string (bytestring)
if err==true then --check
server.Message("Uncompression error" );
return;
end
server.Message("From zip=",UnZip); --print to log