A high performance string format function for Javascript

2006-11-30 updated: I also write another high performance string format function which compiles the pattern into function and cache it for reuse.

Last month, I wrote a logging tool for JS, and to avoid performance depression, I need a string formatter function.
I found that though parse JS toolkit or framework provide string format function, such as Atlas, But they r not quite fast. most r using string. replace (RegEx, func) to replace the placeholder (such as '{n}' or '$ n'), but this function tend to slow, because every match will call the func, and JS function calling is very expensive.
On the contrary, native functions R very fast, so to improve performance, we shocould utilize the native functions as possible as we can. a wonderful example is stringbuilder which use array. join to Concat the string.

So I create my string. Format (), It's very fast.

Usage:

VaR name = 'World ';
VaR result = 'Hello $1! '. Format (name );
// Result = "Hello world! "

VaR letters = string. format ('$1 $2 $3 $4 $5 $6 $7 $8 $9 $10 $11 $12 $13 $14 $15 $16 $17 $18 $19 $20 $21 $22 $23 $24 $25 $26 ', 'A', 'B', 'C', 'D', 'E', 'E', 'F', 'G', 'h', 'I', 'J ', 'k', 'l', 'M', 'n', 'O', 'P', 'Q', 'R', 's','t ', 'U', 'V', 'w', 'x', 'y', 'z ');
// Letters = "abcdefghijklmnopqrstuvwxyz"

The later one almost same fast as the former one, no other implementation can have the same performance as I know.

Note:
1. it's depend on string. replace (RegEx, string), so u can use at most 99 placeholder ($1 to $99), but if the script engine is too old, it maybe only support nine ($1 to $9) or not work at all (eg. JScript before 5.5 ?).
2. Literal $ shocould be escape into $ (two $ ).
3. $ 'and $' will be reomoved, and $ & will replaced into some strange things
4. There is a magic character which you can't use anyway, currently I choose 0x1f (which means Data Separator in ASCII and Unicode ).

2006-11-30 updated:
'$1 1 '. format ('A') result in 'a 1', if you want to strip the space, u can't write '$11 '. format (...) because it will try to match the 11nd parameter, u shocould write '$011 '. format (...) instead.

Source code:

// Copyright (c) He Shi-jun <Hax. SFO at gmail dot com>, 2006
// Below codes can be used under GPL (V2 or later) or lgpl (V2.1 or later) License

If (! String. _ format_separator )...{
String. _ format_separator = string. fromcharcode (0x1f );
String. _ format_args_pattern = new Regexp ('^ [^' + String. _ format_separator + '] *'
+ New array (100). Join ('(? :. ([^ '+ String. _ format_separator +'] *)? '));
}
If (! String. Format)
String. format = function (s )...{
Return array. Prototype. Join. Call (arguments, String. _ format_separator ).
Replace (string. _ format_args_pattern, S );
}
If (! ''. Format)
String. Prototype. format = function ()...{
Return (string. _ format_separator +
Array. Prototype. Join. Call (arguments, String. _ format_separator )).
Replace (string. _ format_args_pattern, this );
}