How often does JavaScript recompile regex literals in functions?

Question

Given this function:

function doThing(values,things){   var thatRegex = /^http:\/\//i; // is this created once or on every execution?   if (values.match(thatRegex)) return values;   return things; } 

How often does the JavaScript engine have to create the regex? Once per execution or once per page load/script parse?

To prevent needless answers or comments, I personally favor putting the regex outside the function, not inside. The question is about the behavior of the language, because I'm not sure where to look this up, or if this is an engine issue.

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EDIT:

I was reminded I didn't mention that this was going to be used in a loop. My apologies:

var newList = []; foreach(item1 in ListOfItems1){    foreach(item2 in ListOfItems2){      newList.push(doThing(item1, item2));   } } 

So given that it's going to be used many times in a loop, it makes sense to define the regex outside the function, but so that's the idea.

_{also note the script is rather genericized for the purpose of examining only the behavior and cost of the regex creation}

Answer 1

From Mozilla's JavaScript Guide on regular expressions:

Regular expression literals provide compilation of the regular expression when the script is evaluated. When the regular expression will remain constant, use this for better performance.

And from the ECMA-262 spec, §7.8.5 Regular Expression Literals:

A regular expression literal is an input element that is converted to a RegExp object (see 15.10) each time the literal is evaluated.

In other words, it's compiled once when it's evaluated as a script is first parsed.

It's worth noting also, from the ES5 spec, that two literals will compile to two distinct instances of RegExp, even if the literals themselves are the same. Thus if a given literal appears twice within your script, it will be compiled twice, to two distinct instances:

Two regular expression literals in a program evaluate to regular expression objects that never compare as === to each other even if the two literals' contents are identical.

...

... each time the literal is evaluated, a new object is created as if by the expression new RegExp(Pattern, Flags) where RegExp is the standard built-in constructor with that name.