JavaScript change in Firefox 5 (not 4), and in other browsers: regular expressions can’t be called like functions

Callable regular expressions

Way back in the day when Netscape implemented regular expressions in JavaScript, it made them callable. If you slapped an argument list after a regular expression, it’d act as if you called RegExp.prototype.exec on it with the provided arguments.

var r = /abc/, res;

res = r("abc");
assert(res.length === 1);

res = r("def");
assert(res === null);

Why? Beats me. I’d have thought .exec was easy enough to type and clearer to boot, myself. Hopefully readers familiar with the history can explain in comments.


Callable regular expressions present one immediate problem to a “naive” implementation: their behavior with typeof. According to ECMAScript, the typeof for any object which is callable should be "function", and Netscape and Mozilla for a long time faithfully implemented this. This tended to cause much confusion in practice, so browsers that implemented callable regular expressions eventually changed typeof to arguably “lie” for regular expressions and return "object". In SpiderMonkey the “fix” was an utterly inelegant hack which distinguished callables as either regular expressions or not, to determine typeof behavior.

Past this, callable regular expressions complicate implementing callability and optimizations of it. Implementations supporting getters and setters (once purely as an extension, now standardized in ES5) must consider the case where the getter or setter is a regular expression and do something appropriate. And of course they must handle regular old calls, qualified (/a/()) and unqualified (({ p: /a/ }).p()) both. Mozilla’s had a solid trickle of bugs involving callable regular expressions, almost always filed as a result of Jesse‘s evil fuzzers (and not due to actual sites breaking).

It’s also hard to justify callable regular expressions as an extension. While ECMAScript explicitly permits extensions, it generally prefers extensions to be new methods or properties of existing objects. Regular expression callability is neither of these: instead it’s adding an internal hook to regular expressions to make them callable. This might not technically be contrary to the spec, but it goes against its spirit.

Regular expressions won’t be callable in Firefox 5

No one’s ever really used callable regular expressions. They’re non-standard, not all browsers implement them, and they unnecessarily complicate implementations. So, in concert with other browser engines like WebKit, we’re making regular expressions non-callable in Firefox 5. (Regular expressions are callable in Firefox 4, but of course don’t rely on this.)

You can experiment with a version of Firefox with these changes by downloading a TraceMonkey nightly build. Trunk’s still locked down for Firefox 4, so it won’t pick up the change until Firefox 4 branches and trunk reopens for changes targeted at the next release. (Don’t forget to use the profile manager if you want to keep the settings you use with your primary Firefox installation pristine.)