Question

I have grouping parentheses () and (?: ) and I need to match even if the expression within the brackets does not match. I’ve seen both | and ? used for this (i.e. (a|b|c|) and (a|b|c)?), but which should be used/is more efficient, and why?

As different JavaScript engines interpret regex differently, I’m specifically using the SpiderMonkey engine. A generalised (both language-wise and engine-wise) answer would be nice however.

Edit: A concrete example is the DuckDuckGo Frequency goodie. Why did the author choose | over ? in this case?

Answer 1

To check performance, see this fiddle.

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Using ? with grouping or | with empty string as an option may lead to unexpected results!

Couple tests:

var myString = "this is a test string"; 
var myRegexp = /(test)?/; 
var match = myRegexp.exec(myString); 
alert(match[0]); // returns empty string

var myString = "this is a string"; 
var myRegexp = /(test)?/; 
var match = myRegexp.exec(myString); 
alert(match[0]); // returns empty string

var myString = "this is a test string"; 
var myRegexp = /(test|)/; 
var match = myRegexp.exec(myString); 
alert(match[0]); // returns empty string

var myString = "this is a string"; 
var myRegexp = /(test|)/; 
var match = myRegexp.exec(myString); 
alert(match[0]); // returns empty string

var myString = "this is a test string"; 
var myRegexp = /(test)/; 
var match = myRegexp.exec(myString); 
alert(match[0]); // returns "test"

This one ends with error:

var myString = "this is a string"; 
var myRegexp = /(test)/; 
var match = myRegexp.exec(myString); 
alert(match[0]); // error

And this one might be a solution for you:

var myString = "this is a test string"; 
var myRegexp = /^(?:.*(test)|(?!.*test))/; 
var match = myRegexp.exec(myString); 
alert(match[1]); // returns "test"

var myString = "this is a string"; 
var myRegexp = /^(?:.*(test)|(?!.*test))/; 
var match = myRegexp.exec(myString); 
alert(match[1]); // returns undefined

Test the above code with this fiddle.