Why JavaScript regex literal execution is slower when it is assigned to a variable?

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My question is related to a previous Stackoverflow post, which says the following two ways have almost the same speed.

But when I test it actually using loop count 1000000, there seems to be a consistent performance. While I expect Example 1 should be faster, but it turns out to be slower than Example 2.

The results of the testing page are here. For example 1, the execution times are: [20.27,19.82, 16.17], but for example 2, they are less:[18.57, 15.48, 15.14].

The results confuse me a lot. Could anyone have a hint about the reason?

var str = "Hello World";
// Example 1
var regExp1 = /[aeiou]+/gi;
for(var i = 0; i < 1000; ++i)
  regExp1.exec(str);
// Example 2
for(var j = 0; j < 1000; ++j)
  /[aeiou]+/gi.exec(str);

It must be slower.

Because every time you go in a new iteration in the loop you re-evaluate the regex before executing it.

This causes a small delay but with large loops, there will be a time difference

In the 1st example, it is already defined and evaluated.

Hence, the only time needed is for the execution.

Regular expressions, Using a regular expression literal, which consists of a pattern enclosed for more information (but slower execution) use the exec() or match() methods. created with an object initializer without assigning it to a variable. In the example above, we used a literal RegEx /help/ to search for the word help in variable str1 and str2. Another example of literal RegEx can be /expect/ , which will match the word expect from

jsperf.com is showing ops/sec, not execution times. Thus the higher the value, the faster the code.

The first version is faster, not slower. You got confusing results by running different number of iterations in one test set. 20.27 ops/s is faster than 18.57 ops/s.

There is no question - caching the regexp in a separate variable is going to be faster than compiling it in each iteration (the V8 VM caches compiled regex'es but apparently not as efficiently as doing it explicitly).

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You Should Learn Regex, Regular Expressions (Regex): One of the most powerful, widely We could replace [0-9] with \d , which will do the same thing (match any digit) 0.0 - Javascript / Node.js / Typescript Regex matching speeds can range from not- very-fast to extremely slow, depending on how well the expression is written. A literal is a fixed value, e.g., 5566, 3.14, "Hello", true, that can be assigned to a variable, or form part of an expression. Types JavaScript is object-oriented.

JavaScript best practices, None of these make much sense — good variable and function names should be easy You can protect those from being overwritten by using an object literal: in another set of parentheses to execute it without assigning any name to it: This includes regular expressions and — more importantly — DOM manipulation. JavaScript : Array literals. In Javascript, an array literal is a list of expressions, each of which represents an array element, enclosed in a pair of square brackets ' [ ] ' . When an array is created using an array literal, it is initialized with the specified values as its elements, and its length is set to the number of arguments specified.

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Comments
  • How are you timing this? I think for the benchmark to be meaningful, it should only be measuring the calls to exec(), and not anything else (the "anything else" not being relevant to the performance of the regex).
  • The link that you've provided for conducting the tests measures Operations/sec and not time elapsed. When I ran the tests, I found that the ops/sec is more for Example 1 than Example 2 which means that method in Example 1 is actually faster than method in Example 2.