Why have two '\' in Regex?

Question

function trim(str) {

    var trimer = new RegExp("(^[\\s\\t\\xa0\\u3000]+)|([\\u3000\\xa0\\s\\t]+\x24)", "g");

    return String(str).replace(trimer, "");

}

why have two '\' before 's' and 't'?

and what's this "[\s\t\xa0\u3000]" mean?

Answer 1

You're using a literal string.

In a literal string, the \ character is used to escape some other chars, for example \n (a new line) or \" (a double quote), and it must be escaped itself as \\. So when you want your string to have \s, you must write \\s in your string literal.

Thankfully JavaScript provides a better solution, Regular expression literals:

var trimer = /(^[\s\t\xa0\u3000]+)|([\u3000\xa0\s\t]+\x24)/g

Answer 2

why have two '\' before 's' and 't'?

In regex the \ is an escape which tells regex that a special character follows. Because you are using it in a string literal you need to escape the \ with \.

and what's this "[\s\t\xa0\u3000]" mean?

It means to match one of the following characters:

• \s white space.
• \t tab character.
• \xa0 non breaking space.
• \u3000 wide space.

This function is inefficient because each time it is called it is converting a string to a regex and then it is compiling that regex. It would be more efficient to use a Regex literal not a string and compile the regex outside the function like the following:

var trimRegex = /(^[\s\t\xa0\u3000]+)|([\u3000\xa0\s\t]+$)/g;

function trim(str) {
    return String(str).replace(trimRegex, "");
}

Further to this \s will match any whitespace which includes tabs, the wide space and the non breaking space so you could simplify the regex to the following:

var trimRegex = /(^\s+)|(\s+$)/g;

Browsers now implement a trim function so you can use this and use a polyfill for older browsers. See this Answer