Regular Expression Expansion

1. RegExp constructor

es5 writing:

var regex = new RegExp('xyz', 'i' );
 // equivalent to 
var regex = /xyz/ i;


var regex = new RegExp(/xyz/ i);
 // equivalent to 
var regex = /xyz/i;

es6 writing:

var regex = new RegExp(/xyz/, 'i');

 

2. Regular method for strings

String objects have 4 methods that can use regular expressions: match(), replace(), search()andsplit()。

ES6 uses these four methods RegExpas instance methods that are all called within the language, so that all regular-related methods are defined on the RegExpobject.

String.prototype.match 调用 RegExp.prototype[Symbol.match]
String.prototype.replace 调用 RegExp.prototype[Symbol.replace]
String.prototype.search calls RegExp.prototype[Symbol.search]
String.prototype.split calls RegExp.prototype[Symbol.split]

3. Lookbehind assertions

es5 lookahead assertion:

Lookahead assertion: x(?=y) (x only matches before y) 
/\d+(?=%)/.exec('100% of US presidents have been male')   // ["100"]

Lookahead Negative Assertion: /x(?!y)/(x can only match if it is not in front of y)
/\d+(?!%)/.exec('that’s all 44 of them')                 // ["44"]

es6 lookbehind assertion:

Line-behind assertion: /(?<=y)x/ matches only the digits after the dollar sign, which should be written as/(?<=\$)\d+/

Negative assertion after the line: /(?<!y)x/ For example, to match only numbers that are not after the dollar sign, write it as /(?<!\$)\d+/.

/(?<=\$)\d+/.exec('Benjamin Franklin is on the $100 bill')  // ["100"]

/(?<!\$)\d+/.exec('it’s is worth about €90')                // ["90"]

The following example is a string replacement using a lookbehind assertion.

const reg= /(?<=\$)foo/g;
'$foo %foo foo'.replace(reg, 'bar');
// '$bar %foo foo'

In the above code, only after the dollar sign foowill be replaced.