JavaScript: RegExp Object

There are 2 ways to use regex for string pattern matching. One is using string's methods. 〔➤see JavaScript String Regex Methods〕 The other way is creating a regex object, and use the regex object's methods.

Creating a RegExp Object

RegExp in JavaScript is technically a “object”. 〔➤see JavaScript: What's Object?〕

To create a regex object, there are 2 ways:

• /pattern/flags → the literal syntax.
• new RegExp("pattern", "flags") → This syntax is useful when you want to generate regex at runtime. ➢ for example: from user input.

Note: when using new RegExp("pattern"), you need to use double backslash in your string argument for any single backslash in your regex. Because, single backslash is used in string to escape the next character. So, when you use double backslash, the regex will just get a single backslash.

// remember to double backslash when using regex constructor

var myRegex1 = new RegExp("\\d+"); // one or more digit
console.log(myRegex1.test("333")); // true. match found

var myRegex2 = new RegExp("\d+"); // WRONG
console.log(myRegex2.test("333")); // false

RegExp Object Methods

Method	Meaning
regex.test(string)	Return true or false. 〔➤see JavaScript: RegExp.prototype.test〕
regex.exec(string)	Return a array of captured patterns, or null. 〔➤see JavaScript: RegExp.prototype.exec〕

RegExp Object Properties

RegExp object has these properties. The most importat one is lastIndex.

Property	Meaning
source	the regex (read only)
global	boolean. whether there's the global flag g in regex. (read only)
ignoreCase	boolean. whether there's the ignore case flag i in regex. (read only)
multiline	boolean. whether there's the multi-line flag m in regex. (read only)
lastIndex	When global flag g is used, the value is the position of end of last match. Next invocation of the method with the same regex object will start from here. (you may want to reset this to 0 in a loop.) When global flag is not used, this value is always 0.

Example:

// example of regex object properties

var myText = 'a x1 X2 c';

var myRegex = new RegExp('\\w\\d+', "ig");

var myResult = myRegex.exec(myText);

// print regex properties

console.log(myRegex.source);    // \w\d+

console.log(myRegex.global);     // true
console.log(myRegex.ignoreCase); // true
console.log(myRegex.multiline);  // false

console.log(myRegex.lastIndex); // 4

// print result

console.log(myResult); // [ 'x1', index: 2, input: 'a x1 X2 c' ]

Regex Result Object Properties

Property	Meaning
index	the beginning position of first occurance.
input	the input string.

Example:

// example of regex result properties

var myText = 'a x1 X2 c';

var myRegex = new RegExp('\\w\\d+', "ig");

var myResult = myRegex.exec(myText);

// print result properties

console.log(myResult.index); // 2
console.log(myResult.input); // a x1 X2 c

Reference

ES5 §15#sec-15.10