More about regular expressions in Javascript

Aah !! Regular expressions

I have got confused most of the times, I need to escape special characters when specifying a regular expression.

Let us take an example of simple date format
dd/mm/yyyy

There are two ways, you can specify your regular expression

// Please note that your regular expression literal object must be surrounded
// between forward slashes as is done below.

// Since forward slash (/) has a special meaning in regular expressions
// it need to be escaped by a backslash (\)
var regex = /^\d{2}\/\d{2}\/\d{4}$/
regex.test("01/04/1975");

/*  / -- Used to signify that a regex literal follows.
 *  ^ - Starts with
 * \d{2} - 2 digits (date)
 * \/  - Escaping the forward slash
 * \d{2} - 2 digits (Month)
 * \/ - Escaping the forward slash
 * \d{4} - 4 digits (Year)
 * $ - end of string.
 * / - specifies the end of regex literal.
 */

// Things become more complex when you want to specify 
// regular expression in a String

// Please note the difference between the regex literal and the string regex
// Here we have to escape the backslash as well. 
// So the  number of backslashes are doubled.

var regex = new RegExp("^\\d{2}\\/\\d{2}\\/\\d{4}");


/*  
 *  ^ - Starts with
 * \\d{2} - 2 digits (date)
 * \\/  - Escaping the forward slash
 * \\d{2} - 2 digits (Month)
 * \\/ - Escaping the forward slash
 * \\d{4} - 4 digits (Year)
 * $ - end of regex.
 */

Please note that you have to escape all those characters which have a special meaning in Regular expressions.
Just place a backslash before that character. (2 backslashes if you are specifying regex as a string literal)
List of characters that need to be escaped are :

[, ], ., ?, *, +, /, \, {, }, |, (, )

3 Responses to More about regular expressions in Javascript

Aashu says:

November 21, 2011 at 14:00

As i understand 'd' here stands for digit then this regex wont give correct dates like it will parse the value 99/99/9999 while its not a valid date.. isn't it?

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Yogesh Gandhi says:

November 21, 2011 at 14:56

Dear aashu,

Yes, you are absolutely right. But this example is not a very precise regular expression for date validation.

If you want to write a exact date validator, just have a look at the link given in the post, 10 java regular expressions that you must know. 🙂

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Yogesh Gandhi says:

December 13, 2012 at 16:24

Please note that it is always better to use ^ in the beginning and $ in the end.

var REAL_NUMBER = new RegExp(“\\d+(\\.\\d+)?”);
alert(REAL_NUMBER.test(“333aad”));

The above code will give you alert true.

Correct code:
var REAL_NUMBER = new RegExp(“^\\d+(\\.\\d+)?$”);

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