JavaScript Regular Expressions
A regular expression is a sequence of characters that forms a search pattern.
The search pattern can be used for text search and text replace operations.
What Is a Regular Expression?
A regular expression is a sequence of characters that forms a search pattern.
When you search for data in a text, you can use this search pattern to describe what you are searching for.
A regular expression can be a single character, or a more complicated pattern.
Regular expressions can be used to perform all types of text search and text replace operations.
Syntax
/pattern/modifiers;
Example
var patt = /w3schools/i;
Example explained:
/w3schools/i is a regular expression.
w3schools is a pattern (to be used in a search).
i is a modifier (modifies the search to be case-insensitive).
Using String Methods
In JavaScript, regular expressions are often used with the two string methods: search() and replace().
The search() method uses an expression to search for a match, and returns the position of the match.
The replace() method returns a modified string where the pattern is replaced.
Using String search() With a Regular Expression
Example
Use a regular expression to do a case-insensitive search for "w3schools" in a string:
var str = "Visit W3Schools";
var n = str.search(/w3schools/i);
The result in n will be:
6
Try it Yourself »Using String search() With String
The search method will also accept a string as search argument. The string argument will be converted to a regular expression:
Example
Use a string to do a search for "W3schools" in a string:
var str = "Visit W3Schools!";
var n = str.search("W3Schools");
Try it Yourself »
Use String replace() With a Regular Expression
Example
Use a case insensitive regular expression to replace Microsoft with W3Schools in a string:
var str = "Visit Microsoft!";
var res = str.replace(/microsoft/i, "W3Schools");
The result in res will be:
Visit W3Schools!
Try it Yourself »
Using String replace() With a String
The replace() method will also accept a string as search argument:
var str = "Visit Microsoft!";
var res = str.replace("Microsoft", "W3Schools");
Try it Yourself »
Did You Notice?
 |
Regular expression arguments (instead of string arguments) can be used in
the methods above. Regular expressions can make your search much more powerful (case insensitive for example). |
|---|
Regular Expression Modifiers
Modifiers can be used to perform case-insensitive more global searches:
| Modifier | Description |
|---|---|
| i | Perform case-insensitive matching |
| g | Perform a global match (find all matches rather than stopping after the first match) |
| m | Perform multiline matching |
Regular Expression Patterns
Brackets are used to find a range of characters:
| Expression | Description |
|---|---|
| [abc] | Find any of the characters between the brackets |
| [0-9] | Find any of the digits between the brackets |
| (x|y) | Find any of the alternatives separated with | |
Metacharacters are characters with a special meaning:
| Metacharacter | Description |
|---|---|
| \d | Find a digit |
| \s | Find a whitespace character |
| \b | Find a match at the beginning or at the end of a word |
| \uxxxx | Find the Unicode character specified by the hexadecimal number xxxx |
Quantifiers define quantities:
| Quantifier | Description |
|---|---|
| n+ | Matches any string that contains at least one n |
| n* | Matches any string that contains zero or more occurrences of n |
| n? | Matches any string that contains zero or one occurrences of n |
Using the RegExp Object
In JavaScript, the RegExp object is a regular expression object with predefined properties and methods.
Using test()
The test() method is a RegExp expression method.
It searches a string for a pattern, and returns true or false, depending on the result.
The following example searches a string for the character "e":
Example
var patt = /e/;
patt.test("The best things in life are free!");
Since there is an "e" in the string, the output of the code above will be:
true
Try it Yourself »
You don't have to put the regular expression in a variable first. The two lines above can be shortened to one:
/e/.test("The best things in life are free!");
Using exec()
The exec() method is a RegExp expression method.
It searches a string for a specified pattern, and returns the found text.
If no match is found, it returns null.
The following example searches a string for the character "e":
Example 1
/e/.exec("The best things in life are free!");
Since there is an "e" in the string, the output of the code above will be:
e
Try it Yourself »
Complete RegExp Reference
For a complete reference, go to our Complete JavaScript RegExp Reference.
The reference contains descriptions and examples of all RegExp properties and methods.
