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
/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 String
The search() method searches a string for a specified value and returns the position of the match:
Example
Use a string to do a search for "W3schools" in a string:
let text = "Visit W3Schools!";
let n = text.search("W3Schools");
The result in n will be:
6
Using String search() With a Regular Expression
Example
Use a regular expression to do a case-insensitive search for "w3schools" in a string:
let text = "Visit W3Schools";
let n = text.search(/w3schools/i);
The result in n will be:
6
Using String replace() With a String
The replace() method replaces a specified value with another value in a string:
let text = "Visit Microsoft!";
let result = text.replace("Microsoft", "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:
let text = "Visit Microsoft!";
let result = text.replace(/microsoft/i, "W3Schools");
The result in res will be:
Visit W3Schools!
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 | Try it |
|---|---|---|
| i | Perform case-insensitive matching | Try it » |
| g | Perform a global match (find all matches rather than stopping after the first match) | Try it » |
| m | Perform multiline matching | Try it » |
Regular Expression Patterns
Brackets are used to find a range of characters:
| Expression | Description | Try it |
|---|---|---|
| [abc] | Find any of the characters between the brackets | Try it » |
| [0-9] | Find any of the digits between the brackets | Try it » |
| (x|y) | Find any of the alternatives separated with | | Try it » |
Metacharacters are characters with a special meaning:
| Metacharacter | Description | Try it |
|---|---|---|
| \d | Find a digit | Try it » |
| \s | Find a whitespace character | Try it » |
| \b | Find a match at the beginning of a word like this: \bWORD, or at the end of a word like this: WORD\b |
Try it »
Try it » |
| \uxxxx | Find the Unicode character specified by the hexadecimal number xxxx | Try it » |
Quantifiers define quantities:
| Quantifier | Description | Try it |
|---|---|---|
| n+ | Matches any string that contains at least one n | Try it » |
| n* | Matches any string that contains zero or more occurrences of n | Try it » |
| n? | Matches any string that contains zero or one occurrences of n | Try it » |
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
const pattern = /e/;
pattern.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
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 as an object.
If no match is found, it returns an empty (null) object.
The following example searches a string for the character "e":
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.
