JavaScript Regular Expressions

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 = /Mytat/i;

Example explained:

/Mytat/i  is a regular expression.

Mytat  is a pattern (to be used in a search).

i  is a modifier (modifies the search to be case-insensitive).

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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.

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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 "Mytat" in a string:  

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Using String search() With a Regular Expression

Example

Use a regular expression to do a case-insensitive search for "mytat" in a string:

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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:

 

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Use String replace() With a Regular Expression

Example

Use a case insensitive regular expression to replace Microsoft with mytat in a string:

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The result in res will be:

Visit Mytat!

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 of a word like this: \bWORD, or at the end of a word like this: WORD\b
\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

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Using The RegExp Object

In JavaScript, the RegExp object is a regular expression object with predefined properties and methods.

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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

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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:

 

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":

Example 1

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