How to Use Regular Expressions in JavaScript
Regular expressions, popularly known as “regex” or “regexp,” are strings that describe a search pattern. You can use regular expressions to check if a string contains a specific pattern, extract information from a string, and replace parts of a string with new text.
Learn the basic syntax of regular expressions and how to use them in JavaScript.
The Basic Syntax of Regular Expressions
There are two ways you can create a regular expression in JavaScript: using a regular expression literal and using theRegExpconstructor.
A regular expression literal consists of a pattern enclosed between forward slashes, followed by an optional flag.
For example:
A flag is an optional parameter that you can add to a regular expression to modify its behavior. For example:
Thegflag indicates that the expression should match all occurrences, not just the first.
You can also create a regular expression using theRegExpconstructor. For example:
TheRegExpconstructor takes two parameters: a pattern—a string or a regular expression literal—and a flag(s).
There are two fairly common flags you’ll use with regular expression in JavaScript:
it’s possible to use flags together in a single expression in any order. For example:
This expression will match all occurrences of “Pattern”, irrespective of the case.
Regular Expression Metacharacters
In regular expressions, certain characters, known as metacharacters, have special meanings. You can use them to match specific types of characters or patterns.
Here are some of the most commonly used metacharacters and their meanings:
Testing a String Against a Regular Expression in JavaScript
In JavaScript, you may test a string against a regular expression using several methods.
This section assumes you have a basic understanding of regular expressions and patterns. If you are uncomfortable with regular expressions, check outa beginner’s guide to regular expressionsfirst.
The test Method
The.test()method returns a boolean indicating whether the regular expression matches the string or not. This method takes a string to perform the search on as an argument. It is particularly useful for simple checks.
This regular expression matches a string that ends with “.com”.
The exec Method
The.exec()method returns an array containing the matched text and any captured groups ornullif it doesn’t find a match. This method takes a string to perform the search on as an argument. It is useful for more complex regular expressions.
The regular expression above matches a string that starts with an optional “(”, three digits, and an optional “)”. It then looks for an optional “-”, “.”, or space, followed by three digits. It finally looks for an optional “-”, “.”, or space followed by four digits at the end of the string.
This regular expression matches phone numbers in the format of “(xxx) xxx-xxxx”, “xxx-xxx-xxxx”, “xxx.xxx.xxxx”, or “xxx xxx xxxx”.
If it finds a match,.exec()returns an array containing the matched text and any captured groups (defined by parentheses). It will include each group as an additional element in the array it returns. This allows you to access specific parts of the matched text, which can help you extract information from a string.
The replace Method
The.replace()method searches for a match between a regular expression and a string and replaces the matched text with a specified replacement text. This is a method of string objects, and it takes a regular expression and a replacement string as arguments.
This example calls thereplace()method on thestringvariable, passing the regular expression,expression. The regular expression will match all occurrences of “The” in the string, irrespective of the case. The call to the replace method instructs it to replace each occurrence with the string “a”.
Performance Considerations While Using Regular Expressions
Although regular expressions help match and manipulate strings, they can also be costly in terms of performance. Making patterns as specific as possible and keeping them simple is vital to keep them performant.
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