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Regular Expressions, often called regex or RegExp, are a powerful way to work with text in JavaScript. They allow developers to define patterns for matching, searching, replacing, and validating strings. In this guide, you’ll learn the fundamentals of JavaScript regular expressions, including patterns, flags, special characters, and assertions.
A regular expression is a sequence of characters that forms a search pattern. In JavaScript, the RegExp object is used for defining and executing these patterns.
A RegExp can be written in two ways:
The /i flag makes the search case-insensitive. You can use regex to find patterns in text, validate input formats, or replace parts of a string.
Flags modify how a pattern behaves. Some of the most common flags include:
Flags can be combined, for example: /pattern/gi.
Character classes define sets or ranges of characters to match.
These are useful for creating flexible patterns that allow variations in text.
Metacharacters have special meanings in regex and are used to define complex patterns.
Quantifiers are used to specify how many times a pattern should occur:
Assertions do not consume characters but specify conditions for matching.
Lookaheads and lookbehinds allow you to check context without including it in the match:
Example:
Once you define a regular expression, you can use several methods to work with strings.
The RegExp object also has useful properties such as:
JavaScript regular expressions are essential for searching, validating, and manipulating text efficiently. By mastering patterns, flags, character classes, metacharacters, and assertions, you gain the ability to process text with high precision.
Regular expressions might look complex at first, but with practice, they become an indispensable part of every developer’s toolkit.
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