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<h3>Introduction</h3><p>Regular Expressions, often called <strong>regex</strong> or <strong>RegExp</strong>, 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.</p><h2><br></h2><h2>1. What Is a JavaScript RegExp</h2><p>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.</p><p>A RegExp can be written in two ways:</p><div class="ql-code-block-container" spellcheck="false"><div class="ql-code-block" data-language="plain"><br></div><div class="ql-code-block" data-language="plain">// Literal notation</div><div class="ql-code-block" data-language="plain">let pattern = /hello/i;</div><div class="ql-code-block" data-language="plain"><br></div><div class="ql-code-block" data-language="plain">// Constructor</div><div class="ql-code-block" data-language="plain">let pattern = new RegExp("hello", "i");</div></div><p>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.</p><h2><br></h2><h2>2. RegExp Flags</h2><p>Flags modify how a pattern behaves. Some of the most common flags include:</p><ol><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span><strong>g</strong> – Global search. Finds all matches instead of stopping after the first.</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span><strong>i</strong> – Case-insensitive search.</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span><strong>m</strong> – Multiline mode. The start ^ and end $ match each line, not just the whole string.</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span><strong>s</strong> – Allows the dot . to match newline characters.</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span><strong>u</strong> – Enables full Unicode support.</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span><strong>y</strong> – Sticky search. Matches starting exactly from the lastIndex position.</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span><strong>d</strong> – Provides match indices (newer feature).</li></ol><p>Flags can be combined, for example: /pattern/gi.</p><h2><br></h2><h2>3. Character Classes and Brackets</h2><p>Character classes define sets or ranges of characters to match.</p><ol><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>[abc] – Matches any one of the characters a, b, or c.</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>[^abc] – Matches any character except a, b, or c.</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>[a-z] – Matches any lowercase letter from a to z.</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>[0-9] – Matches any digit.</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>[A-Za-z0-9_] – Matches any alphanumeric character or underscore.</li></ol><p>These are useful for creating flexible patterns that allow variations in text.</p><h2><br></h2><h2>4. Metacharacters</h2><p>Metacharacters have special meanings in regex and are used to define complex patterns.</p><ol><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>. – Matches any character except a newline.</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>d – Matches any digit (0–9).</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>w – Matches any word character (letters, digits, underscore).</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>s – Matches whitespace (spaces, tabs, line breaks).</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>D, W, S – Negated forms of the above.</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>b – Word boundary.</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>B – Non-word boundary.</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>^ – Matches the start of a string or line.</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>$ – Matches the end of a string or line.</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>| – Acts as OR (alternative between patterns).</li></ol><p><strong>Quantifiers</strong> are used to specify how many times a pattern should occur:</p><ol><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>x* – Zero or more times.</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>x+ – One or more times.</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>x? – Zero or one time.</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>x{n} – Exactly n times.</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>x{n,} – At least n times.</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>x{n,m} – Between n and m times.</li></ol><h2><br></h2><h2>5. Assertions and Boundaries</h2><p>Assertions do not consume characters but specify conditions for matching.</p><ol><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>^pattern – Matches only if the pattern is at the beginning of the string.</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>pattern$ – Matches only if the pattern is at the end of the string.</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>b – Matches a word boundary (start or end of a word).</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>B – Matches a position that is not a word boundary.</li></ol><p><strong>Lookaheads and lookbehinds</strong> allow you to check context without including it in the match:</p><ol><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>x(?=y) – Matches x only if it is followed by y.</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>x(?!y) – Matches x only if it is not followed by y.</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>(?<=y)x – Matches x only if it is preceded by y.</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span>(?<!y)x – Matches x only if it is not preceded by y.</li></ol><p>Example:</p><div class="ql-code-block-container" spellcheck="false"><div class="ql-code-block" data-language="plain"><br></div><div class="ql-code-block" data-language="plain">let pattern = /^Hello/;</div><div class="ql-code-block" data-language="plain">console.log(pattern.test("Hello world")); // true</div><div class="ql-code-block" data-language="plain">console.log(pattern.test("Say Hello"));  // false</div></div><h2><br></h2><h2>6. RegExp Methods and Usage</h2><p>Once you define a regular expression, you can use several methods to work with strings.</p><ol><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span><strong>test()</strong> – Tests for a match and returns true or false.</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span><strong>exec()</strong> – Returns detailed information about the first match or null.</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span><strong>match()</strong> – Returns an array of matches.</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span><strong>replace()</strong> – Replaces matched text with another string.</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span><strong>search()</strong> – Returns the position of the first match.</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span><strong>split()</strong> – Splits a string into an array based on the pattern.</li></ol><p>Example:</p><div class="ql-code-block-container" spellcheck="false"><div class="ql-code-block" data-language="plain"><br></div><div class="ql-code-block" data-language="plain">let text = "I love JavaScript";</div><div class="ql-code-block" data-language="plain">let pattern = /love/i;</div><div class="ql-code-block" data-language="plain">console.log(pattern.test(text));  // true</div></div><p>The RegExp object also has useful properties such as:</p><ol><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span><strong>source</strong> – Returns the text of the pattern.</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span><strong>flags</strong> – Returns the flags used in the expression.</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span><strong>global</strong>, <strong>ignoreCase</strong>, <strong>multiline</strong> – Booleans for respective flags.</li><li data-list="bullet"><span class="ql-ui" contenteditable="false"></span><strong>lastIndex</strong> – Index to start the next match when using the global flag.</li></ol><h2><br></h2><h2>Conclusion</h2><p>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.</p><p>Regular expressions might look complex at first, but with practice, they become an indispensable part of every developer’s toolkit.</p>

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